Dissertations / Theses on the topic 'AGB stars'

Mass loss, in the form of slow stellar winds, is a decisive factor for the evolution of cool luminous giants, eventually turning them into white dwarfs. These dense outflows are also a key factor in the enrichment of the interstellar medium with newly produced elements from the interior of these stars. There are strong indications that these winds are accelerated by radiation pressure on dust grains, but the actual grain species responsible for driving the outflows in M-type Asymptotic Giant Branch stars are still a matter of debate. Observations of dust features in the circumstellar environment of these stars suggest that magnesium-iron silicates are possible wind-drivers. However, the optical properties of these silicate grains are strongly influenced by the Fe-content. Fe-bearing condensates heat up strongly when interacting with the radiation field and therefore cannot form close enough to the star to trigger outflows. Fe-free condensates, on the other hand, have a low absorption cross-section at near-IR wavelengths where AGB stars emit most of their flux.  To solve this conundrum, it has been suggested that winds of M-type AGB stars may be driven by photon scattering on Fe-free silicate grains with sizes comparable to the wavelength of the flux maximum, rather than by true absorption. In this thesis we investigate dynamical models of M-type AGB stars, using Fe-free silicates as the wind-driving dust species. According to our findings these models produce both dynamic and photometric properties consistent with observations. Especially noteworthy are the large photometric variations in the visual band during a pulsation cycle, seen both in the observed and synthetic fluxes. A closer examination of the models reveals that these variations are caused by changes in the molecular layers, and not by changes in the dust. This is a strong indication that stellar winds of M-type AGB stars are driven by dust materials that are very transparent in the visual and near-infrared wavelength regions, otherwise these molecular effects would not be visible.

Post asymptotic giant branch (post-AGB) stars, central stars of planetary nebulae (CSPNe) and planetary nebulae (PNe) are important phases of stellar evolution as the material they feedback is the seed of subsequent star formation in a galaxy. The majority of low and intermediate mass stars are expected to evolve through these channels, however, it is uncertain how many actually do, and at what rate. The Galactic halo, with its older population, provides a direct test of evolutionary models for low mass stars. Birthrate estimates of PNe are uncertain and worse still, are in contradiction with accepted white dwarf (WD) birthrate estimates. Much of the uncertainty stems from the lack of complete samples and poorly determined distance estimates. New surveys such as the Sloan Digital Sky Survey (SDSS), Galaxy Evolutionary Ex- plorer (GALEX) and the INT Photometric H® Survey (IPHAS) have discovered many new PNe and have observed the far edges of the Galaxy. Improved methods of determining distances to CSPNe are presented here, using model atmospheres, evolutionary tracks and high resolution reddening maps utilising these revolution- ary surveys. Locating the CSPN is non-trivial particularly for evolved PNe, as they are ex- tended with their central star often displaced from the centre of the nebula. There- fore, photometric criteria are required to locate the CSPN in the nebula’s field. Synthetic photometry of the CSPNe is derived from spectral energy distributions (SEDs) computed from a grid of model atmospheres covering the parameter range of CSPNe. The SEDs are convolved with filter transmission curves to compute synthetic magnitudes for a given photometric system which are then calibrated with standard stars and WDs. A further project borne out of a search for luminous central stars of faint PNe, resulted in a systematic search for post-AGB stars in the Galactic halo. In this work, new candidate halo post-AGB stars are discovered from a search through the SDSS spectroscopic database. Combined with previously identified halo post- AGB stars, including the results of a sub-sample from the Palomar-Green (PG) survey, the number of observed and predicted populations are compared. The number of observed post-AGB candidates shows a remarkable deficit to expecta- tions. A survey within a subset of the photometric database of SDSS supports the findings of the PG and SDSS spectroscopic surveys. These findings provide strong evidence for a lack of post-AGB stars in the Galac- tic halo and thick disc. A plausible explanation is that a large fraction of stars in these old, metal-poor populations are evolving via alternative channels. The implications of such a result are far reaching with knock on effects for stellar evolutionary theory, galactic evolution and extragalactic redshift estimates.

This report will first give a brief background on asymptotic giant branch (AGB) stars and the characteristics that make them interesting to study. Some methods and tools used in the field are then introduced, before the photometric variability of these stars is investigated. This is achieved by using data from dynamical models of AGB stars with differing chemical abundances. The R, J and K bands of the UBVRI system are specifcally investigated to explore whether these are good candidates for AGB photometric and spectroscopic research. Lastly, the molecular features at these wavelengths are investigated to understand the impact that they have on the photometric variability during the pulsation cycle and which molecules are most prominent in this.

This thesis describes the development of a red tip/tilt and fringe detection system at the Sydney University Stellar Interferometer (SUSI), modelling the instrumental performance and effects of seeing at SUSI, making observations of Mira variable stars and finally modelling the atmospheres of Mira variables with physically self-consistent models. The new SUSI tip/tilt system is based around a CCD detector and has been successfully used to both track the majority of tip/tilt power in median seeing at an R magnitude of 4.5, and to provide seeing measures for post processing. The new fringe-detection system rapidly scans 33 to 140 $\mu$m in delay and detects the fringes using two avalanche-photodiodes. It has been used to acquire fringe data, provide user feedback and to track the fringe group-delay position. The system visibility (fringe visibility for a point source) and throughput were found to be consistent with models of the SUSI optical beam train. Observations were made of a variety of sources, including the Mira variables R Car and RR Sco, which were observed in two orthogonal polarization states. These measurements were the first successful use of Optical Interferometric Polarimetry (OIP), and enabled scattered light to be separated from bright photospheric flux. Dust scattering was found to originate from a thin shell 2-3 continuum radii from these stars, with an optical depth of 0.1 to 0.2 at 900 nm. Physical models of Mira variables including dust formation were developed, providing consistent explanations for these results as well as many other photometric and interferometric observations.

This thesis describes the development of a red tip/tilt and fringe detection system at the Sydney University Stellar Interferometer (SUSI), modelling the instrumental performance and effects of seeing at SUSI, making observations of Mira variable stars and finally modelling the atmospheres of Mira variables with physically self-consistent models. The new SUSI tip/tilt system is based around a CCD detector and has been successfully used to both track the majority of tip/tilt power in median seeing at an R magnitude of 4.5, and to provide seeing measures for post processing. The new fringe-detection system rapidly scans 33 to 140 $\mu$m in delay and detects the fringes using two avalanche-photodiodes. It has been used to acquire fringe data, provide user feedback and to track the fringe group-delay position. The system visibility (fringe visibility for a point source) and throughput were found to be consistent with models of the SUSI optical beam train. Observations were made of a variety of sources, including the Mira variables R Car and RR Sco, which were observed in two orthogonal polarization states. These measurements were the first successful use of Optical Interferometric Polarimetry (OIP), and enabled scattered light to be separated from bright photospheric flux. Dust scattering was found to originate from a thin shell 2-3 continuum radii from these stars, with an optical depth of 0.1 to 0.2 at 900 nm. Physical models of Mira variables including dust formation were developed, providing consistent explanations for these results as well as many other photometric and interferometric observations.

From the main sequence onwards, stars of intermediate masses (1-8 Solar masses) eject a large portion of their mass with rates as high as 0.0001 Solar masses per year during their transition through the Asymptotical Giant Branch (AGB) stage. The outflows are shaped by the same mechanisms that shape the ejecta, which in turn appear to depart from spherical symmetry as early as the AGB stage. The ejecta are then evolving into asymmetrical structures. Stars like that are giant factories of dust, responsible for the enrichment of their surrounding Galactic medium in metals heavier than helium. Depending on their abundances during the AGB stage, the stars are either oxygen-rich or carbon-rich, and as such, the dust produced in their atmospheres is either O-rich or C-rich. The chemical composition of the ejecta, indicates the stellar chemistry at the moment of ejection. The disruption of the spherical symmetry of the mass loss can be caused by fast rotation, stellar magnetic fields or binarity, the latter being the most efficient and favourable mechanism. Such mechanisms can lead to the creation of circumstellar, equatorial, dusty structures, like discs, torii or spirals. Due to their small relative sizes, compared to their surrounding nebulae, they can be studied at best with the use of infrared interferometric techniques. We report the discovery of three such structures in sources at three different evolutionary stages, respectively, with the use of single- and multi-aperture interferometry. In the C-rich AGB star V Hya we imaged via aperture masking in the near-infrared, a complex and possibly orbiting structure, which is embedded within the star's molecular torus. Our MIDI observations in the mid-infrared have revealed, a silicate disc within the symbiotic nebula M2-9 that is currently being shaped by the central binary system within its core, and a C-rich disc-like structure in the born-again star Sakurai's Object, that is also aligned to an asymmetry found in its surrounding planetary nebula. Finally, we compare the properties of the structures found here with those found in the literature in order to establish a relation between late stellar evolution and the existence of dusty structures.

This thesis focuses on the analysis of two UV-bright globular cluster B-type post-Asymptotic Giant Branch (AGB) stars to better understand their evolutionary status. Current theory proposes that the stars have undergone a gas-dust separation based on abundances obtained. Here, ROA 5701, in w Cen and Barnard 29, in M13, are investigated with high-resolution optical spectra. C, N, 0, Mg, AI, Si and S abundance estimates are determined, finding a general metal underabundance relative to young Galactic B-type stars. Their abundance patterns suggested that the stars have not undergone a gas-dust separation, although they may have evolved from the AGB before the third dredge-up. Gas-dust separation was suggested du~ to low Fe abundances, however, Fe abundances for these stars have previously only been estimated from ultraviolet (UV) spectra. Therefore, the role of optical Fe III absorption lines as abundance diagnostics is investigated, using optical spectra for Galactic B-type supergiants and mainsequence stars. With the observed Fe III spectra for a sample of the supergiants, and synthetic spectra from the model atmosphere codes TLUSTY and CMFGEN, non-LTE effects are found to be small. For a sample of the supergiants and main-sequence stars, LTE abundance estimates are obtained, consistent with previous optical studies and the Galactic environment. This thesis recommends a list of Fe III transitions for estimating reliable iron abundances from. early B-type stellar spectra. Following on from the optical Fe III study, for Barnard 29 and ROA 5701, iron abundances are found from both UV and optical spectra, with the UV abundances lower than those expected from the metallicities of the respective clusters. A similar systematic underabundance for UV Fe abundances isfound for other B-type stars in known metallicity environments, e.g. the Magellanic Clouds. The results indicate that Fe III UV lines may yield abundance values which are systematically too low by typically 0.6 dex and hence should be treated with caution.

