Dissertations / Theses on the topic 'Pre-main-sequence'
To see the other types of publications on this topic, follow the link: Pre-main-sequence.
Create a spot-on reference in APA, MLA, Chicago, Harvard, and other styles
Consult the top 50 dissertations / theses for your research on the topic 'Pre-main-sequence.'
Next to every source in the list of references, there is an 'Add to bibliography' button. Press on it, and we will generate automatically the bibliographic reference to the chosen work in the citation style you need: APA, MLA, Harvard, Chicago, Vancouver, etc.
You can also download the full text of the academic publication as pdf and read online its abstract whenever available in the metadata.
Browse dissertations / theses on a wide variety of disciplines and organise your bibliography correctly.
1
Burhan, Mehmet. "Vibrational Stability Of Pre-main Sequence Stars." Master's thesis, METU, 2004. http://etd.lib.metu.edu.tr/upload/12604705/index.pdf.
Full textAbstract:
In this study, vibrational properties and stability of delta-Scuti
like pulsating pre-main sequence stars have been investigated.
Studies were held in the mass range 2-4 Mo
and limited to radial linear adiabatic pulsations. Numerical computations were performed by the
oscillation
program written by Kirbiyik &Al-Murad (1993). The models were selected to be at the latest phases of the pre-main sequence evolution where the luminosity starts to increase. We have limited our calculations upto the end of the radiative inner regions, since at the surface of the star, our adiabatic perturbation computation does not perfectly fit to the relatively thin non-adiabatic convective envelope of the star. The results of the stability analysis showed that the PMS models undergo an instability whose time period is a function of mass. Instability Strip of pulsating PMS stars was re-drawn with comparison to M. Marconi &
F.Palla (1998). The effect of gravitational contraction on stability was also investigated.
2
Hutchinson, Mark Gerard. "Observational studies of pre-main sequence stars." Thesis, Keele University, 1988. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.716860.
Full text
3
Giarrusso, Marina. "Lithium and age of pre-main sequence stars." Doctoral thesis, Università di Catania, 2016. http://hdl.handle.net/10761/3892.
Full textAbstract:
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.
4
Rolph, C. D. "Imaging polarimetry of pre- and post- main sequence objects." Thesis, Durham University, 1990. http://etheses.dur.ac.uk/6257/.
Full textAbstract:
In the first part of this thesis an automated polarimeter is described, and details are given of a dedicated CCD camera system based on a personal computer. The quality of the data produced by these instruments is demonstrated by the results presented in the succeeding chapters. Polarimetric observations of nebulae associated with two pre-MS objects, HH83/Rel7 and GL2591, and two post-MS objects, IRAS 07131-0147 and OH 231.8+4.2, are presented and discussed with reference to previous observations. In each case the location of the exciting source is determined and a simple model is described which explains the observed characteristics of the system. Both HH83/Rel7 and GL2591 are shown to be illuminated by nearby IRAS sources which have no optical counterparts. The nebula associated with HH83/Rel7 is caused by the reflection of radiation off the insides of the walls of a cavity excavated in the dark cloud by outflows from the IRS, and is crossed by a narrow unpolarised jet seen in emission-line radiation. The nebula associated with GL2591 is illuminated at optical wavelengths by both the IRS and a second, visible, source, and is composed of material ejected by the IRS in a discrete period of mass loss. IRAS 07131-0147 and OH 231.8+4.2 are shown to be stars which have evolved off the AGB and which will soon become the central stars of planetary nebulae. The protoplanetary nebulae which have formed as a result of the action of the fast stellar wind on the extended RGE around each star are bipolar and axially symmetric. The fast wind is shown to have ceased in the case of IRAS 07131-0147, but that related to OH 231.8+4.2 is still carrying material away from the star in a highly collimated fashion, producing narrow dusty filaments along the axes of the cavities. High levels of polarisation are measured in both nebulae, which indicates that the scattering particles are much smaller than those in the ISM.A brief comparison of the pre- and post-MS nebulae shows that the two phases of stellar evolution are linked by a number of observationally similar characteristics, and it is thought that similar processes may occur at opposite ends of the evolutionary track. Most notably, circumstellar discs appear to be common at various stages in the stellar life-cycle.
5
Padgett, Deborah Lynne Oke J. Beverley. "Photospheric abundance analysis of low mass pre-main sequence stars /." Diss., Pasadena, Calif. : California Institute of Technology, 1992. http://resolver.caltech.edu/CaltechETD:etd-09202008-104122.
Full text
6
Moe, Maxwell, and Kaitlin M. Kratter. "Dynamical Formation of Close Binaries during the Pre-main-sequence Phase." IOP PUBLISHING LTD, 2018. http://hdl.handle.net/10150/627100.
Full textAbstract:
Solar-type binaries with short orbital periods (P-close equivalent to 1-10. days; a less than or similar to 0.1. au) cannot form directly via fragmentation of molecular clouds or protostellar disks, yet their component masses are highly correlated, suggesting interaction during the pre-main-sequence (pre-MS) phase. Moreover, the close binary fraction of pre-MS stars is consistent with that of their MS counterparts in the field (F-close = 2.1%). Thus, we can infer that some migration mechanism operates during the early pre-MS phase (tau less than or similar to 5 Myr) that reshapes the primordial separation distribution. We test the feasibility of this hypothesis by carrying out a population synthesis calculation which accounts for two formation channels: Kozai-Lidov (KL) oscillations and dynamical instability in triple systems. Our models incorporate (1) more realistic initial conditions compared to previous studies, (2) octupole-level effects in the secular evolution, (3) tidal energy dissipation via weak-friction equilibrium tides at small eccentricities and via non-radial dynamical oscillations at large eccentricities, and (4) the larger tidal radius of a pre-MS primary. Given a 15% triple-star fraction, we simulate a close binary fraction from KL oscillations alone of F-close approximate to 0.4% after tau = 5. Myr, which increases to F-close 0.8% by tau = 5. Gyr. Dynamical ejections and disruptions of unstable coplanar triples in the disk produce solitary binaries with slightly longer periods P approximate to 10-100. days. The remaining approximate to 60% of close binaries with outer tertiaries, particularly those in compact coplanar configurations with log P-out (days) approximate to 2-5 (a(out) < 50 au), can be explained only with substantial extra energy dissipation due to interactions with primordial gas.
7
Johnstone, Colin Philip. "Magnetic fields and X-ray emission in pre-main sequence stars." Thesis, University of St Andrews, 2012. http://hdl.handle.net/10023/3596.
Full textAbstract:
In this thesis, I use numerical models of stellar coronae to investigate coronal magnetic fields, X-ray emission, and accretion geometries of classical T Tauri stars. This is based on recently published Zeeman-Doppler Imaging (ZDI) magnetograms. I also investigate the effects of time-variable eclipsing of stellar flares on their observed lightcurves. I investigate how our ability to model stellar magnetic processes is affected by missing magnetic flux in observed ZDI magnetograms and find that the loss of unresolved small-scale field regions has a significant effect on our ability to model magnetically confined X-ray emitting coronae. However, it has little effect on predicted large scale field structures. I survey the sample of classical T Tauri stars with existing ZDI magnetograms and find that the field complexity is correlated with field strength and stellar rotation rate, such that rapidly rotating stars have weak complex fields, and slowly rotating stars have strong simple fields. It is not clear whether this is a result of the finite resolution of the ZDI technique, magnetic star-disc interactions, or the evolution of pre-main sequence stars. Using observed X-ray emission measures and temperatures for each of these stars, I model the closed X-ray emitting coronae and find that they typically extend several stellar radii from the stellar surface. The coronal extent is primarily determined by the complexity of the magnetic field, with simple fields extending a large distance from the stellar surface, and more complex fields being truncated closer to the stellar surface. Using observed mass accretion rates, I predict circumstellar disc truncation radii for these stars and find that they are typically several stellar radii from the stellar surface, with the locations of accretion footpoints being a strong function of the field strengths and complexities. In several cases, the disc is truncated significantly outside the maximum radius at which the corona can extend. This result is significant as studies into magnetospheric accretion generally assume that the magnetic field has a closed geometry at the inner edge of the disc. The lightcurve of a typical stellar flare consists of a single impulsive rise phase followed by a slower exponential decay. However, a large number of the observed flares do not possess typical morphologies, and instead show multiple rise phases, or large dips in their lightcurves. Using the largest flares observed by the Chandra Orion Ultradeep Project, I show that these atypical lightcurves could have been caused by the time variable eclipsing of typical flares due to the rotation of the host stars. However, this interpretation is unable to account for the large number of atypical lightcurves in the COUP flare sample, and so other physical mechanisms must be involved. Significantly, I find that most flares that are eclipsed by their host stars still retain typical lightcurves showing no obvious signs of eclipsing.
8
Shen, Nancy. "Coronal properties of pre-main sequence stars in the Orion Trapezium Cluster." Thesis, Massachusetts Institute of Technology, 2012. http://hdl.handle.net/1721.1/78488.
Full textAbstract:
Thesis (S.B.)--Massachusetts Institute of Technology, Dept. of Physics, 2012.Cataloged from PDF version of thesis.
Includes bibliographical references (p. 47-48).
Most of the young stars in the Orion Trapezium Cluster are much more peculiar in X-rays than expected, considering their structure, environment, and age. Highly resolved X-ray spectra provide many more details in order to study these peculiarities, specifically with respect to abundances and accretion signatures. We analyzed the high resolution X-ray spectra of six young stars in Orion using data from the Chandra X-Ray Observatory. We fit plasma models to the spectra, calculated temperature-insensitive elemental abundances from individual spectral line fits, determined common elemental abundances to refit the data, and computed confidence maps for pairs of model parameters. These results may provide insights into the physical properties and processes that cause certain stars to exhibit high temperatures in X-rays. This may lead to a better understanding of stellar structure and evolution in young stars.
by Nancy Shen.
S.B.
9
Esau, Claire Frances. "Radiative-transfer modelling of the circumstellar environments of pre-main-sequence stars." Thesis, University of Exeter, 2015. http://hdl.handle.net/10871/22080.
Full textAbstract:
Circumstellar discs of pre-main sequence stars undergo different processes depending on the nature of the circumstellar environment, which is governed by stellar mass. I have performed numerical simulations of the circumstellar regions of classical T~Tauri stars (CTTs) and Herbig~Ae (HAe) stars using the radiative transfer code TORUS in order to test the paradigm of magnetospheric accretion in CTTs, and to ascertain the nature of the material in the inner regions of HAe discs. The process of magnetospheric accretion (MA) involves disc material attaching to stellar magnetic field lines and impacting the photosphere, producing accretion shocks. When the magnetic field is inclined to the star, disc warps form which periodically occult the photosphere. With specific reference to the CTTs AA Tau I perform three-dimensional MA models to study this variability. By comparing synthetic photometry with observational data I show that the geometry of the system can be constrained. I go on to study Balmer line profiles in the context of MA and disc wind outflows. I present three-dimensional models of a system comprising the star, magnetosphere, disc, and disc wind, producing synthetic line profiles and images. Using these profiles I perform time-series fitting to observational data and demonstrate that the mass accretion rate, mass loss rate, and magnetosphere temperature can be constrained. I show that there is a degeneracy between wind temperature and wind acceleration which require alternative methods to constrain further. While an outflow model alternative to a sole disc wind may produce better fits to observations, MA models reproduce various observational features well. Finally I test the hypothesis that refractory grains produce the innermost emission in HAe discs. Focussing on the HAe stars MWC 275 and AB Aur, I perform radiative equilibrium modelling to create synthetic images of these objects from which interferometric visibility profiles are produced. I show that the temperatures at which these refractory grains are required to survive are too high to be physically plausible. I also find that silicate dust is shielded when sufficiently high mass fractions of refractory grains are used, enabling the silicates to survive interior to the classical sublimation radius. While refractory dust may provide a significant contribution to the emission observed in these inner regions, this alone is not sufficient.
10
Jose, Jessy, Jinyoung S. Kim, Gregory J. Herczeg, Manash R. Samal, John H. Bieging, Michael R. Meyer, and William H. Sherry. "STAR FORMATION IN W3—AFGL 333: YOUNG STELLAR CONTENT, PROPERTIES, AND ROLES OF EXTERNAL FEEDBACK." IOP PUBLISHING LTD, 2016. http://hdl.handle.net/10150/621216.
Full textAbstract:
One of the key questions in the field of star formation is the role of stellar feedback on the subsequent star formation process. The W3 giant molecular cloud complex at the western border of the W4 super bubble is thought to be influenced by the massive stars in W4. This paper presents a study of the star formation activity within AFGL. 333, a similar to 104 M-circle dot cloud within W3, using deep JHK(s) photometry obtained from the NOAO Extremely Wide Field Infrared Imager combined with Spitzer IRAC and MIPS photometry. Based on the infrared excess, we identify 812 candidate young stellar objects (YSOs) in the complex, of which 99 are Class I and 713 are Class II sources. The stellar density analysis of YSOs reveals three major stellar aggregates within AFGL. 333, namely AFGL. 333 Main, AFGL. 333 NW1 and AFGL. 333 NW2. The disk fraction within AFGL. 333 is estimated to be similar to 50%-60%. We use the extinction map made from the H - K-s colors of the background stars and CO data to understand the cloud structure and to estimate the cloud mass. From the stellar and cloud mass associated with AFGL. 333, we infer that the region is currently forming stars with an efficiency of similar to 4.5% and at a rate of similar to 2-3M(circle dot) Myr(-1) pc(-2). In general, the star formation activity within AFGL. 333 is comparable to that of nearby low mass star-forming regions. We do not find any strong evidence to suggest that the stellar feedback from the massive stars of nearby W4 super bubble has affected the global star formation properties of the AFGL. 333 region.
11
Skelly, Mairead. "Doppler imaging, differential rotation and H[alpha] emission of pre-main sequence stars." Thesis, Imperial College London, 2009. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.501763.
Full text
12
Rees, Jon Morgan. "Long-lived discs in T associations : pre-main-sequence ages for low-mass stars." Thesis, University of Exeter, 2016. http://hdl.handle.net/10871/24434.
Full textAbstract:
In this thesis, ages have been derived for 4 young clusters by fitting the pre-main-sequence stars with semi-empirical models in colour-magnitude diagrams. Combining these ages with the (consistent) set presented in previous work, the first robust evidence of increased circumstellar disc lifetimes in low-mass, low-density regions is obtained. To obtain this result, the following steps were necessary: • Semi-empirical model isochrones have been constructed in a number of rizJHK photometric systems. These models overcome the issues typically seen in purely theoretical models in which the blue flux of low-mass stars is overestimated. These models are presented in a number of widely used filter sets for the first time, allowing for wider use with new clusters. Additionally the models constructed in previous filter sets have been refined using new observations. • To support the construction of these models, upper-main-sequence fitting is performed for 2 fiducial clusters, and it is demonstrated that the resulting age and distance measurements are consistent with other measures. • A new reduction process for data in the Blanco-DECam system is presented, and it is shown that the DECam photometric system is well characterised. • A photometric method for dereddening stars individually in regions of spatially variable extinction is presented, and applied to the young regions in this study. This method of photometric dereddening can be applied to large numbers of stars, greatly decreasing the time investment needed compared to spectroscopic methods. 2 The ages derived for the young clusters using the semi-empirical models are around a factor 2 older than typically assumed in the literature, which is in-line with that seen for the ages derived for other clusters using the same technique. By considering the disc fraction in these clusters as a function of age, it is shown that Taurus and Chamaeleon show a significant excess of discs compared to a set of massive, dense clusters of similar age. This is clear evidence that discs seem to survive longer in this low-mass, low-density region, giving crucial hints at different disc evolution in these regions. ρ-Oph is a low-mass region with a high stellar density, and so could be used to identify the dominant mechanism leading to these long-lived discs. However the presence of a similar disc excess in ρ-Oph is dependent on the assumed distance, which is currently poorly constrained, and so the dominant mechanism is still unclear.
13
Bell, Cameron Pearce MacDonald. "A critical assessment of ages derived using pre-main-sequence isochrones in colour-magnitude diagrams." Thesis, University of Exeter, 2012. http://hdl.handle.net/10036/4017.
Full textAbstract:
In this thesis a critical assessment of the ages derived using theoretical pre-main-sequence (pre-MS) stellar evolutionary models is presented by comparing the predictions to the low-mass pre-MS population of 14 young star-forming regions (SFRs) in colour-magnitude diagrams (CMDs). Deriving pre-MS ages requires precise distances and estimates of the reddening. Therefore, the main-sequence (MS) members of the SFRs have been used to derive a self-consistent set of statistically robust ages, distances and reddenings with associated uncertainties using a maximum-likelihood fitting statistic and MS evolutionary models. A photometric method (known as the Q-method) for de-reddening individual stars in regions where the extinction is spatially variable has been updated and is presented. The effects of both the model dependency and the SFR composition on these derived parameters are also discussed. The problem of calibrating photometric observations of red pre-MS stars is examined and it is shown that using observations of MS stars to transform the data into a standard photometric system can introduce significant errors in the position of the pre-MS locus in CMD space. Hence, it is crucial that precise photometric studies (especially of pre- MS objects) be carried out in the natural photometric system of the observations. This therefore requires a robust model of the system responses for the instrument used, and thus the calculated responses for the Wide-Field Camera on the Isaac Newton Telescope are presented. These system responses have been tested using standard star observations and have been shown to be a good representation of the photometric system. A benchmark test for the pre-MS evolutionary models is performed by comparing them to a set of well-calibrated CMDs of the Pleiades in the wavelength regime 0.4−2.5 μm. The masses predicted by these models are also tested against dynamical masses using a sample of MS binaries by calculating the system magnitude in a given photometric band- pass. This analysis shows that for Teff ≤ 4000 K the models systematically overestimate the flux by a factor of 2 at 0.5 μm, though this decreases with wavelength, becoming negligible at 2.2 μm. Thus before the pre-MS models are used to derive ages, a recalibration of the models is performed by incorporating an empirical colour-Teff relation and bolometric corrections based on the Ks-band luminosity of Pleiades members, with theoretical corrections for the dependence on the surface gravity (log g). The recalibrated pre-MS model isochrones are used to derive ages from the pre-MS populations of the SFRs. These ages are then compared with the MS derivations, thus providing a powerful diagnostic tool with which to discriminate between the different pre- MS age scales that arise from a much stronger model dependency in the pre-MS regime. The revised ages assigned to each of the 14 SFRs are up to a factor two older than previous derivations, a result with wide-ranging implications, including that circumstellar discs survive longer and that the average Class II lifetime is greater than currently believed.
14
Cox, Erin G., Robert J. Harris, Leslie W. Looney, Hsin-Fang Chiang, Claire Chandler, Kaitlin Kratter, Zhi-Yun Li, Laura Perez, and John J. Tobin. "Protoplanetary Disks in ρ Ophiuchus as Seen from ALMA." IOP PUBLISHING LTD, 2017. http://hdl.handle.net/10150/626413.
Full textAbstract:
We present a high angular resolution (similar to 0 ''.2), high-sensitivity (sigma similar to 0.2 mJy) survey of the 870 mu m continuum emission from the circumstellar material around 49 pre-main-sequence stars in the rho Ophiuchus molecular cloud. Because most millimeter instruments have resided in the northern hemisphere, this represents the largest high-resolution, millimeter-wave survey of the circumstellar disk content of this cloud. Our survey of 49 systems comprises 63 stars; we detect disks associated with 29 single sources, 11 binaries, 3 triple systems, and 4 transition disks. We present flux and radius distributions for these systems; in particular, this is the first presentation of a reasonably complete probability distribution of disk radii at millimeter wavelengths. We also compare the flux distribution of these protoplanetary disks with that of the disk population of the Taurus-Auriga molecular cloud. We find that disks in binaries are both significantly smaller and have much less flux than their counterparts around isolated stars. We compute truncation calculations on our binary sources and find that these disks are too small to have been affected by tidal truncation and posit some explanations for this. Lastly, our survey found three candidate gapped disks, one of which is a newly identified transition disk with no signature of a dip in infrared excess in extant observations.
15
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.
Full text
16
Fang, Min, Jinyoung Serena Kim, Ilaria Pascucci, Dániel Apai, and Carlo Felice Manara. "A CANDIDATE PLANETARY-MASS OBJECT WITH A PHOTOEVAPORATING DISK IN ORION." IOP PUBLISHING LTD, 2016. http://hdl.handle.net/10150/622685.
Full textAbstract:
In this work, we report the discovery of a candidate planetary-mass object with a photoevaporating protoplanetary disk, Proplyd. 133-353, which is near the massive star theta(1) Ori C at the center of the Orion Nebula Cluster (ONC). The object was known to have extended emission pointing away from theta(1) Ori. C, indicating ongoing external photoevaporation. Our near-infrared spectroscopic data and the location on the H-R diagram suggest that the central source of Proplyd. 133-353 is substellar (similar to M9.5) and has a mass probably less than 13 Jupiter mass and an age younger than 0.5 Myr. Proplyd. 133-353 shows a similar ratio of X-ray luminosity to stellar luminosity to other young stars in the ONC with a similar stellar luminosity and has a similar proper motion to the mean one of confirmed ONC members. We propose that Proplyd. 133-353 formed in a very low-mass dusty cloud or an evaporating gas globule near theta(1) Ori C as a second generation of star formation, which can explain both its young age and the presence of its disk.
17
Irwin, J. M. "Observational constraints on pre-main sequence stellar evolution from time-series analysis of open cluster stars." Thesis, University of Cambridge, 2007. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.604961.
Full textAbstract:
Observational constraints on evolutionary models of low-mass stars (≲ 1.0 M⊙) are presently extremely scarce on the pre-main sequence. Recent observational evidence has indicated substantial discrepancies between the predictions of these models and observations made in binary star systems, where the masses can be measured dynamically. It is clear that in order to resolve these issues, a larger sample of precise measurements will be needed to anchor the theory. This work pursues two avenues to do this, using time-series photometric measurements in young open clusters (ages 1 – 200 Myr) obtained as part of the Monitor project, a large-scale survey using 2 – 4 m class telescopes. Stellar rotation periods are readily measured using photometry alone, and yet probe directly a fundamental stellar property: the angular velocity. These measurements place direct constraints on models of rotational evolution, had hence on the stellar models themselves. By examining rotation periods in six of the Monitor open clusters, I show that simple rotational evolution models can partially describe the data, with rapid rotators better-described by a model assuming rotation as a solid body, and slower rotators by a model including differential rotation between the radiative core and convective envelope in stars with masses ≳ 0.4 M⊙, but that more theoretical work is clearly needed to resolve discrepancies between the models and the data. Eclipsing binary systems provide some of the most precise and accurate determination of stellar masses and radii available, from combined analysis of radial velocity and light curves. Searching for these systems is one of the primary science goals of Monitor. Dynamical solutions are presented for four of these systems, and two found to be on the pre-main sequence are compared to the predictions of the models, finding reasonable agreement in the mass-radius plane, but that there may be significant discrepancies in the effective temperatures, as found by several other authors.
18
Elliott, Paul Michael. "Is multiplicity universal? : a study of multiplicity in the young moving groups." Thesis, University of Exeter, 2016. http://hdl.handle.net/10871/21876.
Full textAbstract:
The young moving groups are collections of nearby (<200 pc), young (5-150 Myr) pre-main sequence stars; these stars offer us one of the best opportunities to characterise stellar multiplicity, sub-stellar phenomena, disc evolution and planet formation. Here we present results from a series of multiplicity studies aimed at producing comprehensive multiplicity statistics of the young moving groups. The aim was to compare the derived statistics of the young moving groups to other populations in order to investigate whether the abundance and properties of multiple systems are environment independent. We have combined high-resolution spectroscopy, AO-imaging and direct imaging to identify and characterise multiple systems across a huge range of orbital periods (1- 10e10 day). The observational techniques also allow us to constrain the abundance of multiple systems in these populations by calculating detection limits. We found many similarities (frequency of spectroscopic binaries; frequency, mass-ratio and physical separation of visual binaries) between the young moving groups and both younger and older regions, for multiple systems with physical separations smaller than 1000 au. We did, however, identify a significant number of new wide (>1000 au) companions. We reconciled the apparent excess of wide binary systems, when compared to the field population, by arguing that the wide systems are weakly bound and most likely decaying. By comparing the multiplicity statistics in one particular moving group we showed that the dynamical evolution of non-hierarchical protostars could lead to the population of wide binaries we can observe today. Our results indicate that the majority of low-mass stars form in small groups with 3 or 4 components that undergo significant dynamical evolution. The multiplicity properties of the young nearby moving groups are statistically similar to many other populations, supporting the environment-independent formation of multiple systems.
19
Simon, M. N., I. Pascucci, S. Edwards, W. Feng, U. Gorti, D. Hollenbach, E. Rigliaco, and J. T. Keane. "TRACING SLOW WINDS FROM T TAURI STARS VIA LOW-VELOCITY FORBIDDEN LINE EMISSION." IOP PUBLISHING LTD, 2016. http://hdl.handle.net/10150/622166.
Full textAbstract:
Using Keck/HIRES spectra (Delta v similar to 7 km s(-1)) we analyze forbidden lines of [O I] 6300 angstrom, [O I] 5577 angstrom. and [S II] 6731 angstrom. from 33 T Tauri stars covering a range of disk evolutionary stages. After removing a high-velocity component (HVC) associated with microjets, we study the properties of the low-velocity component (LVC). The LVC can be attributed to slow disk winds that could be magnetically (magnetohydrodynamic) or thermally (photoevaporative) driven. Both of these winds play an important role in the evolution and dispersal of protoplanetary material. LVC emission is seen in all 30 stars with detected [O. I] but only in two out of eight with detected [S. II], so our analysis is largely based on the properties of the [O. I] LVC. The LVC itself is resolved into broad (BC) and narrow (NC) kinematic components. Both components are found over a wide range of accretion rates and their luminosity is correlated with the accretion luminosity, but the NC is proportionately stronger than the BC in transition disks. The full width at half maximum of both the BC and NC correlates with disk inclination, consistent with Keplerian broadening from radii of 0.05 to 0.5 au and 0.5 to 5 au, respectively. The velocity centroids of the BC suggest formation in an MHD disk wind, with the largest blueshifts found in sources with closer to face-on orientations. The velocity centroids of the NC, however, show no dependence on disk inclination. The origin of this component is less clear and the evidence for photoevaporation is not conclusive.
20
Allen, Thomas S. "A Multi-wavelength survey of the Young Stellar Cluster Cep OB3b." University of Toledo / OhioLINK, 2014. http://rave.ohiolink.edu/etdc/view?acc_num=toledo1404697742.
Full text
21
Long, Feng, Gregory J. Herczeg, Ilaria Pascucci, Emily Drabek-Maunder, Subhanjoy Mohanty, Leonardo Testi, Daniel Apai, et al. "An ALMA Survey of CO Isotopologue Emission from Protoplanetary Disks in Chamaeleon I." IOP PUBLISHING LTD, 2017. http://hdl.handle.net/10150/625159.
Full textAbstract:
The mass of a protoplanetary disk limits the formation and future growth of any planet. Masses of protoplanetary disks are usually calculated from measurements of the dust continuum emission by assuming an interstellar gas-to-dust ratio. To investigate the utility of CO as an alternate probe of disk mass, we use ALMA to survey (CO)-C-13 and (CO)-O-18 J = 3-2 line emission from a sample of 93 protoplanetary disks around stars and brown dwarfs with masses from 0.03 to 2 M-circle dot in the nearby Chamaeleon I star-forming region. We detect (CO)-C-13 emission from 17 sources and (CO)-O-18 from only one source. Gas masses for disks are then estimated by comparing the CO line luminosities to results from published disk models that include CO freeze-out and isotope-selective photodissociation. Under the assumption of a typical interstellar medium CO-to-H-2 ratio of 10(-4), the resulting gas masses are implausibly low, with an average gas mass of similar to 0.05M(Jup) as inferred from the average flux of stacked (CO)-C-13 lines. The low gas masses and gas-to-dust ratios for Cha I disks are both consistent with similar results from disks in the Lupus star-forming region. The faint CO line emission may instead be explained if disks have much higher gas masses, but freeze-out of CO or complex C-bearing molecules is underestimated in disk models. The conversion of CO flux to CO gas mass also suffers from uncertainties in disk structures, which could affect gas temperatures. CO emission lines will only be a good tracer of the disk mass when models for C and CO depletion are confirmed to be accurate.
22
Sergison, Darryl James. "Untangling the signals : investigating accretion and photometric variability in young stars." Thesis, University of Exeter, 2015. http://hdl.handle.net/10871/18720.
Full textAbstract:
In this thesis, an assessment is made of the value of optical CMDs as a useful diagnostic of the accretion properties of young stars. An analysis has been made of the phenomena that we observe and their effect on the position of stars in the CMD. Limitations and potential biases have been identified and evaluated. Variability causes some luminosity spread at a given colour in optical CMDs. A detailed characterisation of variability has been performed which places strong constraints on the magnitudes and the timescales on which the variability is seen. On timescales 15 minutes, almost no variability is detected (at levels greater than ≈ 0.2%) in the i band for a sample of ≈ 700 disc-bearing young stellar objects (YSOs). This suggests that the variability predicted by some accretion shock models is either very weak or not present. On hours to days timescales the optical variability in most stars is well described by a simple power law. The amplitude of the variability, a ∝ f−k, where f is the frequency of the variability in days. Disc-bearing and discless YSOs exhibit median values of k of 0.85 ± 0.02 and 0.95 ± 0.03 respectively, the uncertainity being the error on the median. The power law is valid up to a certain timescale (tmax) at which point the variability amplitude does not increase any further. tmax is found to be 1.50 ± 0.07 days and 1.41 ± 0.10 days for disc-bearing and discless stars respectively. Disc-bearing stars show greater variability amplitudes than the discless stars. However, it is notable that the variability timescale and power spectrum exponent are remarkably similar. This implies that the amplitude of the variability is driven by the physics of the underlying process, but that the timescales are instead driven by geometric effects. For disc-bearing stars, the highest amplitude variables are the accreting stars, which often appear to vary in the CMD along lines that correspond to changes in accretion luminosity. Four disc-bearing stars (approximately 0.5% of the disc-bearing sample) in Cep OB3b show extreme variability on timescales of years. Three (possible EXor candidates), show long-timescale changes that have a dramatic effect on their CMD position. However their small numbers mean that the overall impact on the CMDs of young associations is small. Variability on timescales of the rotational period and shorter adds uncertainty to age estimates of individual stars that are calculated by comparison with PMS models. Having provided a detailed description of variability and its impact on the CMD, it is clear that there are further significant mechanisms that affect the positions of YSOs in the CMD. I show that the spread in luminosity seen in the Orion Nebula Cluster and NGC 2264 could not be explained by accretion at rates of M ̇ ≥ 5 × 10−4 M⊙ yr−1 occurring within the protostellar phase of YSO evolution. Thus it appears that CMDs are not a useful diagnostic for study of the accretion histories of YSOs. The wavelength dependence of the extinction by dust within the inner regions of YSO discs is shown to differ from that seen in the ISM. Typically the wavelength dependence of the extinction is given by RV ≈5-8, compared with the value of RV ≈3.1 typical of the ISM. The interpretation is that grain growth has occurred. The location of this material within the ‘snow line’ implies that grains have coalesced rather than simply gaining an ice mantle. This is evidence for the beginning of planet formation. The effect of the high value of RV on the CMD is to add additional uncertainty of 0.1 mag to photometric measurements that have been corrected for the effects of extinction. Accretion luminosity is shown to be the dominant signal in the luminosity spread seen in CMDs of young associations. Stars which exhibit excess flux in the U band or Hα are displaced in CMD space. The accretion vector is shown to be a significant blueward shift in colour accompa- nied by a modest brightening in the g, g − i CMD. Accretion results in a luminosity spread as stars are displaced blueward below the PMS locus. This effect is not seen in non-accreting disc-bearing stars. Examination of the underlying excess luminosity spectrum for 15 accreting stars shows that the colour of the emission excess is not consistent across the sample. Thus to quantify the effect of accretion luminosity on CMD positions for individual stars, moderate resolution spectra are required with a large range in wavelength. This accretion luminosity may systematically bias estimates of PMS ages. A simple mitigation is to exclude accreting stars from age analysis. U band and Hα flux excesses are shown to vary independently by ≈ 1 dex on timescales shorter than the rotation period of the star. The relation between U band flux excess and veiling at 7000Å also appears to be variable. This implies that single epoch measurements of these parameters will add an uncertainty of ≈ 1 dex on accretion rates derived from them. Accretion rates derived from either U or Hα excess should be calculated from a mean of several photometric measurements, separated by significant fractions of the rotation period of the star. In most stars, the veiling at 7000Å is shown not to be a good measure for the calculation of the accretion rate. Despite providing a detailed characterisation of phenomena that influence the positions of YSOs in the CMD, there exists some residual luminosity spread at a given Teff that cannot be explained by variability on any timescale, extinction uncertainties or accretion luminosity. This residual spread should provide an opportunity to study an as-yet uncharacterised aspect of young stars.
23
Min, M., J. Bouwman, C. Dominik, L. B. F. M. Waters, K. M. Pontoppidan, S. Hony, G. D. Mulders, et al. "The abundance and thermal history of water ice in the disk surrounding HD 142527 from the DIGIT Herschel Key Program." EDP SCIENCES S A, 2016. http://hdl.handle.net/10150/622159.
Full textAbstract:
Context. The presence or absence of ice in protoplanetary disks is of great importance to the formation of planets. By enhancing solid surface density and increasing sticking efficiency, ice catalyzes the rapid formation of planetesimals and decreases the timescale of giant planet core accretion. Aims. In this paper, we analyze the composition of the outer disk around the Herbig star HD 142527. We focus on the composition of water ice, but also analyze the abundances of previously proposed minerals. Methods. We present new Herschel far-infrared spectra and a re-reduction of archival data from the Infrared Space Observatory (ISO). We modeled the disk using full 3D radiative transfer to obtain the disk structure. Also, we used an optically thin analysis of the outer disk spectrum to obtain firm constraints on the composition of the dust component. Results. The water ice in the disk around HD 142527 contains a large reservoir of crystalline water ice. We determine the local abundance of water ice in the outer disk (i.e., beyond 130AU). The re-reduced ISO spectrum differs significantly from that previously published, but matches the new Herschel spectrum at their common wavelength range. In particular, we do not detect any significant contribution from carbonates or hydrous silicates, in contrast to earlier claims. Conclusions. The amount of water ice detected in the outer disk requires similar to 80% of oxygen atoms. This is comparable to the water ice abundance in the outer solar system, comets, and dense interstellar clouds. The water ice is highly crystalline while the temperatures where we detect it are too low to crystallize the water on relevant timescales. We discuss the implications of this finding.
24
Pascucci, I., L. Testi, G. J. Herczeg, F. Long, C. F. Manara, N. Hendler, G. D. Mulders, et al. "A STEEPER THAN LINEAR DISK MASS–STELLAR MASS SCALING RELATION." IOP PUBLISHING LTD, 2016. http://hdl.handle.net/10150/622163.
Full textAbstract:
The disk mass is among the most important input parameter for every planet formation model to determine the number and masses of the planets that can form. We present an ALMA 887 mu m survey of the disk population around objects from similar to 2 to 0.03 M-circle dot in the nearby similar to 2 Myr old Chamaeleon I star-forming region. We detect thermal dust emission from 66 out of 93 disks, spatially resolve 34 of them, and identify two disks with large dust cavities of about 45 au in radius. Assuming isothermal and optically thin emission, we convert the 887 mu m flux densities into dust disk masses, hereafter M-dust. We find that the M-dust-M* relation is steeper than linear and of the form M-dust proportional to (M*)(1.3-1.9), where the range in the power-law index reflects two extremes of the possible relation between the average dust temperature and stellar luminosity. By reanalyzing all millimeter data available for nearby regions in a self-consistent way, we show that the 1-3 Myr old regions of Taurus, Lupus, and Chamaeleon. I share the same M-dust-M* relation, while the 10 Myr old Upper. Sco association has a steeper relation. Theoretical models of grain growth, drift, and fragmentation reproduce this trend and suggest that disks are in the fragmentation-limited regime. In this regime millimeter grains will be located closer in around lower-mass stars, a prediction that can be tested with deeper and higher spatial resolution ALMA observations.
25
Bayo, Amelia, Viki Joergens, Yao Liu, Robert Brauer, Johan Olofsson, Javier Arancibia, Paola Pinilla, et al. "First Millimeter Detection of the Disk around a Young, Isolated, Planetary-mass Object." IOP PUBLISHING LTD, 2017. http://hdl.handle.net/10150/624481.
Full textAbstract:
OTS44 is one of only four free-floating planets known to have a disk. We have previously shown that it is the coolest and least massive known free-floating planet (similar to 12 M-Jup) with a substantial disk that is actively accreting. We have obtained Band 6 (233 GHz) ALMA continuum data of this very young disk-bearing object. The data show a clear unresolved detection of the source. We obtained disk-mass estimates via empirical correlations derived for young, higher-mass, central (substellar) objects. The range of values obtained are between 0.07 and 0.63 M-circle plus (dust masses). We compare the properties of this unique disk with those recently reported around higher-mass (brown dwarfs) young objects in order to infer constraints on its mechanism of formation. While extreme assumptions on dust temperature yield disk-mass values that could slightly diverge from the general trends found for more massive brown dwarfs, a range of sensible values provide disk masses compatible with a unique scaling relation between M-dust and M* through the substellar domain down to planetary masses.
26
Lim, Beomdu, Hwankyung Sung, Jinyoung S. Kim, Michael S. Bessell, Narae Hwang, and Byeong-Gon Park. "A CONSTRAINT ON THE FORMATION TIMESCALE OF THE YOUNG OPEN CLUSTER NGC 2264: LITHIUM ABUNDANCE OF PRE-MAIN SEQUENCE STARS." IOP PUBLISHING LTD, 2016. http://hdl.handle.net/10150/622162.
Full textAbstract:
The timescale of cluster formation is an essential parameter in order to understand the formation process of star clusters. Pre-main sequence (PMS) stars in nearby young open clusters reveal a large spread in brightness. If the spread were considered to be a result of a real spread in age, the corresponding cluster formation timescale would be about 5-20 Myr. Hence it could be interpreted that star formation in an open cluster is prolonged for up to a few tens of Myr. However, difficulties in reddening correction, observational errors, and systematic uncertainties introduced by imperfect evolutionary models for PMS stars can result in an artificial age spread. Alternatively, we can utilize Li abundance as a relative age indicator of PMS star to determine the cluster formation timescale. The optical spectra of 134 PMS stars in NGC 2264 have been obtained with MMT/Hectochelle. The equivalent widths have been measured for 86 PMS stars with a detectable Li line (3500 < T-eff [K] <= 6500). Li abundance under the condition of local thermodynamic equilibrium (LTE) was derived using the conventional curve of growth method. After correction for non-LTE effects, we find that the initial Li abundance of NGC 2264 is A(Li)= 3.2 +/- 0.2. From the distribution of the Li abundances, the underlying age spread of the visible PMS stars is estimated to be about 3-4 Myr and this, together with the presence of embedded populations in NGC 2264, suggests that the cluster formed on a timescale shorter than 5 Myr.
27
Kastner, Joel H., David A. Principe, Kristina Punzi, Beate Stelzer, Uma Gorti, Ilaria Pascucci, and Costanza Argiroffi. "M STARS IN THE TW HYA ASSOCIATION: STELLAR X-RAYS AND DISK DISSIPATION." IOP PUBLISHING LTD, 2016. http://hdl.handle.net/10150/621232.
Full textAbstract:
To investigate the potential connection between the intense X-ray emission from young low-mass stars and the lifetimes of their circumstellar planet-forming disks, we have compiled the X-ray luminosities (L-X) of M stars in the similar to 8 Myr old TW Hya Association (TWA) for which X-ray data are presently available. Our investigation includes analysis of archival Chandra data for the TWA binary systems TWA 8, 9, and 13. Although our study suffers from poor statistics for stars later than M3, we find a trend of decreasing L-X/L-bol with decreasing T-eff for TWA M stars, wherein the earliest-type (M0-M2) stars cluster near log(L-X/L-bol) approximate to -3.0 and then log(L-X/L-bol) decreases, and its distribution broadens, for types M4 and later. The fraction of TWA stars that display evidence for residual primordial disk material also sharply increases in this same (mid-M) spectral type regime. This apparent anticorrelation between the relative X-ray luminosities of low-mass TWA stars and the longevities of their circumstellar disks suggests that primordial disks orbiting early-type M stars in the TWA have dispersed rapidly as a consequence of their persistent large X-ray fluxes. Conversely, the disks orbiting the very lowest-mass pre-MS stars and pre-MS brown dwarfs in the Association may have survived because their X-ray luminosities and, hence, disk photoevaporation rates are very low to begin with, and then further decline relatively early in their pre-MS evolution.
28
Eisner, J. A., J. M. Bally, A. Ginsburg, and P. D. Sheehan. "PROTOPLANETARY DISKS IN THE ORION OMC1 REGION IMAGED WITH ALMA." IOP PUBLISHING LTD, 2016. http://hdl.handle.net/10150/621374.
Full textAbstract:
We present ALMA observations of the Orion Nebula that cover the OMC1 outflow region. Our focus in this paper is on compact emission from protoplanetary disks. We mosaicked a field containing similar to 600 near-IR-identified young stars, around which we can search for sub-millimeter emission tracing dusty disks. Approximately 100 sources are known proplyds identified with the Hubble Space Telescope. We detect continuum emission at 1 mm wavelengths toward similar to 20% of the proplyd sample, and similar to 8% of the larger sample of near-IR objects. The noise in our maps allows 4 sigma detection of objects brighter than similar to 1.5 mJy, corresponding to protoplanetary disk masses larger than 1.5 M-J (using standard assumptions about dust opacities and gas-to-dust ratios). None of these disks are detected in contemporaneous CO(2-1) or (CO)-O-18(2-1) observations, suggesting that the gas-to-dust ratios may be substantially smaller than the canonical value of 100. Furthermore, since dust grains may already be sequestered in large bodies in Orion Nebula cluster (ONC) disks, the inferred masses of disk solids may be underestimated. Our results suggest that the distribution of disk masses in this region is compatible with the detection rate of massive planets around M dwarfs, which are the dominant stellar constituent in the ONC.
29
Vieira, Rodrigo Georgetti. "Envoltórios circunstelares de estrelas jovens de massa intermediária." Universidade de São Paulo, 2012. http://www.teses.usp.br/teses/disponiveis/14/14131/tde-08112012-160406/.
Full textAbstract:
As estrelas Herbig Ae/Be (HAeBe) representam os objetos de massa intermediária (2-10 Msol) na pré-sequência principal. Algumas de suas propriedades físicas são pouco compreendidas até o momento. Somente o estudo conjunto das informações fornecidas em diversos comprimentos de onda pode revelar as características do material circunstelar destes objetos. O objetivo deste trabalho de doutorado é analisar sob vários aspectos a estrutura, a composição e a evolução destes ambientes circunstelares. Para realização deste estudo, adotamos a amostra de candidatas a estrelas HAeBe detectadas pelo Pico dos Dias Survey (Vieira et al. 2003). Evitamos as possíveis contaminações desta amostra por estrelas em estágios mais avançados utilizando diagramas de cores, estimativas de extinção e características espectrais. A química da poeira circunstelar foi analisada a partir das propriedades dos espectros ISO disponíveis para nossos objetos. O perfil espectral do silicato em torno de 10 microns revelou características evolutivas do material circunstelar. O status evolutivo dos objetos mais embebidos foi determinado por meio de estimativas da massa de seus envoltórios circunstelares. Este estudo indicou que a maior parte desta sub-amostra se encontra no estágio intermediário entre a Classe 0 (Menv>>M*) e a Classe I (Menv
30
Ksoll, Victor Francisco [Verfasser], and Ralf [Akademischer Betreuer] Klessen. "Characterising Pre-Main-Sequence Stars in the Large Magellanic Cloud with Machine and Deep Learning Techniques / Victor Francisco Ksoll ; Betreuer: Ralf Klessen." Heidelberg : Universitätsbibliothek Heidelberg, 2021. http://d-nb.info/1237270855/34.
Full text
31
Banzatti, A., K. M. Pontoppidan, C. Salyk, G. J. Herczeg, Dishoeck E. F. van, and G. A. Blake. "THE DEPLETION OF WATER DURING DISPERSAL OF PLANET-FORMING DISK REGIONS." IOP PUBLISHING LTD, 2017. http://hdl.handle.net/10150/623096.
Full textAbstract:
We present a new velocity-resolved survey of 2.9 mu m spectra of hot H2O and OH gas emission from protoplanetary disks, obtained with the Cryogenic Infrared Echelle Spectrometer at the VLT (R similar to 96,000). With the addition of archival Spitzer-IRS spectra, this is the most comprehensive spectral data set of water vapor emission from disks ever assembled. We provide line fluxes at 2.9-33 mu m that probe from the dust sublimation radius at similar to 0.05 au out to the region of the water snow line. With a combined data set for 55 disks, we find a new correlation between H2O line fluxes and the radius of CO gas emission, as measured in velocity-resolved 4.7 mu m spectra (R-co), which probes molecular gaps in inner disks. We find that H2O emission disappears from 2.9 mu m (hotter water) to 33 mu m (colder water) as R-co increases and expands out to the snow line radius. These results suggest that the infrared water spectrum is a tracer of inside-out water depletion within the snow line. It also helps clarify an unsolved discrepancy between water observations and models by finding that disks around stars of M-star > 1.5M(circle dot) generally have inner gaps with depleted molecular gas content. We measure radial trends in H2O, OH, and CO line fluxes that can be used as benchmarks for models to study the chemical composition and evolution of planet-forming disk regions at 0.05-20 au. We propose that JWST spectroscopy of molecular-gas may be used as a probe of inner disk gas depletion, complementary to the larger gaps and holes detected by direct imaging and by ALMA.
32
Fang, Min, Jinyoung Serena Kim, Ilaria Pascucci, Dániel Apai, Lan Zhang, Aurora Sicilia-Aguilar, Miguel Alonso-Martínez, Carlos Eiroa, and Hongchi Wang. "NGC 1980 Is Not a Foreground Population of Orion: Spectroscopic Survey of Young Stars with Low Extinction in Orion A." IOP PUBLISHING LTD, 2017. http://hdl.handle.net/10150/623807.
Full textAbstract:
We perform a spectroscopic survey of the foreground population in Orion. A with MMT/Hectospec. We use these data, along with archival spectroscopic data and photometric data, to derive spectral types, extinction values, and masses for 691 stars. Using the Spitzer Space Telescope data, we characterize the disk properties of these sources. We identify 37 new transition disk (TD) objects, 1 globally depleted disk candidate, and 7 probable young debris disks. We discover an object with a mass of. less than 0.018-0.030 M-circle dot, which harbors a flaring disk. Using the Ha emission line, we characterize the accretion activity of the sources with disks, and confirm that the. fraction of accreting TDs is lower than that of optically thick disks (46% +/- 7% versus 73% +/- 9%, respectively). Using kinematic data from the Sloan Digital Sky Survey and APOGEE INfrared Spectroscopy of the Young Nebulous Clusters program (IN-SYNC), we confirm that the foreground population shows similar kinematics to their local molecular clouds and other young stars in the same regions. Using the isochronal ages, we find that the foreground population has a median age of. around 1-2 Myr, which is similar to that of other young stars in Orion. A. Therefore, our results argue against the presence of a large and old foreground cluster in front of Orion. A.
33
Shan, Yutong, Jennifer C. Yee, Brendan P. Bowler, Lucas A. Cieza, Benjamin T. Montet, Héctor Cánovas, Michael C. Liu, et al. "The Multiplicity of M Dwarfs in Young Moving Groups." IOP PUBLISHING LTD, 2017. http://hdl.handle.net/10150/627110.
Full textAbstract:
We image 104 newly identified low-mass (mostly M-dwarf) pre-main sequence (PMS) members of nearby young moving groups (YMGs) with Magellan Adaptive Optics (MagAO) and identify 27 stellar binaries with instantaneous projected separation as small as 40 mas. Fifteen were previously unknown. The total number of multiple systems in this sample including spectroscopic and visual binaries from the literature is 36, giving a raw stellar multiplicity rate of at least 35(-4)(+5)% for this population. In the separation range of roughly 1-300 au in which infrared AO imaging is most sensitive, the raw multiplicity rate is at least 24(-4)(+5)% for binaries resolved by the MagAO infrared camera (Clio). The M-star subsample of 87 stars yields a raw multiplicity of at least 30(-4)(+5)% over all separations, 21(-4)(+5)% for secondary companions resolved by Clio from 1 to 300 au (23(-4)(+5)% for all known binaries in this separation range). A combined analysis with binaries discovered by the Search for Associations Containing Young stars shows that stellar multiplicity fraction as a function of mass over the range of 0.2 to 1.2M(circle dot) appears to be linearly flat, in contrast to the field, where multiplicity increases with mass. After bias corrections are applied, the multiplicity of low-mass YMG members (0.2-0.6M(circle dot)) is in excess of the field. The overall multiplicity fraction is also consistent with being constant in age and across YMGs, which suggests that multiplicity rates for this mass range are largely set by 10 Myr without appreciable evolution thereafter.
34
Canty, James Ignatius. "Investigating the properties of brown dwarfs using intermediate-resolution spectroscopy." Thesis, University of Hertfordshire, 2015. http://hdl.handle.net/2299/15194.
Full textAbstract:
This thesis is an investigation into some properties of brown dwarfs using medium-resolution spectroscopy. In the first part of the thesis, I address the issue of parameter degeneracy in brown dwarfs. In the course of my analysis, I derive a gravity-sensitive spectral index which can be used, statistically at least, to differentiate populations of young objects from field dwarfs. The index is also capable of finding the difference between a population of ~1 Myr objects and a population of ~10 Myr objects and may be used to separate low-mass members from foreground and background objects in young clusters and associations. The second part of my thesis is an investigation into the major opacity sources in the atmospheres of late T dwarfs. I look particularly at CH4 and NH3 absorption features in the near-infrared spectra of these objects. In my analysis, I identify new absorption features produced by these molecules. I also correct features which had previously been wrongly identified. This has been made possible by the use of high quality data, together with a new CH4 synthetic line list, which is more complete at these temperatures than any previously available list.
35
McCarthy, Kyle. "Characterizing the Nearest Young Moving Groups." UKnowledge, 2015. http://uknowledge.uky.edu/physastron_etds/30.
Full textAbstract:
Moving groups are associations of stars which originated from the same star forming region. These groups are typically young (< 200 Myr) since they have not dissipated into the galactic field population. Over the last 15 years, roughly 10 such moving groups have been found with distances < 150 pc (7 with distances < 100 pc), each with a unique velocity and position.
This work first investigates the likelihood to resolve star from two moving groups (AB Doradus and Beta Pictoris) using high spacial resolution optical interferrometry and found 5 AB Doradus stars and 1 Beta Pictoris star with declinations > -30 could be spacially resolved.
To more deeply characterize individual groups, we used the 2.7m telescope at the McDonald Observatory to observe 10 proposed AB Doradus stars and 5 proposed Octans-Near stars (3 probable members, 2 possible) with high resolution (R ~60,000) optical spectroscopy. Each group is characterized in three ways: (1) Chemical analysis to determine the homogeneity among members, (2) Kinematic traceback to determine the origin, and (3) Isochrone fitting to determine the age. We find the 8 stars in our AB Doradus sample are chemically homogeneous with [M/H] = -0.03 ± 0.06 dex, traceback to an age of 125 Myr, and the stars in this mass range are on the main sequence. The two deviants are a metal rich, potentially younger member and a metal poor, young star likely not associated with AB Doradus.
In our Octans-Near sample, we find the 3 probable members have [M/H] = -0.06 ± 0.11, the stars do not trace back to a common origin, and the probable members are on the main sequence. In addition to these tests, we found that the probable members are slightly more lithium depleted than the Pleiades, implying an age between 125 and 200 Myr.
Finally, we investigate systematic trends in fundamental stellar parameters from the use of different techniques. Preliminary results find differences in temperatures between interferrometric and spectroscopic techniques to be a function of temperature with a interferrometric temperatures being cooler by an average of 36 ± 115 K. We also calculated the chemical abundances as a function of condensation temperature for our moving group sample and predict 2 stars in AB Doradus could represent the initial star forming environment and discuss the implications for planet hosting stars in nearby moving groups.
This updated characterization technique allows for a deeper understanding of the moving group environment. As future, high precision instruments emerge in astronomy (Jame Webb Space Telescope, GAIA, 30m class telescopes), moving groups are ideal targets since these associations will help us understand star forming regions, stellar evolution at young ages, constrain stellar evolutionary models, and identify planetary formation and evolution mechanisms.
36
Lomax, Jamie R., John P. Wisniewski, Carol A. Grady, Michael W. McElwain, Jun Hashimoto, Tomoyuki Kudo, Nobuhiko Kusakabe, et al. "CONSTRAINING THE MOVEMENT OF THE SPIRAL FEATURES AND THE LOCATIONS OF PLANETARY BODIES WITHIN THE AB AUR SYSTEM." IOP PUBLISHING LTD, 2016. http://hdl.handle.net/10150/622048.
Full textAbstract:
We present a new analysis of multi-epoch, H-band, scattered light images of the AB Aur system. We use a Monte Carlo radiative transfer code to simultaneously model the system's spectral energy distribution (SED) and H-band polarized intensity (PI) imagery. We find that a disk-dominated model, as opposed to one that is envelope-dominated, can plausibly reproduce AB Aur's SED and near-IR imagery. This is consistent with previous modeling attempts presented in the literature and supports the idea that at least a subset of AB Aur's spirals originate within the disk. In light of this, we also analyzed the movement of spiral structures in multi-epoch H-band total light and PI imagery of the disk. We detect no significant rotation or change in spatial location of the spiral structures in these data, which span a 5.8-year baseline. If such structures are caused by disk-planet interactions, the lack of observed rotation constrains the location of the orbit of planetary perturbers to be >47 au.
37
Watson, Dan M., Nuria P. Calvet, William J. Fischer, W. J. Forrest, P. Manoj, S. Thomas Megeath, Gary J. Melnick, et al. "EVOLUTION OF MASS OUTFLOW IN PROTOSTARS." IOP PUBLISHING LTD, 2016. http://hdl.handle.net/10150/622058.
Full textAbstract:
We have surveyed 84 Class 0, Class I, and flat-spectrum protostars in mid-infrared [Si II], [Fe II], and [S I] line emission, and 11 of these in far-infrared [O I] emission. We use the results to derive their mass. outflow rates, (M) over dot(w). Thereby we observe a strong correlation of (M) over dot(w) with bolometric luminosity, and with the inferred mass accretion rates of the central objects, (M) over dot(a), which continues through the Class 0 range the trend observed in Class II young stellar objects. Along this trend from large to small mass. flow rates, the different classes of young stellar objects lie in the sequence Class 0-Class I/flat-spectrum-Class II, indicating that the trend is an evolutionary sequence in which (M) over dot(a) and (M) over dot(w) decrease together with increasing age, while maintaining rough proportionality. The survey results include two that. are key tests of magnetocentrifugal outflow-acceleration mechanisms: the distribution of the outflow/accretion branching ratio b = (M) over dot(w)/(M) over dot(a), and limits on the distribution of outflow speeds. Neither rules out any of the three leading outflow-acceleration, angular-momentum-ejection mechanisms, but they provide some evidence that disk winds and accretion-powered stellar winds (APSWs) operate in many protostars. An upper edge observed in the branching-ratio distribution is consistent with the upper bound of b = 0.6 found in models of APSWs, and a large fraction (31%) of the sample have a. branching ratio sufficiently small that only disk winds, launched on scales as large as several au, have been demonstrated to account for them.
38
Herczeg(沈雷歌), Gregory J., Subo Dong, Benjamin J. Shappee, 平) Ping Chen(陈, Lynne A. Hillenbrand, Jessy Jose, Christopher S. Kochanek, et al. "THE ERUPTION OF THE CANDIDATE YOUNG STAR ASASSN-15QI." IOP PUBLISHING LTD, 2016. http://hdl.handle.net/10150/622161.
Full textAbstract:
Outbursts on young stars are usually interpreted as accretion bursts caused by instabilities in the disk or the star-disk connection. However, some protostellar outbursts may not fit into this framework. In this paper, we analyze optical and near-infrared spectra and photometry to characterize the 2015 outburst of the probable young star ASASSN-15qi. The similar to 3.5mag brightening in the V band was sudden, with an unresolved rise time of less than one day. The outburst decayed exponentially by 1mag for 6. days and then gradually back to the pre-outburst level after 200 days. The outburst is dominated by emission from similar to 10,000K gas. An explosive release of energy accelerated matter from the star in all directions, seen in a spectacular cool, spherical wind with a maximum velocity of 1000 km s(-1). The wind and hot gas both disappeared as the outburst faded and the source returned to its quiescent F-star spectrum. Nebulosity near the star brightened with a delay of 10-20 days. Fluorescent excitation of H-2 is detected in emission from vibrational levels as high as v = 11, also with a possible time delay in flux increase. The mid-infrared spectral energy distribution does not indicate the presence of warm dust emission, though the optical photospheric absorption and CO overtone emission could be related to a gaseous disk. Archival photometry reveals a prior outburst in 1976. Although we speculate about possible causes for this outburst, none of the explanations are compelling.
39
Fedele, Davide. "Structure and Evolution of Protoplanetary Disks." Doctoral thesis, Università degli studi di Padova, 2008. http://hdl.handle.net/11577/3427197.
Full textAbstract:
The scope of this thesis is to investigate the structure and evolution of protoplanetary disk. High resolution observational techniques such as high resolution optical/infrared spectroscopy and infrared interferometry are well suited for this purpose. High resolution spectroscopy allow to resolve the velocity profile of disk emission lines and determine some important parameters such as the disk geometry and the physical conditions of the line emitting region. Infrared interferometry allows to spatially resolve and constraint the disk geometry within the planet forming region.
The work presented here aims at contributing to the comprehension of the disk structure and evolution at three different evolutionary stages: 1) the early phase when the system is still (partially) embedded in a remnant of the molecular cloud; 2) the so-called Class II phase (from the classification of Lada 1987). At this stage gas and dust evolve rapidly leading to drastic changes of the disk structure; 3) the transition phase from Class II to Class III when gas and dust are dissipated leaving, eventually, a planetary system.
During the early phases of disk evolution the star-disk- envelope system experience powerful instability which are related to rapid enhancement of the mass accretion rate on a timescale of few months. These events are recognizable as so-called FU Orionis outbursts, in which the optical brightness of the system can increase by 4 or more magnitudes. The mass accretion rate increases from 10-7-- 10-8 Myr to 10-3 --10-4 Myr. Statistical studies suggest that young low-mass stars experience several FU Orionis outburst.
In late 2003, the young star V1647 Orionis in the L1630 Ori cloud within the Orion B molecular cloud went into outburst. The outburst shares some properties of the FU Orionis outburst. Following spectro-photometric observations confirmed the nature of the outburst as a disk-instability event. We also find, for the first time, probe of a direct link between an accretion event and the ejection of an Herbig-Haro object (HH).
During the Class II phase dust coagulation and grain growth occur. This is the first step of planet formation. We applied high resolution optical spectroscopy and infrared interferometry to direct compare gas and dust emission from the disk surface of three protoplanetary disks. This study gives some insight on the relative distribution of gas and dust in disk and on the temporal evolution of the two components. A physical decoupling of gas and dust may occur leading to changes in the relative structure of the two (different scale height) and to rapid settling of dust on the disk midplane. This may increase the dust-to-gas mass ratio in the disk interior and, according to recent simulation, may trigger the formation of planetesimals via gravitational instability.
The transition phase from a Class II to a Class III system is characterized by various processes which dissipate the disk material. In particular, viscous accretion and photo-evaporation are very efficient in removing disk material and planet formation is likely in competition with disk dispersion. For this reason, a fundamental quantity is the mass accretion timescale, i.e. the time at which the disk accretion phase ceases. In turn, the time at which the disk accretion phase ceases is a strong constraint on the gas dissipation timescale, relevant for the formation of giant planets. We have observed a number of young stellar clusters of different age aimed at tracing the
evolution viscous accretion with time. The preliminary results show that the accretion seems to cease at similar age of the dust dissipation, i.e. within 5 -- 10 Myr.
40
Czanik, Robert Johann. "An optical study of the high mass star forming region RCW 34 / Robert Johann Czanik." Thesis, North-West University, 2013. http://hdl.handle.net/10394/9102.
Full textAbstract:
This study consisted of an optical photometric and spectroscopic analysis on a 7′ 7′ field around
the Southern high mass star forming region RCW 34. A previous study on RCW 34 in the NIR discov-
ered many deeply embedded young stellar objects which were suspected to be T Tauri stars and which
justified further investigation. The data used in this study consisted of three sets, the first two are
photometric and spectroscopic data sets which were obtained during the first two weeks of February
2002. A third data set of spectroscopic observations was obtained by the author during the second week
of 2011 of selected candidates using results from the NIR study and from the photometric data sets.
All of the spectroscopy was conducted with the long slit spectrograph on the 1.9-m telescope and the
photometry with DANDICAM on the 1.0-m telescope at the South African Astronomical Observatory
(SAAO) in Sutherland. Objectives accomplished in the course of this study were to understand, ob-
tain, reduce and interpret photometric and long slit spectroscopic CCD images. From the photometric
results 57 stars showed excess blue emission on a colour-colour diagram which could be generated by
circumstellar matter. The spectroscopic study showed 5 stars that showed H emission and 2 with
strong Li absorption lines which confirm the suspicions of the NIR study about T Tauri stars in the
region. All of the stars from the spectroscopic study in 2011 were identified as low-mass K or M type
stars. Using colour-magnitude diagrams it was possible to see that the majority of the stars in the
cluster are low-mass pre-main sequence stars. The stars matching between the optical and NIR filters
were plotted on NIR colour-colour diagrams showing that the 5 stars that had H emission lines also
had NIR colours characteristic to T Tauri stars. Out of the 5 stars that showed H emission, 2 were
found to be classical T Tauris and three were found to be weak line T Tauris.Thesis (MSc (Space Physics))--North-West University, Potchefstroom Campus, 2013
41
Da, Rio Nicola, Jonathan C. Tan, Kevin R. Covey, Michiel Cottaar, Jonathan B. Foster, Nicholas C. Cullen, John Tobin, et al. "IN-SYNC. V. Stellar Kinematics and Dynamics in the Orion A Molecular Cloud." IOP PUBLISHING LTD, 2017. http://hdl.handle.net/10150/625776.
Full textAbstract:
The kinematics and dynamics of young stellar populations enable us to test theories of star formation. With this aim, we continue our analysis of the SDSS-III/APOGEE IN-SYNC survey, a high-resolution near-infrared spectroscopic survey of young clusters. We focus on the Orion A star-forming region, for which IN-SYNC obtained spectra of similar to 2700 stars. In Paper IV we used these data to study the young stellar population. Here we study the kinematic properties through radial velocities (vr). The young stellar population remains kinematically associated with the molecular gas, following a similar to 10 km s(-1) gradient along the filament. However, near the center of the region, the vr distribution is slightly blueshifted and asymmetric; we suggest that this population, which is older, is slightly in the foreground. We find evidence for kinematic subclustering, detecting statistically significant groupings of colocated stars with coherent motions. These are mostly in the lower-density regions of the cloud, while the ONC radial velocities are smoothly distributed, consistent with it being an older, more dynamically evolved cluster. The velocity dispersion sigma(v) varies along the filament. The ONC appears virialized, or just slightly supervirial, consistent with an old dynamical age. Here there is also some evidence for ongoing expansion, from a v(r)-extinction correlation. In the southern filament, sigma(v) is similar to 2-3 times larger than virial in the L1641N region, where we infer a superposition along the line of sight of stellar subpopulations, detached from the gas. In contrast, sv decreases toward L1641S, where the population is again in agreement with a virial state.
42
Thalmann, C., M. Janson, A. Garufi, A. Boccaletti, S. P. Quanz, E. Sissa, R. Gratton, et al. "RESOLVING THE PLANET-HOSTING INNER REGIONS OF THE LkCa 15 DISK." IOP PUBLISHING LTD, 2016. http://hdl.handle.net/10150/621504.
Full textAbstract:
LkCa 15 hosts a pre-transitional disk as well as at least one accreting protoplanet orbiting in its gap. Previous disk observations have focused mainly on the outer disk, which is cleared inward of similar to 50 au. The planet candidates, on the other hand, reside at orbital radii around 15 au, where disk observations have been unreliable until recently. Here, we present new J-band imaging polarimetry of LkCa 15 with SPHERE IRDIS, yielding the most accurate and detailed scattered-light images of the disk to date down to the planet-hosting inner regions. We find what appear to be persistent asymmetric structures in the scattering material at the location of the planet candidates, which could be responsible at least for parts of the signals measured with sparse-aperture masking. These images further allow us to trace the gap edge in scattered light at all position angles and search the inner and outer disks for morphological substructure. The outer disk appears smooth with slight azimuthal variations in polarized surface brightness, which may be due to shadowing from the inner disk or a two-peaked polarized phase function. We find that the near-side gap edge revealed by polarimetry matches the sharp crescent seen in previous ADI imaging very well. Finally, the ratio of polarized disk to stellar flux is more than six times larger in the J-band than in the RI bands.
43
Sicilia-Aguilar, A., A. Oprandi, D. Froebrich, M. Fang, J. L. Prieto, K. Stanek, A. Scholz, et al. "The 2014–2017 outburst of the young star ASASSN-13db." EDP SCIENCES S A, 2017. http://hdl.handle.net/10150/626256.
Full textAbstract:
Context. Accretion outbursts are key elements in star formation. ASASSN-13db is a M5-type star with a protoplanetary disk, the lowest-mass star known to experience accretion outbursts. Since its discovery in 2013, it has experienced two outbursts, the second of which started in November 2014 and lasted until February 2017. Aims. We explore the photometric and spectroscopic behavior of ASASSN-13db during the 2014-2017 outburst. Methods. We use high- and low-resolution spectroscopy and time-resolved photometry from the ASAS-SN survey, the LCOGT and the Beacon Observatory to study the light curve of ASASSN-13db and the dynamical and physical properties of the accretion flow. Results. The 2014-2017 outburst lasted for nearly 800 days. A 4.15 d period in the light curve likely corresponds to rotational modulation of a star with hot spot(s). The spectra show multiple emission lines with variable inverse P-Cygni profiles and a highly variable blue-shifted absorption below the continuum. Line ratios from metallic emission lines (Fe I/Fe II, Ti I/Ti II) suggest temperatures of similar to 5800-6000 K in the accretion flow. Conclusions. Photometrically and spectroscopically, the 2014-2017 event displays an intermediate behavior between EXors and FUors. The accretion rate (<(M)over dot> = 1-3 x 10(-7) M-circle dot/yr), about two orders of magnitude higher than the accretion rate in quiescence, is not significantly different from the accretion rate observed in 2013. The absorption features in the spectra suggest that the system is viewed at a high angle and drives a powerful, non-axisymmetric wind, maybe related to magnetic reconnection. The properties of ASASSN-13db suggest that temperatures lower than those for solar-type stars are needed for modeling accretion in very-low-mass systems. Finally, the rotational modulation during the outburst reveals that accretion-related structures settle after the beginning of the outburst and can be relatively stable and long-lived. Our work also demonstrates the power of time-resolved photometry and spectroscopy to explore the properties of variable and outbursting stars.
44
Dong, Ruobing, and Jeffrey Fung. "What is the Mass of a Gap-opening Planet?" IOP PUBLISHING LTD, 2017. http://hdl.handle.net/10150/624383.
Full textAbstract:
High-contrast imaging instruments such as GPI and SPHERE are discovering gap structures in protoplanetary disks at an ever faster pace. Some of these gaps may be opened by planets forming in the disks. In order to constrain planet formation models using disk observations, it is crucial to find a robust way to quantitatively back out the properties of the gap-opening planets, in particular their masses, from the observed gap properties, such as their depths and widths. Combining 2D and 3D hydrodynamics simulations with 3D radiative transfer simulations, we investigate the morphology of planet-opened gaps in near-infrared scattered-light images. Quantitatively, we obtain correlations that directly link intrinsic gap depths and widths in the gas surface density to observed depths and widths in images of disks at modest inclinations under finite angular resolution. Subsequently, the properties of the surface density gaps enable us to derive the disk scale height at the location of the gap h, and to constrain the quantity M-p(2)/alpha, where Mp is the mass of the gap-opening planet and a characterizes the viscosity in the gap. As examples, we examine the gaps recently imaged by VLT/SPHERE, Gemini/GPI, and Subaru/HiCIAO in HD 97048, TW Hya, HD 169142, LkCa. 15, and RX J1615.3-3255. Scale heights of the disks and possible masses of the gap-opening planets are derived assuming each gap is opened by a single planet. Assuming a = 10(-3), the derived planet masses in all cases are roughly between 0.1 and 1M(J).
45
Mawet, Dimitri, Élodie Choquet, Olivier Absil, Elsa Huby, Michael Bottom, Eugene Serabyn, Bruno Femenia, et al. "CHARACTERIZATION OF THE INNER DISK AROUND HD 141569 A FROM KECK/NIRC2 L-BAND VORTEX CORONAGRAPHY." IOP PUBLISHING LTD, 2017. http://hdl.handle.net/10150/625206.
Full textAbstract:
HD 141569 A is a pre-main sequence B9.5 Ve star surrounded by a prominent and complex circumstellar disk, likely still in a transition stage from protoplanetary to debris disk phase. Here, we present a new image of the third inner disk component of HD 141569 A made in the L' band (3.8 mu m) during the commissioning of the vector vortex coronagraph that has recently been installed in the near-infrared imager and spectrograph NIRC2 behind the W.M. Keck Observatory Keck II adaptive optics system. We used reference point-spread function subtraction, which reveals the innermost disk component from the inner working distance of similar or equal to 23 au and up to similar or equal to 70 au. The spatial scale of our detection roughly corresponds to the optical and near-infrared scattered light, thermal Q, N, and 8.6 mu m PAH emission reported earlier. We also see an outward progression in dust location from the L' band to the H band (Very Large Telescope/SPHERE image) to the visible (Hubble Space Telescope (HST)/STIS image), which is likely indicative of dust blowout. The warm disk component is nested deep inside the two outer belts imaged by HST-NICMOS in 1999 (at 406 and 245 au, respectively). We fit our new L'-band image and spectral energy distribution of HD 141569 A with the radiative transfer code MCFOST. Our best-fit models favor pure olivine grains and are consistent with the composition of the outer belts. While our image shows a putative very faint point-like clump or source embedded in the inner disk, we did not detect any true companion within the gap between the inner disk and the first outer ring, at a sensitivity of a few Jupiter masses.
46
Evanko, Liberty Rae. "Development of an H alpha index for the detection of PMS candidates in young open clusters /." Diss., CLICK HERE for online access, 2007. http://contentdm.lib.byu.edu/ETD/image/etd1715.pdf.
Full text
47
Manara, C. F., L. Testi, G. J. Herczeg, I. Pascucci, J. M. Alcalá, A. Natta, S. Antoniucci, et al. "X-shooter study of accretion in Chamaeleon I." EDP SCIENCES S A, 2017. http://hdl.handle.net/10150/625828.
Full textAbstract:
The dependence of the mass accretion rate on the stellar properties is a key constraint for star formation and disk evolution studies. Here we present a study of a sample of stars in the Chamaeleon I star-forming region carried out using spectra taken with the ESO VLT/X-shooter spectrograph. The sample is nearly complete down to stellar masses (M-star) similar to 0.1 M-circle dot for the young stars still harboring a disk in this region. We derive the stellar and accretion parameters using a self-consistent method to fit the broadband flux-calibrated medium resolution spectrum. The correlation between accretion luminosity to stellar luminosity, and of mass accretion rate to stellar mass in the logarithmic plane yields slopes of 1.9 +/- 0.1 and 2.3 +/- 0.3, respectively. These slopes and the accretion rates are consistent with previous results in various star-forming regions and with different theoretical frameworks. However, we find that a broken power-law fit, with a steeper slope for stellar luminosity lower than similar to 0.45 L-circle dot and for stellar masses lower than similar to 0.3 M-circle dot is slightly preferred according to different statistical tests, but the single power-law model is not excluded. The steeper relation for lower mass stars can be interpreted as a faster evolution in the past for accretion in disks around these objects, or as different accretion regimes in different stellar mass ranges. Finally, we find two regions on the mass accretion versus stellar mass plane that are empty of objects: one region at high mass accretion rates and low stellar masses, which is related to the steeper dependence of the two parameters we derived. The second region is located just above the observational limits imposed by chromospheric emission, at M-star similar to 0.3-0.4 M-circle dot. These are typical masses where photoevaporation is known to be effective. The mass accretion rates of this region are similar to 10(-10) M-circle dot/yr, which is compatible with the value expected for photoevaporation to rapidly dissipate the inner disk.
48
Galli, Phillip Andreas Brenner. "Determinação de distâncias cinemáticas de estrelas pré-sequência principal em regiões de formação estelar." Universidade de São Paulo, 2012. http://www.teses.usp.br/teses/disponiveis/14/14131/tde-01042013-194055/.
Full textAbstract:
Este trabalho tem como objetivo principal a determinação da distância de estrelas pré-sequência principal em regiões de formação estelar próximas. A determinação precisa da distância individual das estrelas é necessária para obter os principais parâmetros físicos de cada estrela e para investigar a estrutura da Galáxia. Em particular, investigamos as regiões de formação estelar de Lupus e Ophiuchus que contém uma das associações mais ricas em estrelas T Tauri. A grande maioria das estrelas pré-sequência principal nessas regiões não foi observada pelo satélite Hipparcos devido à sua magnitude e também não têm paralaxe trigonométrica medida a partir do solo devido à distância em que se encontram. O procedimento aqui empregado para a obter a distância individual das estrelas baseia-se na estratégia de ponto de convergência e utiliza dados de movimento próprio e velocidade radial. Desenvolvemos uma nova versão do método de ponto de convergência que permite simultaneamente determinar a posição do ponto de convergência e selecionar os membros de um moving group. Partindo dos dados de movimento próprio e o novo método aqui desenvolvido investigamos as propriedades cinemáticas e realizamos uma análise de pertinência das estrelas em cada região estudada o que nos permitiu identificar um moving group com 114 estrelas em Lupus e 55 estrelas em Ophiuchus. Calculamos a distância para cada membro do grupo usando velocidades radiais publicadas, que foram complementadas com novas observações, e a velocidade espacial do moving grup para as estrelas com velocidade radial não conhecida. Calculamos as paralaxes com precisão de 1-2~mas o que implica em um erro relativo médio de 25% nas distâncias obtidas. Finalmente, investigamos as propriedades dos diversos subgrupos e a estrutura tridimensional dos complexos de nuvens em Lupus e Ophiuchus, concluindo que existem efeitos de profundidade importantes. Utilizamos os novos resultados de distância para obter os parâmetros físicos (luminosidade, massa e idade) das estrelas e o diagrama-HR de cada região de formação estelar considerada, confirmando a distribuição de idade diferente das duas subclasses de estrelas T Tauri. Os resultados aqui obtidos representam um primeiro passo no sentido de melhor entender a estrutura das regiões de formação estelar e os estágios iniciais da formação de estrelas e planetas.The main objective of this work is to determine the distance of pre-main sequence stars in nearby star-forming regions. A precise determination of the distance to individual stars is required to accurately determine the main physical parameters of each star and the structure of the Galaxy. Here we investigate the Lupus and Ophiuchus star-forming regions that contain one of the richest associations of T Tauri stars. Most pre-main sequence stars in these regions were neither observed by the Hipparcos satellite due to their magnitude nor have any trigonometric parallax measured from the ground due to their distance. The procedure that we use here to derive the distance to individual stars is based on the convergent point strategy and makes full use of proper motion and radial velocity data. We developed a new version of the convergent point search method that simultaneously determines the convergent point position and selects the most likely members of a moving group. Based on proper motion data and our new method we investigate the kinematic properties and perform a membership analysis of the stars in each star-forming region considered that allows us to identify a moving group with 114 stars in Lupus and 55 stars in Ophiuchus. We calculate the distance of each group member using published radial velocities, which we supplemented with new measurements, and the spatial velocity of the moving group for the remaining stars with unknown radial velocity. We derived parallaxes with accuracies of 1-2 mas yielding the average relative error of 25% on the distances. Finally, we investigate the properties of the various subgroups and the three dimensional structure of the Lupus and Ophiuchus cloud complex and conclude that significant depth effects exist. We use the new distances to refine the physical parameters (luminosity, mass and age) of stars and the HR-diagram for each star-forming region considered confirming the different age distribution of the two T Tauri subclasses. These results represent a first step towards better understanding the structure of star-forming regions and the early stages of star and planet formation.
49
Berger, Jean-Philippe. "Interférométrie et formation stellaire : Perspectives pour une instrumentation en optique intégrée." Université Joseph Fourier (Grenoble), 1998. http://www.theses.fr/1998GRE10213.
Full textAbstract:
Je présente les premières observations infrarouges des étoile FU Ori et AB Aur obtenues avec les interféromètres longue base IOTA(Umass, SAO) et PTI (JPL/NASA). Ces deux étoiles sont des prototypes des étoiles pré-séquence principale de faible masse (FU Ori) et de masse intermédiaire (AB Aur). Les deux sources sont résolues avec une résolution de l'ordre de l'unité astronomique. Divers scénarios sont envisagés. Les observations sont tout à fait compatibles avec la présence d'un disque d'accrétion autour de FU Ori. