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Dissertations / Theses on the topic 'Stars'

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Published: 4 June 2021
Last updated: 1 August 2024

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1

Moeckel, Nickolas Barry. "Massive stars, disks, and clustered star formation." Connect to online resource, 2008. http://gateway.proquest.com/openurl?url_ver=Z39.88-2004&rft_val_fmt=info:ofi/fmt:kev:mtx:dissertation&res_dat=xri:pqdiss&rft_dat=xri:pqdiss:3303877.

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2

Belles, Pierre-Emmanuel Aime Marcel. "Formation of stars and star clusters in colliding galaxies." Thesis, University of Hertfordshire, 2013. http://hdl.handle.net/2299/10312.

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Mergers are known to be essential in the formation of large scale structures and to have a significant role in the history of galaxy formation and evolution. Besides a morphological transformation, mergers induce important bursts of star formation. These starburst are characterised by high Star Formation Efficiencies (SFEs) and Specific Star Formation Rates, i.e., high Star Formation Rates (SFR) per unit of gas mass and high SFR per unit of stellar mass, respectively, compared to spiral galaxies. At all redshifts, starburst galaxies are outliers of the sequence of star-forming galaxies defined by spiral galaxies. We have investigated the origin of the starburst-mode of star formation, in three local interacting systems: Arp 245, Arp 105 and NGC7252. We combined high-resolution JVLA observations of the 21-cm line, tracing the Hi diffuse gas, with UV GALEX observations, tracing the young star-forming regions. We probe the local physical conditions of the Inter- Stellar Medium (ISM) for independent star-forming regions and explore the atomic-to-dense gas transformation in different environments. The SFR/H i ratio is found to be much higher in central regions, compared to outer regions, showing a higher dense gas fraction (or lower Hi gas fraction) in these regions. In the outer regions of the systems, i.e., the tidal tails, where the gas phase is mostly atomic, we find SFR/H i ratios higher than in standard Hi-dominated environments, i.e., outer discs of spiral galaxies and dwarf galaxies. Thus, our analysis reveals that the outer regions of mergers are characterised by high SFEs, compared to the standard mode of star formation. The observation of high dense gas fractions in interacting systems is consistent with the predictions of numerical simulations; it results from the increase of the gas turbulence during a merger. The merger is likely to affect the star-forming properties of the system at all spatial scales, from large scales, with a globally enhanced turbulence, to small scales, with possible modifications of the initial mass function. From a high-resolution numerical simulation of the major merger of two spiral galaxies, we analyse the effects of the galaxy interaction on the star forming properties of the ISM at the scale of star clusters. The increase of the gas turbulence is likely able to explain the formation of Super Star Clusters in the system. Our investigation of the SFR–H i relation in galaxy mergers will be complemented by highresolution Hi data for additional systems, and pushed to yet smaller spatial scales.
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3

Davey, Stephen. "Irradiation of the secondary star in cataclysmic variable stars." Thesis, University of Sussex, 1994. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.386388.

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4

Souto, D., K. Cunha, D. A. Garcia-Hernandez, O. Zamora, C. Allende Prieto, V. V. Smith, S. Mahadevan, et al. "Chemical Abundances of M-Dwarfs from the Apogee Survey. I. The Exoplanet Hosting Stars Kepler-138 and Kepler-186." IOP PUBLISHING LTD, 2017. http://hdl.handle.net/10150/624381.

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We report the first detailed chemical abundance analysis of the exoplanet-hosting M-dwarf stars Kepler-138 and Kepler-186 from the analysis of high-resolution (R similar to 22,500) H-band spectra from the SDSS-IV-APOGEE survey. Chemical abundances of 13 elements-C, O, Na, Mg, Al, Si, K, Ca, Ti, V, Cr, Mn, and Fe-are extracted from the APOGEE spectra of these early M-dwarfs via spectrum syntheses computed with an improved line list that takes into account H2O and FeH lines. This paper demonstrates that APOGEE spectra can be analyzed to determine detailed chemical compositions of M-dwarfs. Both exoplanet-hosting M-dwarfs display modest sub-solar metallicities: [Fe/H](Kepler-138) = -0.09 +/- 0.09 dex and [Fe/H](Kepler-186) = -0.08 +/- 0.10 dex. The measured metallicities resulting from this high-resolution analysis are found to be higher by similar to 0.1-0.2 dex than previous estimates from lower-resolution spectra. The C/O ratios obtained for the two planet-hosting stars are near-solar, with values of 0.55 +/- 0.10 for Kepler-138 and 0.52 +/- 0.12 for Kepler-186. Kepler-186 exhibits a marginally enhanced [Si/Fe] ratio.
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5

Johnston, Katharine G. "Observational signatures of massive star formation : an investigation of the environments in which they form, and the applicability of the paradigm of low-mass star formation." Thesis, University of St Andrews, 2011. http://hdl.handle.net/10023/1895.

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This thesis presents both a study of the cluster-scale environments in which massive stars form, investigating in particular how the ionized gas in these regions relates to the molecular star-forming material, as well as detailed studies of two luminous forming stars, AFGL 2591 and IRAS 20126+4104, to determine whether they are forming similarly to their low-mass counterparts. The results of this work include the identification of 35 HII regions (20 newly discovered) via a radio continuum survey of ionized gas towards 31 molecular cluster-forming clumps. The observed ionized gas was found to be preferentially associated with the clumps, which were shown to have a range of evolutionary stages. The massive star formation efficiency was determined for the clumps with associated ionized gas, and a relationship was found between the mass of the clumps and the mass of their embedded massive stars. By modelling the SEDs and images of AFGL 2591 and IRAS 20126+4104, it was found that the geometry of their circumstellar material was generally consistent with an envelope plus disk, similar to that expected for low-mass protostars. However, within the central ~1800 AU, the mid-IR images of IRAS 20126+4104 were better described by only a flattened envelope, suggesting that the radiation from IRAS 20126+4104 may be affecting the regions closest to the star. Observations of the ionized and molecular gas towards AFGL 2591 were carried out, and a photoionization code was developed to interpret these observations. The results showed that the observed 3.6 cm emission is likely to be produced by both a shock-ionized jet and a hypercompact HII region that does not appear to have disrupted the jet or the large-scale circumstellar environment. In addition, the C¹⁸O(1-0) emission observed towards AFGL2591 traces the densest parts of the outflow, with the blue-shifted emission exhibiting many of the properties of the outflows from low-mass protostars.
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6

Telleschi, Alessandra Silvia. "Coronal evolution of solar-like stars : X-ray spectroscopy of stars in star-forming regions and the solar neighborhood /." Zürich : ETH, 2007. http://e-collection.ethbib.ethz.ch/show?type=diss&nr=17018.

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7

Nelmes, Susan Grace. "Skyrmion stars." Thesis, Durham University, 2012. http://etheses.dur.ac.uk/5258/.

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Neutron stars are very dense stars composed almost entirely of neutrons. As such, they should be able to be described by Quantum Chromodynamics (QCD). As QCD is a very complicated theory from which it is difficult to produce quantitative results we rely on effective theories to describe QCD physics. It has previously been shown that the Skyrme model, which has topological soliton solutions that can be identified as baryons, is such a low energy effective field theory for QCD. In this thesis, after presenting background material in chapters 1, 2 and 3, we explore the results of attempting to use the theory proposed by Skyrme to model neutron stars by investigating two models. The first, discussed in chapter 4 and based on original research, considers rational map ansatz solutions to the Skyrme model. By coupling the model using this ansatz to gravity and introducing a new way of stacking together the shell-like solutions that form we find minimum energy configurations that are stable models of neutron stars. They are, however, slightly too small to be considered a good model so a second approach is tried. The second model considers Skyrme crystal configurations. By using a relation between the energy per baryon of a Skyrme crystal and its anisotropic deformations we are able to find two equations of state for the crystal. These are combined with a Tolman-Oppenheimer-Volkoff equation, generalised to describe anisotropic deformations, to model neutron stars. We find that below a critical mass all deformations are isotropic and above it they are anisotropic up to a particular maximum mass and that this approach compares well with experimental observations. This second model is described in chapter 5 and is based on original research.
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8

Celizic, Joseph S. "Beyond Stars." Bowling Green State University / OhioLINK, 2009. http://rave.ohiolink.edu/etdc/view?acc_num=bgsu1243878273.

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9

Grigg, Madeline J. "Dog Stars." Bowling Green State University / OhioLINK, 2019. http://rave.ohiolink.edu/etdc/view?acc_num=bgsu1555682074446507.

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10

Frink, Sabine. "Kinematics of T Tauri stars in nearby star forming regions." [S.l. : s.n.], 1999. http://deposit.ddb.de/cgi-bin/dokserv?idn=961689390.

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11

Telting, John Henry. "Be-star discs and non-radial pulsations in rotating stars." [Amsterdam] : Amsterdam : Sterrenkundig Instituut "Anton Pannekoek", Universiteit van Amsterdam ; Universiteit van Amsterdam [Host], 1996. http://dare.uva.nl/document/91370.

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12

Karnath, Nicole. "The Transition Points in Young Stars and Young Star Clusters." University of Toledo / OhioLINK, 2019. http://rave.ohiolink.edu/etdc/view?acc_num=toledo1564763305735395.

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13

Rosen, Anna L. "The Destructive Birth of Massive Stars & Massive Star Clusters." Thesis, University of California, Santa Cruz, 2017. http://pqdtopen.proquest.com/#viewpdf?dispub=10274404.

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The injection of energy and momentum into the interstellar medium by young massive stars’ intense radiation fields and their fast, radiatively driven winds can have a profound influence on their formation and environment. Massive star forming regions are rare and highly obscured, making the early moments of their formation difficult to observe. Instead, we must turn to theory to elucidate the physics involved in the formation of massive stars and massive star clusters (MSCs), which can host thousands of massive stars. In my thesis, I developed analytical and numerical techniques to study the formation of massive stars and how stellar wind feedback affects the dynamics of gas that surrounds MSCs. To estimate the initial rotation rates of massive stars at birth, I developed a protostellar angular momentum evolution model for accreting protostars to determine if magnetic torques can spin down massive stars during their formation. I found that magnetic torques are insufficient to spin down massive stars due to their short formation times and high accretion rates. Radiation pressure is likely the dominate feedback mechanism regulating massive star formation. Therefore detailed simulation of the formation of massive stars requires an accurate treatment of radiation. For this purpose, I developed a new, highly accurate radiation algorithm that properly treats the absorption of the direct radiation field from stars and the re-emission and processing by interstellar dust. With this new tool, I performed a suite of three-dimensional adaptive mesh refinement radiation-hydrodynamic simulations of the formation of massive stars from collapsing massive pre-stellar cores. I found that mass is channeled to the massive star via dense infalling filaments that are uninhibited by radiation pressure and gravitational and Rayleigh-Taylor instabilities. To determine the importance of stellar wind feedback in young MSCs, I used observations to constrain a range of kinetic energy loss channels for the hot gas produced by the shock-heating of stellar winds to explain the low X-ray luminosities observed in Hii regions. I demonstrated that the energy injected by stellar winds is not a significant contributor to stellar feedback in young MSCs.

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14

Glenn, Jason 1968. "Millimeter-wave polarimetry of star formation regions and evolved stars." Diss., The University of Arizona, 1997. http://hdl.handle.net/10150/282440.

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A new λ = 1.3 mm polarimeter, Cyclops, was constructed to make observations of dust continuum emission from star formation regions. The polarization of the inner arcminute of DR 21 was mapped with Cyclops. The polarization percentage and position angle are remarkably constant, indicating a uniform magnetic field throughout the cloud. Turbulent gas motions are a more significant source of support against self gravity in the cloud core than thermal pressure or magnetic fields. The polarization toward the cloud core increases slightly from λ = 100 μm to λ = 2 mm and is consistent with the standard dust composition of silicates and graphite. A small continuum polarization survey of cloud cores with embedded protostars was made with Cyclops and combined with observations from the literature. There is no clear tendency for any preferred alignment of cloud core elongations with respect to magnetic field lines, especially for the bright, high mass star forming regions. This confirms that the massive cloud cores are magnetically supercritical. The magnetic field lines appear randomly oriented with respect to the local Galactic plane position angles, implying that the random component of the Galactic magnetic field dominates the spiral component in this sample. Three-σ upper limits of 0.4%, 1.2%, and 1.2% were placed on the polarization of the HCO⁺ J = 1-0 emission line from the DR 21 and Mon R2 molecular outflows, and the CS J = 2-1 line from the IRAS 16293-2422 molecular outflow, respectively. These polarizations are an order of magnitude lower than predicted by theoretical models. In the case of DR 21, the lack of polarization is probably due to a disordered magnetic field in clumpy, turbulent gas, although multiple scattering may also diminish the polarization. CS J = 2-1 polarizations of 0.9% ± 0.1% and 5.1% ± 1.5% were observed from the envelopes of the evolved stars IRC+10216 and CRL 2688, respectively. An anisotropic optical depth to escape of infrared photons from the central star, perhaps caused by a toroidal dust distribution, could generate the IRC+10216 polarization.
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15

Robitaille, Thomas Pierre. "Star formation across the galaxy : observations and modelling of the spectral energy distributions of young stars /." St Andrews, 2009. http://hdl.handle.net/10023/733.

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16

Pozzo, Monica. "The effect of high-mass stars on low-mass star formation." Thesis, Keele University, 2001. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.366445.

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17

Stewart, Chantal Juanita Michelle. "In the stars." Master's thesis, University of Cape Town, 2013. http://hdl.handle.net/11427/13940.

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This is a book about the nature of reality and the illusions we embrace to smooth our paths through the vagaries of life in an uncertain world. It is about the dichotomies which exist in life and how our perceptions of life define our realities. It uses the stars, moon and sky as a tool to explore these differences by counterpoising the two main characters as having different views of the same subject. Thus Gabriel is an astronomer with a rational scientific approach to the universe while Lena, though brought up in a scientific tradition, has, through her experiences and personality, learnt to question this approach and developed a more aesthetic and mythological approach to the universe. While this is explored in the tensions between her and Gabriel, her own reality is questioned through her interaction with Kingston Ingovazana Mabilu. Although she is drawn to his philosophical views on life, in the end she has misgivings about the extremes to which these beliefs will eventually take her and is unable to commit to cultural beliefs which are so different from her own. The different perceptions are also explored in relation to ideas about health, medicine, causes of illness and their treatments. This brings into opposition the two prevalent paradigms in Southern Africa of the Western medical view and the alternative traditional view. The book tries to demonstrate how these are both valid with the characters showing justifications for their own points of view. Finally, the relationship between Lena and Gabriel focuses on the delicate fragility of relationships between people, in their attempts to understand each other, to communicate, to trust and to truly know each other. In the end, these challenges may be insurmountable. The themes of the book were inspired by the everyday dichotomies of life in Southern Africa; the different languages, religions and world views. It seemed important to explore issues of life and death, health and illness, particularly in a country where, less than a decade ago, the president of the country declared that HIV was not caused by a virus. The constructs of madness also warranted elucidation, as different cultures view these behaviours very differently. The second theme of astronomy versus mythology of the universe was inspired in part by Ben Okri in Starbook and The Famished Road, where magical realism opens up new ideas about the nature of reality in an African context. I also noted the tension between these ideas and the advanced technology of the Square Kilometre Array to be developed in South Africa. The book merely hints at the political past of South Africa but concentrates on the present paradigm of differing beliefs.
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18

Rohozen, Amy N. "Stars and Satellites." Kent State University Honors College / OhioLINK, 2016. http://rave.ohiolink.edu/etdc/view?acc_num=ksuhonors1462107958.

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19

Tinney, Christopher G. Mould Jeremy Reid Neill. "The faintest stars /." Diss., Pasadena, Calif. : California Institute of Technology, 1993. http://resolver.caltech.edu/CaltechETD:etd-09242008-091045.

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20

Cook, Bradley. "Among the stars." [Boise, Idaho] : Boise State University, 2009. http://scholarworks.boisestate.edu/td/36/.

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21

Robitaille, Thomas P. "Star formation across the galaxy : observations and modelling of the spectral energy distributions of young stars." Thesis, University of St Andrews, 2009. http://hdl.handle.net/10023/733.

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In the last few decades, the emergence of large-scale infrared surveys has led to a revolution in the study of star formation. In particular, NASA’s Spitzer Space Telescope has recently carried out mid- and far-infrared observations of numerous star formation regions with unprecedented resolution and sensitivity, and has uncovered thousands of forming stars. In combination with present and future large-scale near-infrared and sub-mm surveys, spectral energy distributions from near-infrared to mm wavelengths will be available for these thousands of young stars. Never before has there been such a wealth of multi-wavelength data for so many young stars. Traditional techniques for studying the physical properties of young stars through their spectral energy distributions have usually focused either on the analysis of many sources using simple observational diagnostics such as colours or spectral indices, or on the analysis of a few sources through the detailed modelling of their full spectral energy distributions. The work presented in the first part of this thesis aims to bridge these two techniques through the efficient modelling of the spectral energy distributions of many young stars. In particular, the technique developed for this work makes it straightforward to find out how well different physical parameters are constrained, whether any parameters are degenerate, and whether additional data would resolve the degeneracies. In the second part of this thesis, a census of intrinsically red sources observed by Spitzer in the Galactic plane is presented, including a catalogue of over 11,000 likely young stellar objects. This sample of sources is the largest uniformly selected sample of young stars to date, and effectively provides a map of the sites of star formation in the mid-plane of the Milky-Way. In parallel, this census has uncovered over 7,000 candidate asymptotic giant branch stars, of which over 1,000 are variable at 4.5 or 8.0 microns.
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22

Roark, Jacob Brian. "The Deconfinement Phase Transition in Neutron Stars and Proto-Neutron Stars." Kent State University / OhioLINK, 2018. http://rave.ohiolink.edu/etdc/view?acc_num=kent1542979864566784.

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23

Bozhinova, Inna. "TOYS : time-domain observations of young stars." Thesis, University of St Andrews, 2017. http://hdl.handle.net/10023/12014.

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Stars form inside clouds of molecular gas and dust. In the early stages of stellar evolution the remainders of the initial cloud form a circumstellar disk. For the next few million years the disk will slowly dissipate via accretion, outflows, photoevaporation and planet growth while the star makes its way onto the Main Sequence. This stage of a star's life is referred to as the T Tauri phase and is characterised by high-level spectrophotometric variability. This thesis aims to study and map out the environments of T Tauri stars down to the very low mass regime by the means of time-domain monitoring. Different physical processes in the system manifest themselves as variability on different time- scales as well as produce characteristic spectroscopic and photometric features at various wave- lengths. In order to study young stellar objects in depth, the observing campaigns presented in this work were designed to cover a large range of time-scales - minutes, hours, days and months. Combining all the data, this thesis establishes a baseline of over a decade for some objects. The observations also cover a wide range of wavelengths from the optical to the mid-infrared part of the spectrum. The star RW Aur experienced two long-lasting dimming events in 2010 and 2014. This thesis presents a large collection of spectral and photometric measurements carried out just before and during the 2014 event. Spectral accretion signatures indicate no change in the accretion activity of the system. Photometry indicates that parallel to the dimming in the optical the star becomes brighter in the mid-infrared. The observations in this work combined with literature data suggest that the origin of the 2014 event is most likely obscuration of the star by hot dust from the disk being lifted into the disk wind. Very low mass stars (< 0.4 M⊙) are the most common type of star in the Galaxy. In order to understand the early stages of stellar evolution we must study young very low mass stars. This work investigates the photometric and spectroscopic variability of seven brown dwarfs in star forming regions near σ Ori and ε Ori. All targets exhibit optical photometric variability between from 0.1 to over 1.0 magnitude that persists on a time-scale of at least one decade. Despite the photometric variability no change in the spectral type is measured. In the cases where the stars are accreting, modelling of the spectral changes suggest the accretion flow is more homogeneous and less funnelled compared to Sun-like T Tauri stars. The non-accreting variables are more plausibly explained by obscuration by circumstellar material, possibly a ring made out of multiple clouds of dust grains and pebbles with varying optical depths. The star-disk systems studied in this thesis have some broader implications for star and planet formation theory. The case-study of RW Aur has unambiguously demonstrated that the planet- forming environment is very dynamic and can change dramatically on short time-scales, which in turn would have implications for the diversity of planetary systems found in the Galaxy. The Orion stars have shown that the current theory for the T Tauri stage of stellar evolution is valid down to the very low mass regime. The seven dwarfs are a good example for the evolutionary path of circumstellar disks, showing the transition from gas-high, flared accretion disks (σ Ori) to dust-dominated, depleted, structured debris disks (ε Ori).
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24

Oh, Seungkyung [Verfasser]. "The Dynamical Ejections of Massive Stars from Young Star Clusters / Seungkyung Oh." Bonn : Universitäts- und Landesbibliothek Bonn, 2017. http://d-nb.info/1139048805/34.

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25

Gregory, Scott G. "T Tauri stars : mass accretion and X-ray emission." Thesis, St Andrews, 2007. http://hdl.handle.net/10023/336.

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26

Brunsden, Emily. "The Music of the Stars : Spectroscopy of Pulsations in gamma Doradus Stars." Thesis, University of Canterbury. Astronomy, 2013. http://hdl.handle.net/10092/8373.

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The mysteries of the interior structures of stars are being tackled with asteroseismology. The observable parameters of the surface pulsations of stars inform us of the interior characteristics of numerous classes of stars. The main-sequence gamma Doradus stars, just a little hotter than the Sun, offer the potential of determining stellar structure right down to the core. To determine the structural profile of a star, the observed frequencies and a full geometric description must be determined. This is only possible with long-term spectroscopic monitoring and careful analysis of the pulsation signature in spectral lines. This work seeks to identify the pulsational geometry of several gamma Doradus stars and to identify areas of improvement for current observation, analysis and modelling techniques. More than 4500 spectra were gathered on five stars for this purpose. For three stars a successful multi-frequency and mode identification solution was determined and significant progress has been made towards the understanding of a binary system involving a gammaDoradus star. A hybrid gamma Doradus/\delta Scuti pulsator was also intensely monitored and results from this work raise important questions about the classification of this type of star. Current analysis techniques were found to be fit-for-purpose for pure gamma Doradus stars, but stars with complexities such as hybrid pulsations and/or fast rotation require future development of the current models.
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27

Lai, David K. "Metal-poor stars : the fingerprints of the first stars and early galaxy /." Diss., Digital Dissertations Database. Restricted to UC campuses, 2008. http://uclibs.org/PID/11984.

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28

Chen, Yang. "Evolution of Very Low Mass Stars and Very Massive Stars in PARSEC." Doctoral thesis, SISSA, 2015. http://hdl.handle.net/20.500.11767/4879.

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During my Ph.D. study, I have been concentrating on the evolutionary tracks and atmosphere models of very low mass stars (VLMSs; ∼ 0.1 − 0.6 Msun) and very massive stars (VMSs; ∼ 12−350 Msun), based on the PAdova-TRieste Stellar Evolution Code (PARSEC). For the very low mass models, it appeared that the previous models computed with PARSEC were unable to correctly predict some basic observational relations. These relations include the mass–radius relation and the colour-magnitude relations of cool dwarfs. We replace the approximate boundary conditions used in PARSEC with those provided by more realistic atmosphere modelling. We implement the T – tau relations from Phoenix/BT-Settl model atmospheres as the outer boundary conditions in the PARSEC code, finding that this change alone reduces the discrepancy in the mass–radius relation from 8 to 5 per cent. Furthermore, we convert the theoretical quantities to the magnitudes and colors with the stellar spectral libraries from Phoenix/BT-Settl. We compare the models with multi–band photometries of clusters Praesepe, M 67, 47 Tuc and NGC 6397, showing that the use of T – tau relations clearly improves the description of the optical colours and magnitudes. However, using both Kurucz and Phoenix model spectra, the models are still systematically fainter and bluer than the observations. We then apply a shift to the above T – tau relations, increasing from 0 at Teff = 4730 K to ∼14 per cent at Teff = 3160 K, to reproduce the observed mass–radius relation of dwarf stars. Taking this experiment as a calibration of the T – tau relations, we can reproduce the optical and near-infrared CMDs of low mass stars in the old metal–poor globular clusters NGC 6397 and 47 Tuc, and in the intermediate–age and young Solar–metallicity open clusters M 67 and Praesepe. Thus, we extend PARSEC models using this calibration, providing VLMS models that are more suitable for the lower main sequence stars over a wide range of metallicities and wavelengths. For the very massive stars, the Padova models were computed more than 20 years ago and were not distributed to the community because suitable bolometric corrections for these models were not yet implemented. In this project, we complement the PARSEC data base with the stellar evolutionary tracks of massive stars, from the pre-main sequence phase to the central Carbon ignition. We consider a broad range of metallicities, 0.0001≤ Z ≤ 0.04 and initial masses up to Mini =350 Msun. The main difference with respect to our previous models of massive stars is the adoption of a recent formalism accounting for the mass-loss enhancement when the ratio of the stellar luminosity to the Eddington luminosity, Γe , approaches unity. With this new formalism, the models are able to reproduce the Humphreys-Davidson limit observed in the Galactic and Large Magellanic Cloud colour-magnitude diagrams, without an ad hoc mass-loss enhancement. We also follow the predictions of recent wind models indicating that the metallicity dependence of the mass-loss rates becomes shallower when Γ e approaches unity. We thus find that massive stars may suffer from substantial mass-loss even at low metallicity. We also predict that the Humphreys-Davidson limit should become brighter at decreasing metallicity. We supplement the evolutionary tracks with new theoretical bolometric correction tables, useful for comparing tracks and isochrones with the observations. For this purpose, we homogenize existing stellar atmosphere libraries of hot and cool stars (PoWR, ATLAS9 and Phoenix) and add, where needed, new atmosphere models computed with WM-basic. The model grids are fully adequate in mass, age and metallicity for performing detailed investigations of the properties of very young stellar systems in both local and distant galaxies. The new tracks supersede the previous old Padova models of massive stars. Therefore, my work together with the already updated PARSEC models of the other masses could depict the full evolution of stars across the mass range from ∼ 0.08 Msun to 350 Msun and over a wide range of metallicity from super-Solar (Z = 0.04) to extreme metal-poor (Z = 0.0001). Consequently, they would provide paramount information for studies not only on stars or star clusters but also on galaxy formation and evolution.
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29

Gill, Stuart P. D. "The evolution of a dark halo substructure." Australasian Digital Thesis Program, 2005. http://adt.lib.swin.edu.au/public/adt-VSWT20061009.115152/index.html.

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Thesis (Ph.D) - Swinburne University of Technology, [Faculty of Information & Communication Technologies], 2005.
A dissertation presented in total fulfillment of the requirements of for the degree of Doctor of Philosophy, [Faculty of Information and Communication Technologies], Swinburne University of Technology, 2005. Typescript. Bibliography p. 145-154.
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30

Wilking, B. A., C. J. Lada, and E. R. Young. "IRAS Observations of the Rho Ophiuchi Infrared Cluster: Spectral Energy Distributions and Luminosity Function." Steward Observatory, The University of Arizona (Tucson, Arizona), 1988. http://hdl.handle.net/10150/623919.

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31

Rosén, Lisa. "Spectropolarimetry of Magnetic Stars." Thesis, Uppsala universitet, Observationell astrofysik, 2011. http://urn.kb.se/resolve?urn=urn:nbn:se:uu:diva-155686.

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A star´s magnetic field is important although stellar magnetic field generation is not completely understood. It´s necessary to investigate different types of stars and stars of different ages to learn how the magnetic fields are generated and how they affect the star and its surroundings. In this thesis I investigated six stars, HD 171488, GQ Lup, Tau Boo, HR 1099, HD 216489 and AU Mic with the help of high resolution circular polarization observations. They are all cool stars with relatively weak magnetic fields spanning in age from a young T-Tauri star, (GQ Lup), to an evolved subgiant, (HR 1099). Some of them have never before been investigated in terms of polarization. To obtain polarization profiles the Least Squares Deconvolution, LSD, technique was applied. A magnetic field was detected for all stars except Tau Boo, probably because the S/N ratio was too low. The values of the mean longitudinal magnetic field varied from a few G for Tau Boo up to -300 G for GQ Lup. GQ Lup also had two emission lines of HeI with even higher negative values up to -2000 G. The shape of the LSD polarization profiles indicates that AU Mic might have a dipole like field not aligned with the rotation axis, while GQ Lup showed the same polarity in all observations, possibly indicating a stable magnetic structure being observed pole-on. The two binaries, (HR 1099 and HD 216489), seemed to have complex fields, while HD 171488 seemed to have an azimuthal field.
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32

Cannon, Robert Charles. "Stars with compact cores." Thesis, University of Cambridge, 1992. https://www.repository.cam.ac.uk/handle/1810/252023.

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33

Grundstrom, Erika Dawn. "Hot Stars with Disks." Digital Archive @ GSU, 2007. http://digitalarchive.gsu.edu/phy_astr_diss/19.

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The evolutionary paths of the massive O and B type stars are often defined by angular momentum transformations that involve circumstellar gas disks. This circumstellar gas is revealed in several kinds of observations, and here I describe a series of investigations of the hydrogen line emission from such disk using detailed studies of five massive binaries and a survey of 128 Be stars. By examining three sets of spectra of the active mass-transfer binary system RY Scuti, I determined masses of 7.1 +/- 1.2 M_sun for the bright supergiant and 30.0 +/- 2.1 M_sun for the massive companion that is hidden by an accretion torus. I also present a cartoon model of the complex mass flows in the system. Using optical spectroscopy and X-ray flux data, I investigated the mass transfer processes in four massive X-ray binaries (a massive B star with mass flowing onto a compact, neutron star companion). The B-supergiant system LS I +65 010 transfers mass via stellar winds. I find the X-ray flux modulates with the orbital period. In the other three X-ray binary systems (LS I +61 303, HDE 245770, and X Persei), an outflowing circumstellar disk is responsible for the mass transfer, and in all three systems, the disk appears to be truncated by gravitational interactions with the compact companion. The disk in the microquasar system LS I +61 303 is limited in radius by the periastron separation and an increase in both H-alpha equivalent width and X-ray flux following periastron may be due to a density wave in the disk induced by tidal forces. Observations of HDE 245770 document what appears to be the regeneration of a circumstellar disk. The disk of X Persei appears to have grown to near record proportions and the X-ray flux has dramatically increased. Tidal interaction may generate a spiral density wave in the disk and cause an increase in H-alpha equivalent width and mass transfer to the compact companion. During the course of the analysis of the X-ray binaries, I developed numerical models for estimating the size of the Be star disks using just the H-alpha equivalent width. Finally, I present the results of a three year spectroscopic survey of both the H-alpha and H-gamma regions of 128 Be stars. I find that the median fractional variation in the equivalent width of the disk emission lines is 15% over a two year period. I also find that two-thirds of the sample displays evidence of Fe II emission or absorption resulting from surrounding circumstellar material. Many candidates for non-radial pulsation and binary systems are also found. Spectra and notes for all of the sample stars are presented in an appendix.
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34

Boyajian, Tabetha Suzanne. "Sizing Up the Stars." Digital Archive @ GSU, 2009. http://digitalarchive.gsu.edu/phy_astr_diss/34.

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For the main part of this dissertation, I have executed a survey of nearby, main sequence A, F, and G-type stars with the CHARA Array, successfully measuring the angular diameters of forty-four stars to better than 4% accuracy. The results of these observations also yield empirical determinations of stellar linear radii and effective temperatures for the stars observed. In addition, these CHARA-determined temperatures, radii, and luminosities are fit to Yonsei-Yale isochrones to constrain the masses and ages of the stars. These quantities are compared to the results found in Allende Prieto & Lambert (1999), Holmberg et al. (2007), and Takeda (2007), who indirectly determine these same properties by fitting models to observed photometry. I find that for most cases, the models underestimate the radius of the star by ~12%, while in turn they overestimate the effective temperature by ~ 1.5 - 4%, when compared to my directly measured values, with no apparent correlation to the star's metallicity or color index. These overestimated temperatures and underestimated radii in these works appear to cause an additional offset in the star's surface gravity measurements, which consequently yield higher masses and younger ages, in particular for stars with masses greater than ~ 1.3 M_sol. Alternatively, these quantities I measure are also compared to direct measurements from a large sample of eclipsing binary stars in Andersen (1991), and excellent agreement is seen within both data sets. Finally, a multi-parameter solution is found to fit color-temperature-metallicity values of the stars in this sample to provide a new calibration of the effective temperature scale for these types of stars. Published work in the field of stellar interferometry and optical spectroscopy of early-type stars are presented in Appendix D and E, respectively.
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35

Izzard, R. G. "Nucleosynthesis in binary stars." Thesis, University of Cambridge, 2004. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.604980.

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Galactic chemical evolution models require stellar nucleosynthesis yields as input data. Stellar evolution models are used to calculate such yields but do not take into account the fact that many stars are in binaries. The computing time required to explore the binary star parameter space is usually considered to be prohibitively large. Therefore binaries, except for type Ia supernovae and novae which are included in an ad hoc way, are ignored in most galactic chemical evolution models. In this dissertation synthetic nucleosynthesis models are developed which approximate full stellar evolution models. Cunning methods are employed to model shell burning in low- and intermediate-mass stars while high-mass stars have their surface abundances fitted to their mass. Explosive yields are fitted to published results. The synthetic nucleosynthesis model, with the addition of algorithms to deal with mass transfer in binaries, is coupled to a rapid binary star evolution code. The use of a synthetic model speeds up the calculation of stellar yields by a factor of about 107 and extends the analysis to binary stars. Single- and binary-star yields are calculated for a range of initial mass and separation distributions. A change in the primary or single-star mass distribution is most significant. Changing the secondary mass or separation distribution has a smaller effect. Consideration is then given to variation of the input physics to determine which free parameters are important for the calculation of yields from single and binary stars. It is found that certain parameters are important for some isotopes. Future prospects are then briefly discussed.
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36

Cunha, Margarida Maria Salvador. "Pulsations of magnetized stars." Thesis, University of Cambridge, 1999. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.624434.

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37

Ng, Chi-yung, and 吳志勇. "Theories of strange stars." Thesis, The University of Hong Kong (Pokfulam, Hong Kong), 2001. http://hub.hku.hk/bib/B29868646.

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38

Nieves, John A. "Ashes from Falling Stars." Scholar Commons, 2006. http://scholarcommons.usf.edu/etd/3766.

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This thesis is a book length collection of poetry—all original and by the author. The book has three chapters, each with a different mode of expressing the work’s overall theme: the remnants of unfulfilled wishes. The first chapter deals with ordinary or mundane manifestations of the theme. The second chapter covers extraordinary, but still feasible, variations on the theme. The final chapter deals with subconscious versions of these unfulfilled wishes. It is far more surreal than the other two chapters and exists in a sort of dream-reality. The poetry included in this work is all free verse. There are narrative and lyric poems present, along with other experimental modes of poetry. Even though plot threads run through some individual poems, the overall collection shares only thematic unity. The work, as the title implies, seeks to call attention the fragmentation of dead dreams after hope has burned away. In a sense then, this collection could be seen as a post-modern work. The poems are arranged within the chapters to either create groupings around minithemes, or to create sharp contrast. The order represented in this collection is an attempt to maximize impact. The cast of characters in this collection of poems is meant to be representative of the characters present in city life. There are drunks and prostitutes, mothers and fathers, lovers and ex-lovers, husbands and wives, sisters and brothers, schoolchildren, writers, grandparents, lawyers, politicians, photographers, gamblers and even corpses. These characters populate a world where unanswered wishes are as legitimate as minutes and hours as instruments to gauge the passing of lives—a poetic rendering of Earth at the beginning of the twenty-first century.
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39

Bushby, Paul James. "Nonlinear dynamos in stars." Thesis, University of Cambridge, 2004. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.615668.

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40

Millman, Eric B. "The Stars of David." ScholarWorks@UNO, 2015. http://scholarworks.uno.edu/td/1987.

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The Stars of David is based on the true story of a woman whose love of baseball stood above all. Set in the midst of the Great Depression, Jackie Austin, disgusted by the chauvinistic expectations of her impoverished father, sets off on her own to play for whatever team that will have her. That team proves to be the barnstorming House of David Baseball Club, an ascetic religious commune struggling to regain past glory after a decade of tragedy and shame. Outsiders and freaks to the rest of the world, these new "Stars" of David must learn to work together on the field in order to prosper in life. Can they succeed in the staunchly traditional, largely racist world of Depression-era Major League Baseball? Or will they, too, be whitewashed by time?
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41

Turek, Robert O. "Polygons, stars, and clusters." Thesis, Virginia Polytechnic Institute and State University, 1986. http://hdl.handle.net/10919/101328.

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One technique for displaying a set of quantitative variables is to represent the set as a polygon. Such displays allow the observer to visualize complex information quickly, as a whole. Polygon displays have been employed to display information for analysis, status, or presentation. An experimental investigation was undertaken to ascertain the effect of variation in certain visual features of the display on the consistency with which people categorize information presented as polygons. Variables included background information of the display, shading, and form. Subjects performed a categorization task on two sets of data; the results are analyzed for consistency between individuals and for consistency with certain standard clustering algorithms. The effects of distinctive portions of the figures on the judgment of similarity, and of the nature of the data and of interactions of combinations of the variables used in the experiment on the consistency of clustering were noted. Implications for the design of polygon displays are discussed.
M.S.
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42

Palladino, Chiara. "Numbers, winds and stars." Universitätsbibliothek Leipzig, 2017. http://nbn-resolving.de/urn:nbn:de:bsz:15-qucosa-221565.

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43

Ng, Chi-yung. "Theories of strange stars /." Hong Kong : University of Hong Kong, 2001. http://sunzi.lib.hku.hk:8888/cgi-bin/hkuto%5Ftoc%5Fpdf?B23316548.

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44

Harko, Tiberiu. "Properties of strange stars." Thesis, Hong Kong : University of Hong Kong, 2001. http://sunzi.lib.hku.hk:8888/cgi-bin/hkuto%5Ftoc%5Fpdf?B23242280.

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45

Doyle, Marianne T. "HOPCAT : a complete southern sky neutral hydrogen-optical galaxy catalogue and the star formation rate-density relation /." [St. Lucia, Qld.], 2006. http://www.library.uq.edu.au/pdfserve.php?image=thesisabs/absthe20100.pdf.

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46

Oskinova, Lidia M., Richard Ignace, and D. P. Huenemoerder. "X-ray Diagnostics of Massive Star Winds." Digital Commons @ East Tennessee State University, 2016. https://dc.etsu.edu/etsu-works/2703.

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Observations with powerful X-ray telescopes, such as XMM-Newton and Chandra, significantly advance our understanding of massive stars. Nearly all early-type stars are X-ray sources. Studies of their X-ray emission provide important diagnostics of stellar winds. High-resolution X-ray spectra of O-type stars are well explained when stellar wind clumping is taking into account, providing further support to a modern picture of stellar winds as non-stationary, inhomogeneous outflows. X-ray variability is detected from such winds, on time scales likely associated with stellar rotation. High-resolution X-ray spectroscopy indicates that the winds of late O-type stars are predominantly in a hot phase. Consequently, X-rays provide the best observational window to study these winds. X-ray spectroscopy of evolved, Wolf-Rayet type, stars allows to probe their powerful metal enhanced winds, while the mechanisms responsible for the X-ray emission of these stars are not yet understood.
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47

Hamaguchi, Kenji. "X-ray Study of the Intermediate Mass Young Stars Herbig Ae/Be Stars." 京都大学 (Kyoto University), 2001. http://hdl.handle.net/2433/150824.

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48

Naso, Luca. "Magnetic Fields in Proto-Neutron Stars and in Accretion Discs Around Neutron Stars." Doctoral thesis, SISSA, 2009. http://hdl.handle.net/20.500.11767/4267.

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Themain characters of this thesis are themagnetic field, the plasma velocity field, the turbulentmagnetic resistivity and the numerical codes. They act on two different stages and on two different levels and occasionaly there are other bit players, e.g. the α-effect, the quenching, the differential rotation, themagnetic streamfunction, themagnetic Reynolds number, the Interactive Data Language and even ZEUS. All of them are led by the same invisible hand with the purpose of understanding better the intricate topic of the magnetic field - plasma relation. The two stages of the scene could not be more different, in one case everything is done in less than a minute inside a proto-neutron star soon after a supernova explosion, in the other case there is no time evolution at all and an equilibrium configuration is looked for inside a disc ofmatter spiraling around a neutron star. Nevertheless the same set of equations can describe the behaviour of the characters on both stages, this set is composed of the equations of the electromagnetic field plus the fluid equations. However knowing that the answers to all of your questions are written inside only one book, does not mean that you are able to read that book ... It is at this moment that the numerical codes come into the scene, offering you a way of translating the book in a language that you know. Unfortunately they like playing tricks and you cannot trust their translations unless you take many precautions every time. Eventually, after the equations have been solved, comes the art of interpreting the results; a task that might seem quite simple in comparison with the difficulties overcome on the path to get there, but that requires a deep knowledge of what has already been done and a good intuition about what can possibly happen later on. We do not presume to have made big leaps forward in the process of understanding the behaviour of the magnetic field in the cases considered here, nonetheless thanks to our simplified models we were able to grasp the fundamental aspects of the phenomena being considered, to gain some insights and to propose new falsifiable ideas. At the same time we have also developed new tools for making our models more elaborate and realistic. Therefore we expect to find even more characters in the future Chapters of this analysis, but that is another story, and will be told another time.
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49

De, Rosa Robert John. "The volume-limited A-star survey : exploring the multiplicity of intermediate mass stars." Thesis, University of Exeter, 2012. http://hdl.handle.net/10036/3544.

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I present the results of the first volume-limited adaptive optics multiplicity survey of A-type stars. Using high-resolution observations obtained using adaptive optics instruments at five observatories, I have estimated the frequency of binary companions to a sample of 233 A-type stars within 60 parsecs. The survey is complete within a projected separation range of $56 < a{\rm [AU]} \le 891$, and a companion mass ratio range of $0.05 \le q \le 1.00$, corresponding to the bottom of the Main Sequence for companions to A-type stars. The multiplicity fraction of the sample, when corrected for completeness, is measured to be $24.2\% \pm 3.5\%$, continuing the trend of increasing multiplicity as a function of increasing primary mass reported in previous volume-limited surveys of Solar-type and M-dwarf primaries. A companion mass ratio and separation distribution are constructed over the restricted separation range, and are compared with previous observations of lower-mass primaries and theoretical predictions. The mass ratio distribution is strongly skewed towards lower-mass companions, consistent with the formation of binary companions within large circumstellar disks, while the shape of the separation distribution is not fully resolved due to the incompleteness of the survey. Over the separation range to which the observations are sensitive, a significant separation dependence on the mass ratio is observed, with a greater frequency of lower-mass companions measured at wider separations. Using this large dataset of adaptive optics observations, I have also explored the unexplained X-ray detection of a subset of A-type stars, whose interior structures theoretically preclude the generation of X-rays. By constructing two similarly sensitive samples of X-ray and non X-ray detected early-type stars, I have tested the hypothesis that an unresolved, lower-mass companion is responsible for the emission. A comparison of the fraction of targets with resolved binary companions within both samples reveals a significantly higher multiplicity fraction for the X-ray detected sample of A-type stars, consistent with the prediction of this companion hypothesis. I have also studied the orbital motions of thirteen A-type stars, and determined the dynamical mass of each system. These dynamical mass estimates are compared with theoretical values from stellar evolutionary models, with discrepancies between these values within several systems used as evidence of unresolved companions, providing potential targets for future interferometric and spectroscopic observations.
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Szigeti, László, Szabolcs Mészáros, Verne V. Smith, Katia Cunha, Nadège Lagarde, Corinne Charbonnel, D. A. García-Hernández, et al. "12C/13C isotopic ratios in red-giant stars of the open cluster NGC 6791." OXFORD UNIV PRESS, 2018. http://hdl.handle.net/10150/627135.

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Carbon isotope ratios, along with carbon and nitrogen abundances, are derived in a sample of 11 red-giant members of one of the most metal-rich clusters in the Milky Way, NGC 6791. The selected red-giants have a mean metallicity and standard deviation of [Fe/H] = +0.39 +/- 0.06 (Cunha et al. 2015). We used high-resolution H-band spectra obtained by the SDSS-IV Apache Point Observatory Galactic Evolution Experiment. The advantage of using high-resolution spectra in the H band is that lines of CO are well represented and their line profiles are sensitive to the variation of C-12/C-13. Values of the C-12/C-13 ratio were obtained from a spectrum synthesis analysis. The derived C-12/C-13 ratios varied between 6.3 and 10.6 in NGC 6791, in agreement with the final isotopic ratios from thermohaline-induced mixing models. The ratios derived here are combined with those obtained for more metal poor red-giants from the literature to examine the correlation between C-12/C-13, mass, metallicity, and evolutionary status.
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