The notion that low- to intermediate-mass young stellar objects (YSOs) gain mass at a constant rate during the early stages of their evolution appears to be challenged by observations of YSOs suffering sudden increases of the rate at which they gain mass from their circumstellar discs. Also, this idea that stars spend most of their lifetime with a low accretion rate and gain most of their final mass during short-lived episodes of high accretion bursts, helps to solve some long-standing problems in stellar evolution. The original classification of eruptive variables divides them in two separate subclasses known as FU Orionis stars (FUors) and EX Lupi stars (EXors). In this classical view FUors are at an early evolutionary stage and are still gaining mass from their parent envelopes, whilst EXors are thought to be older objects only surrounded by an accretion disc. The problem with this classical view is that it excludes younger protostars which have higher accretion rates but are too deeply embedded in circumstellar matter to be observed at optical wavelengths. Optically invisible protostars have been observed to display large variability in the near-infrared. These and some recent discoveries of new eruptive variables, show characteristics that can be attributed to both of the optically-defined subclasses of eruptive variables. The new objects have been proposed to be part of a new class of eruptive variables. However, a more accepted scenario is that in fact the original classes only represent two extremes of the same phenomena. In this sense eruptive variability could be explained as arising from one physical mechanism, i.e. unsteady accretion, where a variation in the parameters of such mechanism can cause the different characteristics observed in the members of this class. With the aim of studying the incidence of episodic accretion among young stellar objects, and to characterize the nature of these eruptive variables we searched for high amplitude variability in two multi-epoch infrared surveys: the UKIDSS Galactic Plane Survey (GPS) and the Vista Variables in the Via Lactea (VVV). In order to further investigate the nature of the selected variable stars, we use photometric information arising from public surveys at near- to farinfrared wavelengths. In addition we have performed spectroscopic and photometric follow-up for a large subset of the samples arising from GPS and VVV. We analyse the widely separated two-epoch K-band photometry in the 5th, 7th and 8th data releases of the UKIDSS Galactic Plane Survey. We find 71 stars with _K > 1 mag, including 2 previously known OH/IR stars and a Nova. Even though the mid-plane is mostly excluded from the dataset, we find the majority (66%) of our sample to be within known star forming regions (SFRs), with two large concentrations in the Serpens OB2 association (11 stars) and the Cygnus-X complex (27 stars). The analysis of the multi-epoch K-band photometry of 2010-2012 data from VVV covering the Galactic disc at |b| < 1◦ yields 816 high amplitude variables, which include known variables of different classes such as high mass X-ray binaries, Novae and eclipsing binaries among others. Remarkably, 65% of the sample are found concentrated towards areas of star formation, similar to the results from GPS. In both surveys, sources in SFRs show spectral energy distributions (SEDs) that support classification as YSOs. This indicates that YSOs dominate the Galactic population of high amplitude infrared variable stars at low luminosities and therefore likely dominate the total high amplitude population. Spectroscopic follow-up allows us to confirm the pre-main sequence nature of several GPS and VVV Objects. Most objects in both samples show spectroscopic signatures that can be attributed to YSOs undergoing high states of accretion, such as veiling of photospheric features and CO emission, or show FUor-like spectra. We also find a large fraction of objects with 2.12 μm H2 emission that can be explained as arising from shock-excited emission caused by molecular outflows. Whether these molecular outflows are related to outbursts events cannot be confirmed from our data. Adding the GPS and VVV spectroscopic results, we find that between 6 and 14 objects are new additions to the FUor class from their close resemblance to the near-infrared spectra of FUors, and at least 23 more objects are new additions to the eruptive variable class. For most of these we are unable to classify them into any of the original definitions for this variable class. In any case, we are adding up to 37 new stars to the eruptive variable class which would double the current number of known objects. We note that most objects are found to be deeply embedded optically invisible stars, thus increasing the number of objects belonging to this subclass by a much larger factor. In general, objects in our samples which are found to be likely eruptive variable stars show a mixture of characteristics that can be attributed to both of the optically-defined classes. This agrees well with the recent discoveries in the literature. Finally, we are able to derive a first rough estimate on the incidence of episodic accretion among class I YSOs in the star-forming complex G305. We find that _ 9% of such objects are in a state of high accretion. This number is in agreement with previous theoretical and observational estimates among class I YSOs.

Central stars of planetary nebulae are low-mass stars on the brink of their final evolution towards white dwarfs. Because of their surface temperature of above 25,000 K their UV radiation ionizes the surrounding material, which was ejected in an earlier phase of their evolution. Such fluorescent circumstellar gas is called a "Planetary Nebula". About one-tenth of the Galactic central stars are hydrogen-deficient. Generally, the surface of these central stars is a mixture of helium, carbon, and oxygen resulting from partial helium burning. Moreover, most of them have a strong stellar wind, similar to massive Pop-I Wolf-Rayet stars, and are in analogy classified as [WC]. The brackets distinguish the special type from the massive WC stars. Qualitative spectral analyses of [WC] stars lead to the assumption of an evolutionary sequence from the cooler, so-called late-type [WCL] stars to the very hot, early-type [WCE] stars. Quantitative analyses of the winds of [WC] stars became possible by means of computer programs that solve the radiative transfer in the co-moving frame, together with the statistical equilibrium equations for the population numbers. First analyses employing models without iron-line blanketing resulted in systematically different abundances for [WCL] and [WCE] stars. While the mass ratio of He:C is roughly 40:50 for [WCL] stars, it is 60:30 in average for [WCE] stars. The postulated evolution from [WCL] to [WCE] however could only lead to an increase of carbon, since heavier elements are built up by nuclear fusion. In the present work, improved models are used to re-analyze the [WCE] stars and to confirm their He:C abundance ratio. Refined models, calculated with the Potsdam WR model atmosphere code (PoWR), account now for line-blanketing due to iron group elements, small scale wind inhomogeneities, and complex model atoms for He, C, O, H, P, N, and Ne. Referring to stellar evolutionary models for the hydrogen-deficient [WC] stars, Ne and N abundances are of particular interest. Only one out of three different evolutionary channels, the VLTP scenario, leads to a Ne and N overabundance of a few percent by mass. A VLTP, a very late thermal pulse, is a rapid increase of the energy production of the helium-burning shell, while hydrogen burning has already ceased. Subsequently, the hydrogen envelope is mixed with deeper layers and completely burnt in the presence of C, He, and O. This results in the formation of N and Ne. A sample of eleven [WCE] stars has been analyzed. For three of them, PB 6, NGC 5189, and [S71d]3, a N overabundance of 1.5% has been found, while for three other [WCE] stars such high abundances of N can be excluded. In the case of NGC 5189, strong spectral lines of Ne can be reproduced qualitatively by our models. At present, the Ne mass fraction can only be roughly estimated from the Ne emission lines and seems to be in the order of a few percent by mass. Furthermore, using a diagnostic He-C line pair, the He:C abundance ratio of 60:30 for [WCE] stars is confirmed. Within the framework of the analysis, a new class of hydrogen-deficient central stars has been discovered, with PB 8 as its first member. Its atmospheric mixture resembles rather that of the massive WNL stars than of the [WC] stars. The determined mass fractions H:He:C:N:O are 40:55:1.3:2:1.3. As the wind of PB 8 contains significant amounts of O and C, in contrast to WN stars, a classification as [WN/WC] is suggested.
Zentralsterne Planetarischer Nebel sind massearme Sterne kurz vor ihrer finalen Entwicklung zu Weißen Zwergen. Aufgrund ihrer Oberflächentemperatur von über 25 000 K sind sie in der Lage, durch Abstrahlung von UV-Licht das sie umgebende Material, welches in einer vorigen Phase ihrer Entwicklung abgestoßen wurde, zu ionisieren. Das solchermaßen zum Leuchten angeregte Gas bezeichnet man als Planetarischen Nebel. Etwa ein Zehntel der galaktischen Zentralsterne sind wasserstoffarm. Im Allgemeinen besteht die Oberfläche dieser Zentralsterne aus einer Mischung der Elemente Helium, Kohlenstoff und Sauerstoff, welche z.T. durch Heliumbrennen erzeugt wurden. Die meisten dieser Sterne haben darüberhinaus einen starken Sternwind, ähnlich den massereichen Pop-I-Wolf-Rayet-Sternen und werden in Analogie zu diesen als [WC] klassifiziert, wobei die eckigen Klammern der Unterscheidung von den massereichen WC-Sternen dienen. Qualitative Analysen der Spektren von [WC]-Sternen lassen eine Entwicklungssequenz dieser Sterne von kühleren sogenannten late-type [WC]-Sternen (kurz [WCL]) zu sehr heißen, early-type [WC]-Sternen (kurz [WCE]) vermuten. Mithilfe von Computerprogrammen, die den Strahlungstransport im mitbewegten Beobachtersystem zusammen mit den statistischen Gleichungen der Besetzungszahlen der Ionen im Sternwind rechnen können, wurden quantitative Untersuchungen der Winde von [WC]-Sternen möglich. Erste Analysen mit Modellen ohne Eisenlinien ergaben dabei systematisch unterschiedliche Häufigkeiten für [WCL]- und [WCE]-Sterne. Während sich für [WCL]-Sterne ein Verhältnis der Massenanteile von He:C von etwas 40:50 ergab, fand man für die [WCE]-Sterne ein mittleres Verhältnis von 60:30 für die He:C-Massenanteile. Dabei sollte die Entwicklung von [WCL] nach [WCE] innerhalb einer sehr kurzen Zeit durch Aufheizung infolge der Kontraktion der Hülle erfolgen und nicht mit einer wesentlichen Abnahme der Kohlenstoffhäufigkeit bei gleichzeitiger Zunahme der Heliumhäufigkeit an der Oberfläche einhergehen. Im Rahmen der vorgelegten Arbeit wird untersucht, ob sich mittels verbesserter Modelle für die Atmosphären von [WC]-Sternen das He:C-Häufigkeitsverhältnis der [WCE]-Sterne bestätigt. Elaboriertere Modelle, welche vom Potsdamer WR-Modelatmosphären-Code (PoWR) berechnet werden können, berücksichtigen Line-Blanketing aufgrund von Elementen der Eisengruppe, kleinskalige Windinhomogenitäten und die Elemente He, C, O, H, P, N und Ne. Unter Bezug auf Sternentwicklungsmodelle, die die Ursache der Wasserstoffunterhäufigkeit von [WC]-Sternen erklären, sind insbesondere die Neon- und Stickstoff-Häufigkeiten interessant. Von den drei möglichen Entwicklungskanälen für [WC]-Sterne führt lediglich das VLTP-Szenario zu einer Stickstoff-Überhäufigkeit von einigen Prozent bezogen auf die Masse. Bei einem VLTP, einem very late thermal pulse, handelt es sich um einen plötzlichen, starken Anstieg der Energieproduktion in der helium-brennenden Schale, während das Wasserstoffbrennen bereits zum Erliegen gekommen ist. Infolge eines VLTPs wird sämtlicher Wasserstoff kurz nach dem thermischen Puls in tiefere Schichten gemischt und in Anwesenheit von C, He und O verbrannt. Infolgedessen wird N und auch Ne erzeugt. Bei der Analyse von elf [WCE]-Sternen wurden für drei von ihnen, PB 6, NGC 5189 und [S71d]3, Stickststoffmassenanteile von 1,5 % bestimmt, während für drei andere Sterne solche hohen Stickstoffhäufigkeiten ausgeschlossen werden können. Für NGC 5189 gelang außerdem die qualitative Reproduktion der beobachteten, starken Ne-Spektrallinien mittels unserer Modelle. Zur Zeit lässt sich aus der Stärke der Ne-Emissionslinien der Ne-Massenanteil leider nur abschätzen, er scheint aber im Bereich einiger Prozent zu liegen. Mittels eines diagnostischen He-C-Linienpaares konnte das He:C-Massenverhältnis von 60:30 für [WCE]-Sterne bestätigt werden. Als Ergebnis der Analyse von PB 8 postulieren wir eine neue Klasse von wasserstoffarmen Zentralsternen, die in ihrer Elementzusammensetzung eher an massereiche WNL-Sterne als an [WC]-Sterne erinnern. Die ermittelten Massenanteile H:He:C:N:O betragen 40:55:1.3:2:1.3, der Wind von PB 8 enthält daher im Unterschied zu WN-Sternen signifikante Mengen von O und C. Es wird daher eine Klassifizierung als [WN/WC] vorgeschlagen.

We present an ongoing, five-year systematic search for extragalactic infrared transients, dubbed SPIRITS-SPitzer InfraRed Intensive Transients Survey. In the first year, using Spitzer/IRAC, we searched 190 nearby galaxies with cadence baselines of one month and six months. We discovered over 1958 variables and 43 transients. Here, we describe the survey design and highlight 14 unusual infrared transients with no optical counterparts to deep limits, which we refer to as SPRITEs (eSPecially Red Intermediate-luminosity Transient Events). SPRITEs are in the infrared luminosity gap between novae and supernovae, with [4.5] absolute magnitudes between -11 and -14 (Vega-mag) and [3.6]-[4.5] colors between 0.3 mag and 1.6 mag. The photometric evolution of SPRITEs is diverse, ranging from < 0.1 mag yr(-1) to > 7 mag yr(-1). SPRITEs occur in star-forming galaxies. We present an indepth study of one of them, SPIRITS 14ajc in Messier 83, which shows shock-excited molecular hydrogen emission. This shock may have been triggered by the dynamic decay of a non-hierarchical system of massive stars that led to either the formation of a binary or a protostellar merger.

We have used high-resolution spectra, acquired with UVES@ESO-VLT, to determine the chemical abundances of different samples of AGB and RGB stars in 4 Galactic globular clusters, namely 47Tuc, NGC3201, M22 and M62. For almost all the analyzed AGB stars we found a clear discrepancy between the iron abundance measured from neutral lines and that obtained from single ionized lines, while this discrepancy is not obtained for the RGB samples observed in the same clusters and analyzed with the same procedure. Such a behavior exactly corresponds to what expected in the case of Non-Local Thermodynamical Equilibrium (NLTE) in the star atmosphere. These results have a huge impact on the proper determination of GC chemistry. In fact, one of the most intriguing consequences is that, at odds with previous claims, no iron spread is found in NGC3201 and M22 if the iron abundance is obtained from ionized lines only.

We have used high-resolution spectra, acquired with UVES@ESO-VLT, to determine the chemical abundances of different samples of AGB and RGB stars in 4 Galactic globular clusters, namely 47Tuc, NGC3201, M22 and M62. For almost all the analyzed AGB stars we found a clear discrepancy between the iron abundance measured from neutral lines and that obtained from single ionized lines, while this discrepancy is not obtained for the RGB samples observed in the same clusters and analyzed with the same procedure. Such a behavior exactly corresponds to what expected in the case of Non-Local Thermodynamical Equilibrium (NLTE) in the star atmosphere. These results have a huge impact on the proper determination of GC chemistry. In fact, one of the most intriguing consequences is that, at odds with previous claims, no iron spread is found in NGC3201 and M22 if the iron abundance is obtained from ionized lines only.

Evolved stars and planetary nebulae are rich and varied sites of molecule and dust formation. These objects undergo dramatic mass loss which ultimately enriches the interstellar medium. In this thesis, a number of studies, outlined below, have been undertaken to better understand the chemical and physical properties of these diverse objects. A molecular line survey of a sample of evolved stars and planetary nebulae has been carried out using the Mopra radio telescope, Australia. Transitions with hyperfine structure have been fitted to constrain optical depths. The population diagram method was applied to determine the rotation temperatures of molecules which had multiple transitions available. Column densities have been calculated for all detected species and isotopic ratios measured where possible. The results include the corroboration of the classification of II Lup as a J-type star. The 89.087 GHz HCN maser was detected in IRAS 15082-4808 for the first time from the aforementioned survey, bringing the total number of detections of this maser to ten. The velocity shift of this maser has been measured at −2.0+/-0.9 km/s. Drawing on literature data in addition to the survey data, the variation of maser intensity with pulsation phase has been investigated across all sources for the first time. Comparing these masers with model atmospheres constrains the formation region to between 2 and 4 stellar radii. CO in the circumstellar envelope of II Lup has been modelled using the radiative transfer codes GASTRoNOoM, and ComboCode. The models have demonstrated that a ‘standard’ smooth model does not satisfactorily reproduce the combined CO observations of PACS, JCMT, Mopra and APEX. Two potential solutions are proposed: a discontinuous temperature model, requiring the presence of an efficient cooling molecule that is most effective in the region 75-200 R*, or a variable mass loss model that requires a factor of ten increase inmass loss in the same region. Zinc abundances, a proxy for iron abundances, have been determined for a sample of Galactic planetary nebulae using the [Zn IV] 3.625 μm line. O++/O has been shown to be a reliable ionisation correction factor for Zn3+ from Cloudy photoionisation models. The majority of the sample are sub-solar in [Zn/H] and enriched in [O/Zn]. Zinc abundances as functions of Galactocentric distance have also been investigated and no evidence for a trend has been found.

Thesis (M.S.)--University of Missouri-Columbia, 2006.
The entire dissertation/thesis text is included in the research.pdf file; the official abstract appears in the short.pdf file (which also appears in the research.pdf); a non-technical general description, or public abstract, appears in the public.pdf file. Title from title screen of research.pdf file viewed on (February 6, 2007) Includes bibliographical references.

In this thesis, I present the properties of IRAS 19306+1407 central source and its sur- rounding circumstellar envelope (CSE), from the analysis of near-infrared (near-IR) polarimetry and integral field spectroscopy (IFS), with supporting archived HST im- ages and sub-millimetre (sub-mm) photometry. This is supported by axi-symmetric light scattering (ALS), axi-symmetric radiative transfer (DART) and molecular hy-drogen (H2) shock models. The polarimetric images show that IRAS 19306+1407 has a dusty torus, which deviates from axisymmetry and exhibits a ‘twist’ feature. The DART and ALS modelling shows that the CSE consists of Oxygen-rich sub-micrometre dust grains, with a range in temperature from 130±30 to 40±20 K at the inner and outer radius, respectively, with inner and outer radii of 1.9±0.1×1014 and 2.7±0.1×1015 m. The CSE detached 400±10 years ago and the mass loss lasted 5700±160 years, assuming a constant asymptotic giant branch (AGB) outflow speed of 15 km s−1. The dust mass and total mass of the CSE is 8.9±5×10−4 and 1.8±1.0×10 −1 M⊙, assuming a gas-to-dust ratio of 200. The mass loss rate was 3.4±2.1×10−5 M⊙ year−1. The central source is consistent with a B1I-type star with a radius of 3.8 ± 0.6 R⊙, luminosity of 4500 ± 340 L⊙ at a distance of 2.7 ± 0.1 kpc. A purpose built idl package (fus) was developed and used in the SINFONI IFS data critical final reduction steps. It also produced emission line, kinematic and line ratio images. The IFS observations show that H2 is detected throughout the CSE, located in bright arcs and in the bipolar lobes. The velocity of the H2 is greatest at the end of the lobes. Brγ emission originates from, or close, to the central source – produced by a fast jump (J) shock or photo-ionised atomic gas. The 1-0 S(1)/2- 1 S(1) and 1-0 S(1)/3-2 S(3) ratios were used as a diagnostic and determined that H2 was excited by bow shaped shocks; however, these shock models could not wholly explain the observed rotational and vibrational temperatures. The CDR values were fitted by combining continuous (C) or J-bow shock and fluorescence models, with a contribution from the latter, observed throughout the CSE (5–77 per cent). The majority of shock can be described by a C-bow shock model with B = 0.02 to 1.28 mG. Shocks are predominately seen in the equatorial regions. Polarimetry and IFS highlight a ‘twist’ feature, which could be due to an episodic jet undergoing a recent change in the outflow direction. The sub-arcsecond IFS observations reveal a flocculent structure in the south- east bright arc, consisting of several clumps interpreted as a fast-wind eroding an equatorial torus, possibly forming H2 knots seen in (some) evolved planetary nebulae (PNe). My analysis has effectively constrained the following: spectral type, stellar radius, luminosity and distance, chemistry, dust grain properties, geometry, age, mass loss, excitation mechanism and evolutionary state of the post-AGB star and its surrounding CSE. I conclude that IRAS 19306+1407 is a post-AGB object on the verge becoming a PN.

Stars on the asymptotic giant branch (AGB) lose a significant fraction of their mass to their surroundings through stellar winds. As a result, they are surrounded by circumstellar shells of gas and dust. This stellar mass loss replenishes and enriches the interstellar medium (ISM) with the products of stellar nucleosynthesis, progressively increasing its metallicity and thereby driving galactic chemical evolution. In this thesis I present a comprehensive study of oxygen-rich (O-rich) AGB stars and red supergiants (RSG) observed with the Spitzer Infrared Spectrograph and Infrared Space Observatory Short Wavelength Spectrometer in the Milky Way, the Large and Small Magellanic Clouds, and Galactic globular clusters; focusing on the composition of the dust in the circumstellar envelopes surrounding these stars. Combining spectroscopic and photometric observations with the GRAMS grid of radiative transfer models to derive (dust) mass-loss rates, I detect crystalline silicates in stars with dust mass-loss rates which span over a factor of 1000, down to rates of ~10^{-9} Msun/yr. Detections of crystalline silicates are more prevalent in higher mass-loss rate objects, and our results indicate that the dust mass-loss rate has a greater influence on the crystalline fraction than the gas mass-loss rate, suggesting that thermal annealing of amorphous silicate grains is the primary formation mechanism of crystalline silicates in such environments rather than the direct condensation of crystalline silicates from the gas phase. I also find that metallicity influences the composition of crystalline silicates, with enstatite seen increasingly at low metallicity, while forsterite becomes depleted at these metallicities due to the different chemical composition of the gas. To trace the evolution of alumina and silicate dust along the AGB, I present an alternative grid of MODUST radiative transfer-models for a range of dust compositions, mass-loss rates, dust shell inner radii and stellar parameters. Our analysis shows that the AKARI [11]-[15] versus [3.2]-[7] colour is a robust indicator of the fractional abundance of alumina in O-rich AGB stars. From the modelling, I show that a grain mixture consisting primarily of amorphous silicates, with contributions from amorphous alumina and metallic iron provides a good fit to the observed spectra of O-rich AGB stars in the LMC. In agreement with previous studies, we find a correlation between the dust composition and mass-loss rate; the lower the mass-loss rate the higher the percentage of alumina in the shell. Finally, I present mid-infrared observations of the Local Group dwarf elliptical galaxy M32; where I find a large population of dust-enshrouded stars. These observations will act as a pathfinder for future observations with the JWST and SPICA.

Polarization is a powerful tool for understanding stellar atmospheres and circumstellar environments. Mira and semi-regular variable stars have been observed for decades and some are known to be polarimetrically variable, however, the semi-regular variable V Canes Venatici displays an unusually large, unexplained amount of polarization. We present ten years of optical polarization observations obtained with the HPOL instrument, supplemented by published observations spanning a total interval of about forty years for V CVn. We find that V CVn shows large polarization variations ranging from 1 - 6%. We also find that for the past forty years the position angle measured for V CVn has been virtually constant suggesting a long-term, stable, asymmetric structure about the star. We suggest that this asymmetry is caused by the presence of a stellar wind bow shock and tail, consistent with the star's large space velocity.

In this study we present photometric predictions for C-type Asymptotic Giant Branch Stars (AGB) stars from Eriksson et al. (2014) for the James Webb Space Telescope (JWST) and the Wide-field Infrared Survey Explorer (WISE) instruments. The photometric predictions we have done are for JWST’s general purpose wide-band filters on NIRCam and MIRI covering wavelengths of 0.7 — 21 microns. AGB stars contribute substantially to the integrated light of intermediate-age stellar popula- tions and is a substantial source of the metals (especially carbon) in galaxies. Studies of AGB stars are (among other reasons) important for the understanding of the chemical evolution and dust cycle of galaxies. Since the JWST is scheduled for launch in 2018 it should be a high priority to prepare observing strategies. With these predictions we hope it will be possible to optimize observing strategies of AGB stars and maximize the science return of JWST. By testing our method on Whitelock et al. (2006) objects from the WISE catalog and comparing them with our photometric results based on Eriksson et al. (2014) we have been able to fit 20 objects with models. The photometric data set can be accessed at: http://www.astro.uu.se/AGBmodels/
I den här studien har jag gjort fotometriska förutsägelser för asymptotis- ka jättegrensstjärnor (AGB-stjärnor) av C typ från Eriksson et al. (2014) modifierade för instrument ombord på James Webb Space Telescope (JWST) och Wide-field Infrared Survey Explorer (WISE). AGB-stjärnor bidrar kraftigt till det totala ljuset av stjärnor av intermediär ålder och är också en stor källa till metaller (speciellt kol) i galaxer. Studier av AGB stjärnor är viktiga av flera anledningar, däribland för att förstå den kemiska evolutionen och stoftcykler i galaxer. JWST är planerad att skjutas upp 2018 och fram till dess bör det vara en hög prioritet att förbereda observeringsstrategier. Med den fotometriska datan i den här studien hoppas vi att användare av JWST kommer kunna optimera sina observeringsstrategier av AGB-stjärnor och få ut så mycket som möjligt av sin obseravtionstid med teleskopet. Vi har testat metoden genom att titta på objekt från Whitelock et al. (2006) i WISE-katalogen och jämföra dem med de fotometriska resultaten baserade på modellerna från Eriksson et al. (2014). På detta sett har vi lyckats matcha 20 objekt med modeller. Den fotometriska datan går att ladda ner ifrån: http://www.astro.uu.se/AGBmodels/

Si l'évolution des étoiles simples est relativement bien comprise, l'étude des étoiles binaires, qui représentent la majorité des étoiles, nécessite encore des progrès majeurs, particulièrement en ce qui concerne leurs différents modes d'interactions. Dans ces systèmes, la composition de surface d'une étoile peut être altérée non seulement par l'accrétion d'éléments synthétisés au sein de l'étoile compagnon, mais également par des processus de mélanges internes induits par les forces de marées et d'un transport du moment angulaire. Plusieurs classes d'étoiles post-transfert de masse (les étoiles à baryum, CH et S) possèdent effectivement des compositions de surface caractérisées par la présence d'éléments lourds, tel que le baryum. Ces systèmes sont présumés se former au sein de systèmes binaires incluant une étoile de la branche asymptotique des géantes (appelé étoile AGB). Ces dernières sont des étoiles remarquables qui représentent l'unique site d'une nucléosynthèse particulière. En effet, elles constituent les contributeurs essentiels de la production de fluor ou de baryum. Les étoiles AGB sont également caractérisées par une importante perte de masse par vent qui éjecte progressivement leur enveloppe enrichie en ces éléments. Au sein d'un système binaire, une partie de ce vent est accréditée par l'étoile compagnon et pollue ainsi sa surface, laissant une signature spectrale distincte qui subsistera longtemps après que l'étoile AGB ait disparu. Ce scénario est suggéré comme étant responsable de la formation d'une grande variété d'étoiles chimiquement particulières, tels que les étoiles à baryum.

Cependant, plusieurs propriétés clés de ces systèmes, en particulier leurs distributions de périodes orbitales et d'excentricités, demeurent inexpliquées depuis des décennies. L'incapacité de nos modèles à reproduire ces propriétés orbitales met en évidence notre compréhension limitée des mécanismes d'interaction qui gouvernent l'évolution des systèmes binaires. Plus particulièrement, des mécanismes qui génèrent des orbites excentriques au sein des étoiles à baryum et des systèmes analogues sont requis. Nous examinons ainsi la possibilité qu'à sa naissance l'étoile naine blanche subisse un kick ou que la présence d'un disque entourant le système binaire soit à l'origine des fortes excentricités observées chez les étoiles à baryum. Ces deux mécanismes permettent pour la première fois depuis l'étude de ces systèmes d'apporter une solution à ces problèmes. Il est montré comment comprendre les signatures induites par un compagnon étoile AGB et les corréler avec les propriétés orbitales du système binaire est essentiel pour tester et améliorer notre connaissance de l'évolution des étoiles binaires; l'objectif de ce travail.

Doctorat en Sciences
info:eu-repo/semantics/nonPublished

Diss. (sammanfattning) Stockholm : Stockholms universitet, 2010.
At the time of the doctoral defense, the following papers were unpublished and had a status as follows: Paper 2:submitted. Paper 4: in progress. Härtill 4 uppsatser.

Polarimetry is considered as an important searching tool in Astronomy and it deserves to be a standard observational technique for a variety of astrophysical environments. Many phenomena, in fact, can contribute to the polarisation of radiation and so, its observation can potentially provide information about their basic causes. In particular, in a stellar context high resolution spectropolarimetry can potentially: (i) investigate the physics of the shaping mechanisms of the protoplanetary nebulae (PNe) and the possible interaction between the central star and the very close surrounding, (ii) investigate stellar photospheric inhomogeneities (e.g. pulsations, granulations, hot spots and magnetic activity), (iii) play a key role in the exoplanet research. Despite this, however, the difficulties involved in the measurements and in the theoretical interpretation made this technique not as widespread as it deserves. The main goal of this work is to demonstrate the diagnostic potential of high resolution linear spectropolarimetry. At this purpose, we studied the polarised spectrum of the post-AGB binary system 89 Herculis on the basis of data collected with the high resolution \emph{Catania Astrophysical Observatory Spectropolarimeter} (CAOS) and \emph{HArps-North POlarimeter} (HANPO), in addition to archive data taken with the \emph{Echelle SpectroPolarimtric Device for the Observation of Stars} (ESPaDOnS). We found the existence of linear polarisation in the metal lines in absorption and in some of the metal lines in emission: this is the first \emph{Second Solar Spectrum} ever observed in a star different than the Sun. We have then analyzed the observed polarisation properties. Firstly, regarding the polarisation measured across the absorption lines, we found that the complex Stokes $Q$ and $U$ morphologies vary with the orbital period of the system. We then rule out magnetic field, continuum depolarisation due to pulsations and hot spots as a possible origin. We found that in the framework of optical pumping due to the secondary star, the observed periodic properties of the spectral line polarisation can be justified by two jets, with a flow velocity of a few tens of $km$ $s^{-1}$, at the basis of that hourglass structure characterizing 89 Herculis. Regarding the emission lines, numerical simulations show that these polarised profiles could be formed in an undistrupted circumbinary disk rotating at $\leq 10$ $km$ $s^{-1}$ and with an orientation in the sky in agreement with optical and radio interferometric results. We have concluded that the study of aspherical envelopes, the origin of which is not yet completely understood, of PNe and already present in post-AGBs can benefit from high resolution spectropolarimetry and that this technique can shape envelopes still too far away for interferometry. In addition, we have conducted a large spectropolarimetric survey on a sample of F-G-K bright stars to understand if the presence of intrinsic linear polarisation is a rare case or it is widespread in these type of stars. Surprisingly, we have found linear polarisation across metal lines in absorption for about $71$ $\%$ of stars of our sample. Some of them clearly show temporal variability, to be characterized with further data. Despite this study is neither exhaustive nor conclusive we speculated that the presence of linear polarisation in the stars is not rare: a broad and in-depth spectropolarimetric study is then necessary.

Stars of low and intermediate mass (0.9 ≲ M/M ⊙ ≲ 8) develop an electron-degenerate CO core after the core He-burning phase, and experience the thermally pulsing asymptotic giant branch phase (TP-AGB) as they approach the end of their evolution. Although very short lived, TP-AGB stars are of paramount importance in the study of stellar populations and galaxy evolution. In fact, being intrinsically very bright, they provide a significant contribution to the total luminosity of single stellar populations, and galaxies in general. Most of their radiation is emitted at infrared wavelengths, a spectral range where extinction by dust is small. They are thus very useful as tracers of intermediate age stars, and are often used to characterise stellar populations in external galaxies. The characteristic spectral signatures of TP-AGB stars have been observed even at high redshift, and they are in general visible even at large distances. Additionally, variable TP-AGB stars follow a well defined period-luminosity relation in the near infrared, which makes them a promising distance indicator. TP-AGB stars play a prominent role in the chemical enrichment of galaxies. Repeated third dredge-up events are efficient in bringing nucleosynthesis products (He, C, N, s-process elements) to the stellar surface, which low temperature favours the formation of molecular species. Stellar pulsations induce shock waves that compress the atmosphere and allow for the condensation of dust grains. Being highly opaque, dust is pushed outwards in the interstellar medium by radiation pressure, dragging the enriched gas with itself. The chain of events just described represents a clear example of the various physical processes, poorly understood and heavily entangled, involved in TP-AGB evolution. As a result, in spite of its crucial role across astrophysics, the TP-AGB is one of the least understood phases of stellar evolution. Its modelling is affected by large uncertainties that propagate in the field of extragalactic astronomy, degrading the predicting power of current population synthesis models of galaxies. The present PhD thesis is the result of three years of work within the ERC project STARKEY, which major goal is exactly to provide a physically-sound calibration of the TP-AGB phase as a function of age and metallicity. This is pursued by requiring models to simultaneously reproduce different observables of resolved TP-AGB stellar populations in star clusters and nearby galaxies. The project adopts an all-round theoretical approach that takes into account several, strongly interconnected, key physical processes (convective mixing, stellar winds, dust formation, and stellar pulsation). These calibrated models are used to generate new well-tested grids of stellar tracks, isochrones, chemical yelds of gas and dust, stellar spectra, pulsation models, all made available to the scientific community. The specific subject of the present work is the study of stellar pulsations on the TP-AGB, and was performed by computing a large grid of new pulsation models. Updated models of luminous red giant variables have long been missing from the scientific literature, and a set of models systematically accounting for the variety of properties of TP-AGB stars has never been published. A critical shortcoming of previous models, with the relevant exception of a few selected studies, is that they do not account for surface chemical enrichment. Carbon stars, produced by the dredge-up of carbon, have characteristic spectral features that are dramatically different than those of their O-rich counterpart. This is a consequence of altered molecular equilibria, and the corresponding drastic change in the main sources of molecular opacity. Atmospheric opacities determine stellar radii, thus affecting the pulsation period. It is therefore clear that they need to be consistent with the detailed chemical mixture predicted by evolutionary models. Part of this work was devoted to the inclusion of updated opacities in the modelling of pulsation, a significant improvement with respect to past studies, which generally employed opacity data computed for standard scaled-solar mixtures. As already mentioned, pulsation on the TP-AGB is essential to the enrichment of the interstellar medium. More generally, it is crucial for mass-loss, dust formation, and ultimately evolution. But of course, pulsating red giant stars are important for a number of other reasons. The most luminous ones, the large-amplitude Mira variables, have long been known to follow a period-luminosity (PL) relation that is very clear in the near-infrared bands, and represents a very promising distance indicator (see, e.g., Whitelock, 2013; Huang et al., 2018), especially in view of the forthcoming missions such as JWST. The discovery, during the last two decades, of multiple PL relations in the long-period variables (LPVs) of the Large Magellanic Cloud (Wood et al., 1999) re-ignited the interest for such stars. The different PL relations, or sequences, are assumed to be due to different pulsation modes, which are characterised by distinct period and excitation properties depending on the stellar properties and evolutionary stages. Therefore, observed periods provide an additional constraint, together with other observables, to be matched by theoretical models, allowing us to refine our knowledge of stellar structure and evolution. Observed periods represent also a powerful tool to estimate global parameters such as stellar masses and radii. However, to fully exploit the potential of LPVs, a knowledge of the exact pulsation modes corresponding to each sequence is required. This aim has been pursued by numerous studies in the past decades, with the unfortunate result that two interpretations emerged, both based on valid arguments, but providing mode assignments in contrast with each other. Again, this disagreement is largely due to the use of pulsation models unable to represent the variety of the AGB population of the Magellanic Clouds. In the present work, we present a new, large grid of linear, radial, non-adiabatic pulsation models, with updated opacity data for CNO-varied metal mixtures. The grid covers a wide range of the space of stellar parameters characterising the TP-AGB phase, in terms of total mass, core mass, luminosity, effective temperature, and chemical composition. Models include periods and amplitude growth rates for five radial pulsation modes, from the fundamental to the fourth overtone. Growth rates allow us to predict to a reasonably good accuracy the excitation/stability degree of individual modes, and provide information on their expected observability. The structure of the grid in terms of its defining parameters is based on detailed TP-AGB evolutionary tracks, but the computation of pulsation models is decoupled from evolutionary models. This way, pulsation models are compatible with virtually any output from evolutionary and population synthesis codes, and are going to be made publicly available, filling a long-existing gap. The grid of pulsation models has been implemented in the STARKEY framework to be tested against observations. Our approach involves the simulation of the pulsation properties of a synthetic population of luminous red giant stars. Such a simulation was computed to reproduce the observed photometric and variability properties of AGB stars in the Large Magellanic Cloud (Trabucchi et al., 2017). The results have shown a good degree of agreement between models and observations, and allowed us to provide a new interpretation of the observed PL sequences, essentially solving the past tensions and bringing the previous interpretations into alignment. Our result provides additional information about the open topic of long secondary periods in red giant variables. Moreover, it supports the idea of a connection between faint LPVs and solar-like oscillations in less evolved red giants (see, e.g., Mosser et al., 2013, and references therein), the implications of which would open new frontiers in the study of stellar oscillations. The comparison with observations confirmed that the new models are able to predict pulsation periods of all observed modes with good accuracy. Remarkably, theoretical growth rates are able to account for the observed distribution of overtone modes amplitudes, in spite of the uncertainties in the treatment of the interaction between convection and pulsation. On the other hand, growth rates of the fundamental mode are still affected by large uncertainties, as they are not able to reproduce the observed instability strip. Further studies are required to address in more detail the excitation of pulsation in luminous red giants, with special attention for the fundamental mode. Additional future developments include the use of non-linear models to: (1) constrain models by reproducing observed variability amplitudes, (2) investigate the conditions under which linear models are not appropriate to describe pulsation periods, and (3) provide, for those cases, suitable period corrections as a function of global stellar parameters.
Stars of low and intermediate mass (0.9 ≲ M/M ⊙ ≲ 8) develop an electron-degenerate CO core after the core He-burning phase, and experience the thermally pulsing asymptotic giant branch phase (TP-AGB) as they approach the end of their evolution. Although very short lived, TP-AGB stars are of paramount importance in the study of stellar populations and galaxy evolution. In fact, being intrinsically very bright, they provide a significant contribution to the total luminosity of single stellar populations, and galaxies in general. Most of their radiation is emitted at infrared wavelengths, a spectral range where extinction by dust is small. They are thus very useful as tracers of intermediate age stars, and are often used to characterise stellar populations in external galaxies. The characteristic spectral signatures of TP-AGB stars have been observed even at high redshift, and they are in general visible even at large distances. Additionally, variable TP-AGB stars follow a well defined period-luminosity relation in the near infrared, which makes them a promising distance indicator. TP-AGB stars play a prominent role in the chemical enrichment of galaxies. Repeated third dredge-up events are efficient in bringing nucleosynthesis products (He, C, N, s-process elements) to the stellar surface, which low temperature favours the formation of molecular species. Stellar pulsations induce shock waves that compress the atmosphere and allow for the condensation of dust grains. Being highly opaque, dust is pushed outwards in the interstellar medium by radiation pressure, dragging the enriched gas with itself. The chain of events just described represents a clear example of the various physical processes, poorly understood and heavily entangled, involved in TP-AGB evolution. As a result, in spite of its crucial role across astrophysics, the TP-AGB is one of the least understood phases of stellar evolution. Its modelling is affected by large uncertainties that propagate in the field of extragalactic astronomy, degrading the predicting power of current population synthesis models of galaxies. The present PhD thesis is the result of three years of work within the ERC project STARKEY, which major goal is exactly to provide a physically-sound calibration of the TP-AGB phase as a function of age and metallicity. This is pursued by requiring models to simultaneously reproduce different observables of resolved TP-AGB stellar populations in star clusters and nearby galaxies. The project adopts an all-round theoretical approach that takes into account several, strongly interconnected, key physical processes (convective mixing, stellar winds, dust formation, and stellar pulsation). These calibrated models are used to generate new well-tested grids of stellar tracks, isochrones, chemical yelds of gas and dust, stellar spectra, pulsation models, all made available to the scientific community. The specific subject of the present work is the study of stellar pulsations on the TP-AGB, and was performed by computing a large grid of new pulsation models. Updated models of luminous red giant variables have long been missing from the scientific literature, and a set of models systematically accounting for the variety of properties of TP-AGB stars has never been published. A critical shortcoming of previous models, with the relevant exception of a few selected studies, is that they do not account for surface chemical enrichment. Carbon stars, produced by the dredge-up of carbon, have characteristic spectral features that are dramatically different than those of their O-rich counterpart. This is a consequence of altered molecular equilibria, and the corresponding drastic change in the main sources of molecular opacity. Atmospheric opacities determine stellar radii, thus affecting the pulsation period. It is therefore clear that they need to be consistent with the detailed chemical mixture predicted by evolutionary models. Part of this work was devoted to the inclusion of updated opacities in the modelling of pulsation, a significant improvement with respect to past studies, which generally employed opacity data computed for standard scaled-solar mixtures. As already mentioned, pulsation on the TP-AGB is essential to the enrichment of the interstellar medium. More generally, it is crucial for mass-loss, dust formation, and ultimately evolution. But of course, pulsating red giant stars are important for a number of other reasons. The most luminous ones, the large-amplitude Mira variables, have long been known to follow a period-luminosity (PL) relation that is very clear in the near-infrared bands, and represents a very promising distance indicator (see, e.g., Whitelock, 2013; Huang et al., 2018), especially in view of the forthcoming missions such as JWST. The discovery, during the last two decades, of multiple PL relations in the long-period variables (LPVs) of the Large Magellanic Cloud (Wood et al., 1999) re-ignited the interest for such stars. The different PL relations, or sequences, are assumed to be due to different pulsation modes, which are characterised by distinct period and excitation properties depending on the stellar properties and evolutionary stages. Therefore, observed periods provide an additional constraint, together with other observables, to be matched by theoretical models, allowing us to refine our knowledge of stellar structure and evolution. Observed periods represent also a powerful tool to estimate global parameters such as stellar masses and radii. However, to fully exploit the potential of LPVs, a knowledge of the exact pulsation modes corresponding to each sequence is required. This aim has been pursued by numerous studies in the past decades, with the unfortunate result that two interpretations emerged, both based on valid arguments, but providing mode assignments in contrast with each other. Again, this disagreement is largely due to the use of pulsation models unable to represent the variety of the AGB population of the Magellanic Clouds. In the present work, we present a new, large grid of linear, radial, non-adiabatic pulsation models, with updated opacity data for CNO-varied metal mixtures. The grid covers a wide range of the space of stellar parameters characterising the TP-AGB phase, in terms of total mass, core mass, luminosity, effective temperature, and chemical composition. Models include periods and amplitude growth rates for five radial pulsation modes, from the fundamental to the fourth overtone. Growth rates allow us to predict to a reasonably good accuracy the excitation/stability degree of individual modes, and provide information on their expected observability. The structure of the grid in terms of its defining parameters is based on detailed TP-AGB evolutionary tracks, but the computation of pulsation models is decoupled from evolutionary models. This way, pulsation models are compatible with virtually any output from evolutionary and population synthesis codes, and are going to be made publicly available, filling a long-existing gap. The grid of pulsation models has been implemented in the STARKEY framework to be tested against observations. Our approach involves the simulation of the pulsation properties of a synthetic population of luminous red giant stars. Such a simulation was computed to reproduce the observed photometric and variability properties of AGB stars in the Large Magellanic Cloud (Trabucchi et al., 2017). The results have shown a good degree of agreement between models and observations, and allowed us to provide a new interpretation of the observed PL sequences, essentially solving the past tensions and bringing the previous interpretations into alignment. Our result provides additional information about the open topic of long secondary periods in red giant variables. Moreover, it supports the idea of a connection between faint LPVs and solar-like oscillations in less evolved red giants (see, e.g., Mosser et al., 2013, and references therein), the implications of which would open new frontiers in the study of stellar oscillations. The comparison with observations confirmed that the new models are able to predict pulsation periods of all observed modes with good accuracy. Remarkably, theoretical growth rates are able to account for the observed distribution of overtone modes amplitudes, in spite of the uncertainties in the treatment of the interaction between convection and pulsation. On the other hand, growth rates of the fundamental mode are still affected by large uncertainties, as they are not able to reproduce the observed instability strip. Further studies are required to address in more detail the excitation of pulsation in luminous red giants, with special attention for the fundamental mode. Additional future developments include the use of non-linear models to: (1) constrain models by reproducing observed variability amplitudes, (2) investigate the conditions under which linear models are not appropriate to describe pulsation periods, and (3) provide, for those cases, suitable period corrections as a function of global stellar parameters.

Silicon monoxide maser emission has been detected in many evolved stars in circumstellar envelopes in different vibrationally-excited rotational transitions. It is considered a good tracer to study the dynamics in a region close to the photosphere of the star. We present multi-epoch, total intensity, high-resolution images of 43 GHz, v=1, J=1-0 SiO maser emission toward the Mira variable R Cas. In total we have 23 epochs of data for R Cas at approximate monthly intervals over an optical pulsation phase range of φ = 0.158 to φ = 1.782. These maps show a ring-like distribution of the maser features in a shell, which is assumed to be centred on the star at average radius of 1.6 → 2.3 times the radius of star, R⋆. It is clear from these images that the maser emission is significantly extended around the star. At some epochs a faint outer arc can be seen at about 4 R⋆. The intensity of the emission waxes and wanes during the stellar phase. Some maser features are seen infalling as well as outflowing. We have made initial comparisons of our data with models by Gray et al. (2009). We have investigated the polarization morphology by mapping the linear and circular polarization of SiO masers in the v=1, J=1-0 transition. We found that some of the polarization vectors are either tangential or radial, which indicate a bimodal structure of the linear polarization morphology. Other angles can be seen as well. This is consistent with a radial, stellar-centred magnetic field in the SiO maser shell. We found in some isolated features the fractional linear polarization exceeds 100%. In other features, the polarization angle abruptly flips by 90◦. We found that our data are in the regime that the Zeeman splitting rate g is much greater than the stimulated emission rate R which in turn is greater than the decay rate , which indicates that the solution of Goldreich et al. (1973) can be applied.

I have developed a new general-purpose deterministic 2D radiative transfer code for astrophysical dusty environments named LELUYA (www.leluya.org). It can provide the solution to an arbitrary axially symmetric multi-grain dust distribution around an arbitrary heating source. By employing a new numerical method, the implemented algorithm automatically traces the dust density and optical depth gradients, creating the optimal unstructured triangular grid. The radiative transfer equation includes dust scattering, absorption and emission. Unique to LELUYA is also its ability to self-consistently reshape the sublimation/condensation dust cavity around the source to accommodate for the anisotropic diffuse radiation. LELUYAs capabilities are demonstrated in the study of the asymptotic giant branch (AGB) star IRC+10011. The stellar winds emanating from AGB stars are mostly spherically symmetric, but they evolve into largely asymmetric planetary nebulae during later evolutionary phases. The initiation of this symmetry breaking process is still unexplained. IRC+10011 represents a rare example of a clearly visible asymmetry in high-resolution near-infrared images of the circumstellar dusty AGB wind. LELUYA shows that this asymmetry is produced by two bipolar cones with 1/r0.5 density profile, imbedded in the standard 1/r2 dusty wind profile. The cones are still breaking though the 1/r2 wind, suggesting they are driven by bipolar jets. They are about 200 years old, thus a very recent episode in the final phase of AGB evolution before turning into a proto-planetary nebula, where the jets finally break out from the confining spherical wind. IRC+10011 provides the earliest example of this symmetry breaking thus far.

Since the identi cation of proto-planetary nebulae (PPNe) as transition objects between the asymptotic giant branch stars and planetary nebulae more than two decades ago, astronomers have attempted to characterise these exciting objects. Today many questions still elude a conclusive answer, partly due to the sheer diversity observed within this small subset of stellar objects, and partly due to the low numbers detected. Fortunately, many of these objects display a rich spectrum of emission/absorption lines that can be used as diagnostics for these nebulae. This dissertation presents a study of six PPNe using the relatively new (at NIR wavelengths) integral eld spectroscopy technique. This method has allowed the investigation of distinct regions of these nebulae, and in certain cases the application of magneto-hydrodynamic shock models to the data. The goal of this research has been to investigate the evolution of PPNe by detailed examination of a small sample of objects consisting of a full range of evolutionary types. Near-IR ro-vibrational lines were employed as the primary tool to tackle this problem. In all six sources the 1!0S(1) line is used to map the spatial extent of the H2. In three of these objects the maps represent the rst images of their H2 emission nebulae. In the case of the earliest-type object (IRAS 14331-6435) in this sample, the line map gives the rst image of its nebula at any wavelength. In the only M-type object (OH 231.8+4.2) in the sample, high-velocity H2 is detected in discrete clumps along the edges of the bipolar out ow, while a possible ring of slower moving H2 is found around the equatorial region. This is the rst detection of H2 in such a late-type object but due its peculiarities, it is possibly not representative of what is expected of M-type objects. In IRAS 19500-1709, an intermediate-type object, the line map shows the H2 emission to originate in clumpy structures along the edges of a bipolar shell/out ow. The remaining three objects have all been the subject of previous studies but in each case new H2 lines are detected in this work along with other emission lines (Mg ii, Na i & CO). In the case of IRAS 16594-4656, MHD shock models have been used to determine the gas density and shock velocity. Two new python modules/classes have been written. The rst one to deal with the data cubes, extract ux measurements, rebin regions of interest, and produce line maps. The second class allows the easy calculation of many important parameters, for example, excitation temperatures, column density ratio values, extinction estimates from several line-pairs, column density values, and total mass of the H2. The class also allows the production of input les for the shock tting procedure, and simulated shocks for testing this tting process. A new framework to t NIR shock models to data has been developed, employing Monte Carlo techniques and the extensive computing cluster at the University of Hertfordshire (UH). This method builds on the approach used by many other authors, with the added advantages that this framework provides a method of correctly sampling the shock model parameter space, and providing error estimates on the model t. Using this approach, data from IRAS 16594-4656 have been successfully modelled using the shock models. A full description of this class of stellar objects from such a small sample is not possible due to their diverse nature. Although H2 was detected across the full spectral vi range of post-AGB objects, the phase at which H2 emission begins is still not clear. The only M-type object in this work is a peculiar object and may not be representative of a typical post-AGB star. The H2 PPNe appear to be located at lower Galactic latitudes (b 20 ) than the total PPNe population, possibly pointing to an above average mass and hence younger age of these objects.

Dans cette thèse, j'analyse des observations spectropolarimétriques d'étoiles froides évoluées dans tous les paramètres de Stokes ($I$, $Q$, $U$ et $V$).L'étude des spectres polarisés circulairement de trois étoiles supergéantes rouges (RSG) m'a permis de détecter un faible champ magnétique (de l'ordre de 1 Gauss) à leur surface ainsi que de mettre en évidence l'influence de la forte polarisation linéaire de ces étoiles sur la mesure du champ magnétique.Cette forte polarisation linéaire est d'origine non magnétique et est reliée à la polarisation du continu dont l'observabilité résulte de la présence de structures brisants la symétrie cylindrique du disque stellaire.Après une analyse en profondeur de la polarisation du continu des étoiles RSG, j'utilise un modèle analytique spectropolarimétrique permettant de remonter à la position sur le disque de ces structures, en bon accord avec des observations interférométriques.Les étoiles variables pulsantes montrent elles aussi des signaux polarisés linéairement intenses dont l'origine est la polarisation intrinsèque (où de résonance) associée à certaines raies spectrales.Pour ces étoiles, les gradients de vitesses, introduits par les ondes de chocs se propageants dans leur atmosphère, amplifient la polarisation intrinsèque des raies, un mécanisme d'éclaircissement Doppler, bien connu dans le cas du Soleil, qui offre de nouvelles perspectives dans l'étude de ces objets
In this thesis work, full Stokes ($I$, $Q$, $U$ et $V$) spectropolarimetric observations of cool evolved stars have been analysed.I have measured a weak magnetic field at the surface of three well known red supergiant stars. Furthermore, I have shown that the strong level of linear polarisation, measured for these stars, can lead to some ambiguity in the measurement of magnetic fields.This strong, unmagnetic, linear polarisation originate from depolarisation of the continuum which, in turn, is only detectable because of symmetry-breaking effects on the stellar disk.I study in some details the continuum polarisation of RSG stars and I present a way of mapping symmetry-breaking strucutres through the mean of a spectropolarimetric model. Good agreement with interferometric observations is reached.From their side, pulsating variable stars also show strong linearly polarised features in their spectra. These features are indeed due to resonant scattering polarisation associated to metallic lines, a phenomenon reminiscent of the second solar spectrum.For these stars, the velocity gradients, pertaining to the shock waves propagating through their atmosphere, enhance the intrinsic polarisation of the lines. This Doppler brightening effect, well known in the solar case, offer a new innovative method for the analysis of these stars

Context. Theoretical studies predict that the length scale of convection in stellar atmospheres isproportional to the pressure scale height, which implies that giant and supergiant stars should have convection granules of sizes comparable to their radii. Numerical simulations and the observation of anisotropies on stellar discs agree well with this prediction. Aims. To measure the characteristic sizes of convection structures of models simulated with the CO5BOLD code, to look at how they vary between models and to study their limitations due to numerical resolution. Methods. Fourier analyses are performed to frames from the models to achieve spatial spectral power distributions which are averaged over time. The position of the main peak and the averagevalue of the wavevector are taken as indicators of these sizes. The general shape of the intensity map of the disc in the frame is fitted and subtracted so that it does not contaminate the Fourier analysis. Results. A general relationship of the convection granule size being more or less ten times larger than the pressure length scale is found. The expected wavevector value of the time-averaged spectral power distributions is higher than the position of the main peak. Loose increasing trends with the characteristic sizes by the pressure scale height increasing against stellar mass, radius, luminosity,temperature and gravity are found, while a decreasing trends are found with the radius and modelresolution. Bad resolution subtracts signals on the slope at the side of the main peak towards larger wavevector values and in extreme cases it creates spurious signal towards the end of the spectrum due to artifacts appearing on the frames. Conclusions. The wavevector position of the absolute maximum in the time-averaged spectral power distribution is the best measure of the most prominent sizes in the stellar surfaces. The proportionality constant between granule size and pressure length scale is of the same order ofmagnitude as the one in the literature, however, models present sizes larger than the ones expected, likely because the of prominent features do not correspond to convection granules but to larger features hovering above them. Further studies on models with higher resolution will help in drawing more conclusive results. Appendix. The SIA pipeline takes a set of time-dependent pictures of stellar disks and uses a Fourier Analysis to measure the characteristic sizes of their features and other useful quantities, such as standard deviations or the spatial power distributions of features. The main core of the pipeline consists in identifying the stellar disc in the frames and subtracting their signal from the spatial power distributions through a general fit of the disc intensity. To analyze a time sequence, the SIA pipeline requires at least two commands from the user. The first commandorders the SIA pipeline to read the .sav IDL data structure file where the frame sequence is stored and to produce another .sav file with information on the spectral power distributions, the second command orders the reading of such file to produce two more .sav files, one containing time-averaged size measurements and their deviations while the other breaking down time-dependant information and other arrays used for the calculations. The SIA pipeline has been entirely written in Interactive Data Language (IDL). Most of the procedures used here are original from the SIA pipeline, but a small handfull like ima3_distancetransform.pro, power2d1d.pro, extremum.pro and smooth2d.pro from Bernd Freytag and peaks.pro and compile opt.pro amongst others are actually external.

The report consists in two parts:

1.- The main project, where we apply our pipeline and get scientific results.

2.- The appendix, where a technical description of the pipeline is given.

The imaging of astronomical objects at extremely high angular resolution is an invaluable tool for myriad areas of astronomy, including the study of protoplanetary disks, mass-loss of evolved stars and AGN. But this poses a significant technical challenge as Earth’s turbulent atmosphere massively degrades the resolution achievable by large telescopes. In this thesis the development and implementation of two novel techniques to overcome this seeing limit are presented, building upon the technique of astronomical interferometry. First, differential polarimetry is combined with aperture-masking interferometry to produce diffraction-limited measurements of polarised structure (such as protoplanetary disks and evolved-star mass-loss shells) at precisions well beyond conventional aperture-masking. Observations using this technique are presented, and the development of an entirely new, purpose-built instrument - VAMPIRES - is described (now at the 8 m Subaru telescope). First on-sky tests and science observations are also presented. The second technique described replaces the traditional aperture mask with photonic pupil-remapping technologies. In this instrument - Dragonfly - optical waveguides inscribed in three dimensions within a photonic chip (using laser direct-write) are used to re-map an arbitrary set of telescope sub-apertures into a one-dimensional output, yielding several advantages. The requirement for a non-redundant input is removed, potentially allowing the entire telescope pupil to be utilised, vastly increasing throughput. The waveguides are single-moded, acting as a spatial filter and greatly improving closure phase precision. This output configuration is then ideal for a photonic beam combiner chip or direct fringe production, and spectral dispersion. Various technical challenges and their characterisation and mitigation are also presented. Together, these technologies stand to play a key role in high angular resolution, high contrast astronomical imaging.

Aim. In this project, theoretical models of dust driven winds of asymptotic giant branch (AGB) stars with effective temperatures within a range of 2400 − 3200 [K] and relative carbon-to-oxygen abundance C/O > 1 are studied. The aim is to understand if and how a detailed description of the grain size in winds of carbon rich AGB stars affects the wind formation and wind driving processes. Method. The computations were performed with a well tested FORTRAN code by calculating a grid of 60 models with different stellar parameters using grain size-dependent opacities. The results were then compared with models where the small particle approximation (SPA) had been used. Conclusions. The results indicate a certain dependency on grain size of the wind properties. The results from the computations of the majority of the models show no significant diferences however, especially not for the mass loss rates. Thus earlier computations performed using the SPA need not necessarily to be rejected.

Fluorine abundance in the universe is far from being reproduced by models. In particular, it is strongly underestimated with respect to what experimentally observed in AGB-stars, considered to be the main production sites for 19F, the only stable isotope of Fluorine. AGB-stars are composed by a degenerate CO core surrounded by a He and a H shell. Those shells are then divided by a thin layer (10^-2-10^-3 solar radii). If temperature is T =10^8K, the 14N produced in the CNO cycle can be processed into 19F by the chain of reactions 14N( alpha,gamma)18F(beta + )18N(p, alpha)19F. If the production pattern of 19F is quite clear, the same thing can not be said for its destruction pathways. In AGB-environment two reactions are considered to be the main destructions pattern for 19F: 19F(p, alpha)16O and 19F( alpha,p)22Ne. We focused on this second reaction: try to obtain informations about this reaction is, indeed, very di cult with direct nuclear physics experiments. That is due to the Coulomb barrier, that strongly reduces the cross-section (to some pico-nanobarn). The direct measurements of the 19F(alpha,p)22Ne reaction at lowest energy is at ECM=0.91 MeV. However, given the typical temperatures for a low-mass AGB-stars (2 ·10^8 K, 2- 4 solar masses ), the Gamow window for this reaction is between 390 and 800 keV. For cases like that, in the last 20 years indirect methods have been used. Among these, the Trojan Horse Method is one of the most important for astrophysical measurements. For these reasons we decided perform an experiment at Rudjer Boskovic Institut (Zagreb), that consisted in using a 6Li beam to investigate the two body reaction 19F( alpha,p)22Ne using the 6Li(19F,p 22Ne)2H three-body one. In this why we were able the extract a cross-section in absolute unit, and to calculate the S(E)-factor, for the reaction rate and for the astrophysical impact of this measure. In the end, some preliminary results of the study of the 23Na(p, alpha)20Ne via the 23Na(d,p22Ne)n using THM is also discussed: in particular we were able to extract the two-body cross-section in arbitrary units, and to guess the involved resonances. The reaction 23Na(p,alpha )20Ne reaction is of primary importance for sodium destruction inside the nucleosynthesis path in the A>20 mass region in intermediate mass (4- 8 solar masses ) AGB-stars. This reaction is also involved in the branching point of the Ne-Na cycle, responsible for hydrogen burning at high temperatures (T=20 -100 T_6).

The physics pertaining to the asymptotic giant branch (AGB) phase of stellar evolution has been studied for many years. However, the mechanics behind many characteristics displayed at this stage are still not fully understood. As a member of the Long Period Variable class of stars, AGB stars are invaluable in creating three-dimensional maps of the Milky Way, the Magellanic System and other galaxies with resolved stellar populations. Variable stars can be used to determine radial distances from Earth using their periodic luminosity variations. As this type of star has unknown qualities, models of AGB populations need to be calibrated with observed data. Previous research has derived a best-fitting model using the TRILEGAL code (a TRIdimensional modeL of thE GALaxy). This model was calibrated against single-epoch luminosity functions (LFs) calculated from resolved stellar populations in the Small Magellanic Cloud (SMC). With multi-epoch data now available from the VISTA survey of the Magellanic Clouds (VMC), this best-fitting model can now be compared with the LFs as they vary with time. Firstly, statistical tests are completed to measure the extent of the LF variation between epochs and from the mean LF for both the full VMC AGB catalogue and for the oxygen-rich, carbon-rich and extreme AGB classes. Statistical tests are then performed to measure the similarity between the LFs from different epochs and the simulated LFs, again for the entire sample and the three classes. This investigation shows that, while the current best-fitting model is a good approximation of many individual epochs’ AGB LFs in the SMC to within 3σ, inclusion of multi-epoch data would make for a more robust analysis. In order to do this, it would be desirable to have more epochs with deeper and regular observations that could cover full lightcurves of some of the sources. There also seems to be a statistical difference between the inner and outer areas of the SMC, perhaps due to tidal disruptions. It would be interesting to see the results of a similar study using the LMC, which is less affected by the gravitational influence of its smaller companion.

This thesis was written under the supervision of Maria-Rosa Cioni at the Leibniz Institute for Astrophysics in Potsdam. The presentation was held online due to the COVID-19 pandemic.

Modelling stellar structures and comparing them with observationsis a very important step when it comes to verifying our theories aboutstellar evolution. Three-dimensional reconstruction is therefore impor-tant and in the case of certain stellar types it makes for a large portionof the ongoing research.For this project, three dierent objects and their three-dimensionalmodels were selected for 3D-printing. The systems in question areEta Carinae, 1 Gruis and HD 101584 and the reason behind thischoice is the fact that each object showcases a dierent phase and/orprocess of stellar evolution approaching or during the planetary nebula(PN) stage. On top of that, these objects have been observed using atechnique that allows us to deduce their 3D structure.The three-dimensional models and prints allow us to nd features(such as axial symmetries) that give us more information about themovements inside the system and their consequences on how the stellarstructure has and will evolve.

During these last years, with the advent of modern telescopes and satellites, studies about high redshift galaxies are topics in great development from an observational point of view; photometric surveys permit observations of very far and faint galaxies with bigger efficiency, thus allowing to study the galactic evolution with redshift. Observations show a strongly obscured high-z universe featuring huge quantities of dust (Omont et al. 2001; Shapley et al. 2001; Bertoldi et al. 2003; Robson et al. 2004; Wang et al. 2008; Wang et al. 2008; Gallerani et al. 2010; Michalowski et al. 2010; Michalowski et al. 2010). Dust absorbs the stellar radiation in the UV/optical range of the electromagnetic spectrum and re-emits it in the IR, so it deeply changes the shape of the observed spectral energy distributions (SEDs) of obscured galaxies (Silva et al. 1998; Piovan et al. 2006b; Popescu et al. 2011), hampering their interpretation in terms of the fundamental physical parameters, such as age, metallicity, initial mass function (IMF), mix of stellar populations, star formation history (SFH). Only taking into account all the spectral range of emission it is possible to study the properties of galaxies by means of the evolutionary population synthesis (EPS) models. It is therefore mandatory to calculate evolutionary sequences of spectral models for galaxies of various morphological types, including the effects of dust, both for the local universe and high redshift objects (Narayanan et al. 2010; Jonsson et al. 2010; Grassi et al. 2010; Pipino et al. 2011; Popescu et al. 2011): this approach leads to a growing complexity and typically to a much larger set of parameters influencing the results of the simulations to be then compared with the observations. In this thesis, the effects of dust on galaxies have been deeply studied and considered in both chemical and spectro-photometric simulations of galaxies of different morphological types at many different levels, trying to improve the treatment of dust extinction, emission, formation and evolution in our models. In this work: (a) we introduced and studied the effects of dust in isochrones and SEDs of SSPs with an improved state-of-art treatment of the AGB phase in intermediate and low mass stars, taking into account circumstellar dust shells around them; (b) we developed and tested state-of-art models of galaxies of different morphological types, E, S0, Sa, Sb, Sc, Sd and disks, with the inclusion of all the local and global effects of dust extinction and emission. We presented and tested the models in many different photometric systems, both for the local and high redshift universe; (c) in order to improve upon the information supplied by the chemical code to the EPS, we developed and presented a state-of-art description of the evolution and formation of dust in spiral galaxies, with a complete treatment of dust yields and dust accretion/destruction processes. This refined model should be in future used to support the spectro-photometric models with a more advanced chemical interface. In particular, the Milky Way has been chosen as the ideal laboratory to study the dust cycle (Zhukovska et al. 2008) and its impact on the wider subject of galaxy formation, with the target to later (1) extend the dusty chemical model to ellipticals, intermediate type and starburst galaxies and (2) interface it to our spectro-photometric code.
In questi ultimi anni, lo studio delle galassie ad alto redshift è un argomento in piena evoluzione dal punto di vista osservativo; le survey fotometriche hanno consentito osservazioni di galassie sempre più distanti e deboli con una crescente efficienza, permettendo uno studio via via più approfondito dell’evoluzione galattica al variare del redshift. Le osservazioni mostrano un Universo ad alto-z fortemente oscurato dalle polveri: l’effetto della polvere è di assorbire la radiazione stellare nel range UV/ottico dello spettro elettromagnetico e di riemettere tale radiazione nell’infrarosso, modificando quindi profondamente la forma delle distribuzioni spettrali di energia (SEDs) delle galassie oscurate (Silva et al. 1998; Piovan et al. 2006b; Popescu et al. 2011), e di ostacolare l’interpretazione dei loro parametri fisici fondamentali, quali l’età, la metallicità, la funzione iniziale di massa (IMF), le popolazioni stellari (SSP), la storia di formazione stellare. Solo tenendo conto dell’emissione proveniente da tutte le lunghezze d’onda è possibile studiare le proprietà delle galassie per mezzo della sintesi evolutiva di popolazione (EPS). Ha quindi notevole importanza calcolare sequenze evolutive di modelli spettrali di galassie di diverso tipo morfologico, includendo gli effetti della polvere, sia nell’universo locale che ad alto redshift (Narayanan et al. 2010; Jonsson et al. 2010; Grassi et al. 2010; Pipino et al. 2011; Popescu et al. 2011): questo approccio ovviamente aumenta la complessità del lavoro e porta in genere ad avere un numero molto più elevato di parametri che influenzano i risultati delle simulazioni, da confrontare con le osservazioni. In questa tesi, gli effetti della polvere sulle galassie sono stati profondamente studiati e analizzati sia dal punto di vista chimico che spettrofotometrico, per galassie di diversi tipo morfologici, a vari livelli, cercando di migliorare il trattamento dell’ estinzione, emissione, formazione ed evoluzione della polvere nelle nostre simulazioni. In questo lavoro: (a) abbiamo introdotto e studiato gli effetti della polvere nelle isocrone e nelle popolazioni stellari, introducendo un trattamento allo stato dell’arte della fase di ramo asintotico delle giganti (AGB), per stelle di massa piccola e intermedia, considerando le shells di polveri circondanti tali oggetti; (b) abbiamo sviluppato e testato modelli state-of-art di galassie di diversi tipi morfologici, (E, S0, Sa, Sb, Sc, Sd e dischi), includendo tutti gli effetti locali e globali legati all’estinzione e emissione delle polveri. Abbiamo presentato e testato i modelli in diversi sistemi fotometrici, sia per l’Universo locale che ad alto redshift; (c) allo scopo di migliorare le informazioni fornite dal codice chimico al codice di sintesi di popolazione, abbiamo sviluppato e presentato una descrizione attuale dell’evoluzione e della formazione della polvere nelle galassie a spirale, trattando in modo completo gli yields di polvere e i processi di accrescimento/distruzione. Questo modello raffinato sarà poi in futuro utilizzato come base per i modelli spettrofotometrici, che includano un’interfaccia chimica più avanzata. In particolare, la Via Lattea stato scelta come il laboratorio ideale per studiare il ciclo della polvere (Zhukovska et al. 2008) e il susseguente impatto dello stesso sulla formazione delle galassie, con l’obiettivo di (1) estendere il modello chimico polveroso alle galassie ellittiche, di tipo intermedio e starburst e (2) interfacciarlo al nostro codice spettro-fotometrico.

Carbon is the basis for life, as we know it, but its origin is still largely unclear. Carbon-rich Asymptotic Giant Branch (AGB) stars (carbon stars) play an important rôle in the cosmic matter cycle and may contribute most of the carbon in the Galaxy. In this thesis it is explored how the dust-driven mass loss of these stars depends on the basic stellar parameters by computing a large grid of wind models. The existence of a critical wind regime and mass-loss thresholds for dust-driven winds are confirmed. Furthermore, a steep dependence of mass loss on carbon excess is found. Exploratory work on the effects of different stellar metallicities and the sizes of dust grains shows that strong dust-driven winds develop also at moderately low metallicities, and that typical sizes of dust grains affect the wind properties near a mass-loss threshold. It is demonstrated that the mass-loss rates obtained with the wind models have dramatic consequences when used in models of carbon-star evolution. A pronounced superwind develops soon after the star becomes carbon rich, and it therefore experiences only a few thermal pulses as a carbon star before the envelope is lost. The number of dredge-up events and the thermal pulses is limited by a self-regulating mechanism: each thermal pulse dredges up carbon, which increases the carbon excess and hence also the mass-loss rate. In turn, this limits the number of thermal pulses. The mass-loss evolution during a thermal pulse (He-shell flash) is considered as an explanation of the observations of so-called detached shells around carbon stars. By combining models of dust-driven winds with a stellar evolution model, and a simple hydrodynamic model of the circumstellar envelope, it is shown that wind properties change character during a He-shell flash such that a thin detached gas shell can form by wind-wind interaction. Finally, it is suggested that carbon stars are responsible for much of the carbon in the interstellar medium, but a scenario where high-mass stars are major carbon producers cannot be excluded. In either case, however, the carbon abundances of the outer Galactic disc are relatively low, and most of the carbon has been released quite recently. Thus, there may neither be enough carbon, nor enough time, for more advanced carbon-based life to emerge in the outer Galaxy. This lends some support to the idea that only the mid-part of the Galactic disc can be a “Galactic habitable zone”, since the inner parts of the Galaxy are plagued by frequent supernova events that are presumably harmful to all forms of life.

Most of the physical processes driving the Thermally-Pulsing Asymptotic Giant Branch (TP-AGB) evolution are not yet fully understood and they need to be modelled with parametrised descriptions. The uncertainties of the models affect the interpretation of the spectrophotometric properties of galaxies up to high-redshift. In the framework of the ERC - STARKEY project, the aim of this Thesis is to constrain the uncertain parameters, i.e. third dredge-up and mass-loss, that still affect the TP-AGB models. To this purpose, I perform detailed simulations of AGB star populations in the Small Magellanic Cloud (SMC) based on robust measurements of the space-resolved star formation history as derived from the deep near-infrared photometry of the VISTA survey of the Magellanic Clouds. I compare the resulting synthetic catalogues with high-quality observations of resolved stellar populations in the infrared passbands of 2MASS and Spitzer. A large grid of TP-AGB evolutionary tracks is computed with several combinations of third dredge-up and mass-loss prescriptions. By requiring the models to reproduce the star counts and the luminosity functions of the observed Oxygen-, Carbon-rich and extreme-AGB stars, I put quantitative constraints on the efficiencies of the third dredge-up and mass-loss. The observed luminosity functions in all the available infrared photometric filters are successfully reproduced by two set of models, one with a relatively high mass-loss efficiency for Oxygen-rich stars and the second with a lower mass-loss efficiency and a lower efficiency of the third dredge-up for the more massive TP-AGB stars, i.e. initial masses larger than three solar masses. On the basis of the best-fitting model I present a complete characterisation of the AGB population in terms of stellar parameters, including the predicted mass-loss rates, initial masses, and Carbon-to-Oxygen ratio. I use the TP-AGB models calibrated in the SMC to model the population of Long Period Variables (LPVs) in the Large Magellanic Cloud as observed by Gaia. The remarkable agreement between models and observations allows us to guide the interpretation of a new observational diagram that is able to photometrically distinguish the evolutionary stages, the initial masses and the chemical type of these stars. In the context of the Large Synoptic Survey Telescope (LSST) science collaboration, I produce catalogues containing the synthetic photometry of the Magellanic Clouds in the Gaia and LSST filters. These catalogues, together with the all-sky simulations of the Milky Way will be made available to the community through the NOAO Data Lab to help defining the observing strategy of the LSST mini-surveys. In addition, I simulate samples of AGB stars in Local Group dwarf galaxies and find a general agreement with the data. However, to properly consider these objects in the TP-AGB models calibration, the simulations should be improved to take into account the crowding effects and the different areas used for the star formation histories derivation and the AGB stars identification. Finally, the products of this work, namely calibrated stellar isochrones and pulsation periods of LPVs, will be publicly available and ready to use for the interpretation of the data coming from present and future observing facilities. The calibrated TP-AGB models may be included in population synthesis models used to probe the integrated light of galaxies in the extragalactic Universe.
La maggior parte dei processi fisici che controllano l'evoluzione delle stelle di ramo asintotico nella fase dei pulsi termici (TP-AGB) non sono ancora stati pienamente compresi e devono essere modellati con descrizioni parametriche. Le incertezze dei modelli hanno un impatto sull'interpretazione delle proprietà spettrofotometriche delle galassie fino ad alto redshift. Nel contesto del progetto ERC - STARKEY, lo scopo di questa Tesi è di vincolare i parametri incerti, ovvero il terzo dredge-up e la perdita di massa nei modelli stellari AGB. A tal fine, ho effettuato dettagliate simulazioni di popolazioni stellari della Piccola Nube di Magellano basate su robuste misure della storia di formazione stellare spazialmente risolta, derivata dalla fotometria infrarossa della survey VISTA delle Nubi di Magellano. Ho confrontato i risultati dei cataloghi sintetici con accurate osservazioni delle popolazioni stellari risolte nelle bande infrarosse di 2MASS e Spitzer. Un'estesa griglia di tracce evolutive TP-AGB è stata calcolata con diverse prescrizioni di perdita di massa e terzo dredge-up. Imponendo ai modelli di riprodurre i conteggi stellari e le funzioni di luminosità osservate delle stelle AGB e delle sottoclassi di AGB ricche di ossigeno, ricche di carbonio e delle stelle più arrossate, ho posto vincoli quantitativi sull'efficienza del terzo dredge-up e della perdita di massa. Le funzioni di luminosità osservate in tutti i filtri infrarossi disponibili sono riprodotte con successo da due set di modelli: il primo ha una perdita di massa con efficienza relativamente alta per le stelle ricche di ossigeno e il secondo ha un'efficienza di perdita di massa inferiore e una ridotta efficienza del terzo dredge-up per le stelle TP-AGB più massicce, ovvero con massa iniziale maggiore di circa tre masse solari. Sulla base di quest'ultimo set di modelli ho presentato una completa caratterizzazione della popolazione AGB della Piccola Nube di Magellano in termini di parametri stellari, inclusi i tassi di perdita di massa previsti, i valori di massa iniziale e rapporto carbonio ossigeno. Ho utilizzato i modelli TP-AGB calibrati nella Piccola Nube di Magellano per simulare le variabili a lungo periodo nella Grande Nube di Magellano osservate da Gaia. Il notevole accordo tra modelli e osservazioni ha permesso di guidare l'interpretazione di un nuovo diagramma osservativo capace di distinguere fotometricamente gli stadi evolutivi, le masse iniziali e il tipo chimico di queste stelle. Nel contesto della collaborazione scientifica Large Synoptic Survey Telescope (LSST), ho prodotto cataloghi contenenti la fotometria sintetica delle Nubi di Magellano nei filtri di Gaia e LSST. Questi cataloghi, assieme alle simulazioni della Galassia, saranno rese disponibili alla comunità scientifica tramite NOAO Data Lab per aiutare a definire la strategia osservativa dei programmi "mini-survey" di LSST. Inoltre, ho simulato alcuni campioni di stelle AGB in galassie locali e ho trovato un buon accordo tra dati e modelli. Tuttavia, per poter considerare questi oggetti nella calibrazione dei modelli TP-AGB è necessario migliorare le simulazioni per tenere conto degli effetti del crowding e delle diverse regioni utilizzate per la derivazione della storia di formazione stellare e l'identificazione delle stelle AGB. I prodotti di questo lavoro, ovvero isocrone stellari calibrate e periodi di pulsazione delle variabili a lungo periodo, saranno resi pubblicamente disponibili per interpretare dati osservativi già disponibili e dati che verranno rilasciati dai futuri telescopi. I modelli TP-AGB calibrati potranno essere inclusi in modelli di sintesi di popolazione utilizzati per analizzare la luce integrata di galassie nell'Universo extragalattico.

Thesis (Ph.D.)--University of Toledo, 2005.
Typescript. "A dissertation [submitted] as partial fulfillment of the requirements of the Doctor of Philosophy degree in Physics." Bibliography: leaves 328-333.

The expectation to date the age of low mass pre-main sequence stars from lithium has been tested by comparing the observed lithium and the predicted abundance by evolutionary models. The test, in this thesis, has been applied on a sample of binary systems whose components have a well known mass or whose mass ratio has been exactly established. The common metallicity and the coevality of the two components of a system are strong constraints to determine the age on the basis of evolutionary codes. To achieve reliable results, by an observational campaign, I have doubled the sample of stars presenting the necessary information for the analysis. Stellar parameters have been determined with the most precise and accurately tested techniques: high resolution spectroscopy along a very large wavelength range and numerical solution of the radiative transfer equation. As to the evolutionary code, I have implemented FRANEC with the very accurate reaction rates as determined with the most reliable experimental technique, the Trojan Horse Method. Since for PMS stars the agreement between observed and predicted lithium abundance can be obtained just tuning the external convective efficiency, I have computed a database of models for different values of the mixing length parameter. Age determination of stars has been carried out by adopting what is nowadays believed to be the most powerful statistical method in the field, the Bayesian analysis. I have extended in an original way this statistical method from binary system with known masses to the most common double lined spectroscopic binaries.

The Hubble Space Telescope archive contains a large collection of images of eta Carinae, and this paper analyses those most suitable for measuring its expanding Homunculus Nebula. Multiple intensity tracings through the Homunculus reveal the fractional increase in the overall size of the nebula; this avoids registration uncertainty, mitigates brightness fluctuations, and is independent of previous methods. Combining a 13 yr baseline ofWide Field Planetary Camera 2 images in the F631N filter, with a 4 yr baseline of Advanced Camera for Surveys/ High Resolution Channel images in the F550M filter, yields an ejection date (assuming linear motion) of 1847.1 (+/- 0.8 yr). This result improves the precision, but is in excellent agreement with the previous study by Morse et al., that used a shorter time baseline and a different analysis method. This more precise date is inconsistent with ejection during a periastron passage of the eccentric binary. Ejection occurred well into the main plateau of the Great Eruption, and not during the brief peaks in 1843 and 1838. The age uncertainty is dominated by a real spread in ages of various knots, and by some irregular brightness fluctuations. Several knots appear to have been ejected decades before or after the mean date, implying a complicated history of mass-loss episodes outside the main bright phase of the eruption. The extended history of mass ejection may have been largely erased by the passage of a shock through clumpy ejecta, as most material was swept into a thin shell with nearly uniform apparent age.

This thesis examines the changing nature of Hollywood stardom and how this is informed by an emergent celebrity culture. Through several case studies this study augments older forms of analysis with Bourdieu’s concept of capital to create a new model of stardom that can accommodate recent cultural developments. In chapter one four key forms of capital are identified. After contextualising this new model within the history of classic Hollywood and older academic approaches to stardom in chapter two, the analysis of Nicole Kidman’s star text in chapter three shows how her image has evolved to combine all forms of cultural capital and as such exemplifies an entirely new formulation of the Hollywood film star. Chapter four applies this analysis to the small screen, with the case studies of Michael J. Fox and Sarah Jessica Parker showing how some performers are able to accrue cultural capital by simultaneously working in film and television, establishing television as a legitimate site for Hollywood stardom and its associated capital. In chapter five a case study of Brand Beckham shows how the capital of contemporary celebrity can be effectively deployed in order to generate a similar allure to that of the classic Hollywood star and with it a similar level of Hollywood power. The final chapter examines the simultaneous unravelling of one brand and the creation of another in light of the increasing power of the fan within celebrity culture. A detailed study of Britney Spears’s presence on perezhilton.com highlights the involvement of the audience as producers of her image and demonstrates how new technologies can be used to create an entirely new form of fame for the gossip columnist, which in turn has been appropriated by the Hollywood system as the next site for legitimate fame.

I study mid-infrared and star formation properties of AGN samples using infrared observations, and star-forming galaxies using radio observations in order to investigate the link between star formation, AGN activity and radio luminosity. I present the results of these investigations in this thesis. I carried out an analysis of four complete samples of radio-loud AGN (3CRR, 2Jy, 6CE and 7CE) using near- and mid-IR data taken by the Wide-Field Infrared Survey Explorer (WISE). The combined sample consists of quasars and radio galaxies, and covers a redshift range 0:003 < z < 3:395. The dichotomy in the mid-IR properties of low- and high-excitation radio galaxies (LERGs - HERGs) is analysed using large complete samples. The results show that a division in the accretion modes of powerful LERGs and HERGs clearly stands out in the mid-IR radio plane. Evaluation of the positions of the sample objects in WISE colour-colour diagrams shows that widely used WISE colour cuts are not completely reliable in selecting AGN. I examined the link between AGN activity and star formation by constructing matched samples of local (0 < z < 0:6) radio-loud and radio-quiet AGN in the Herschel-ATLAS fields. AGN accretion and jet powers in these active galaxies are traced by [OIII] emission-line and radio luminosity, respectively. Star formation properties were derived using Herschel 250-_m and stellar mass measurements are taken from the SDSS-MPA-JHU catalogue. The stacking analyses show that star formation rates (SFRs) and specific star formation rate (SSFRs) of both radio-loud and radio-quiet AGN increase with increasing AGN power but that radio-loud AGN tend to have lower SFR. Additionally, radio-quiet AGN are found to have approximately an order of magnitude higher SSFRs than radio-loud AGN for a given level of AGN power. The difference between the star formation properties of radio-loud and -quiet AGN is also seen in samples matched in stellar mass. I also investigated the relationship between SFR and low-frequency radio luminosity observed in star-forming galaxies. I used a sample of star-forming galaxies in the 19 local Universe selected from the SDSS-MPA-JHU catalogue. LOFAR observations of the Herschel-ATLAS North Galactic Pole field (NGP) were carried out as part of the LOFAR surveys Key Science Project at an effective frequency of 150 MHz, which provided low-frequency radio luminosity of sample galaxies. SFRs of galaxies in the sample were derived using MAGPHYS spectral energy distribution (SED) fitting. The results of this study show that the slope of L150/SFR is less than unity and not universal for all star-forming galaxies (SFGs) in the local Universe (0 < z < 0:3). The slope of the L150/SFR relation is also found to be steeper than the L1:4/SFR relation, probably due to the contribution from thermal radio emission at 1.4 GHz. If the L150=SFR relation for strongly star-forming objects is explained naively by electron calorimetry, I conclude that low luminosity sources are not ideal calorimeters and differ from the main locus of SFGs at low redshifts. The different gradients we obtain for the far- IR/radio correlation using samples selected at different frequencies reveal the selection effects on relations derived in this thesis.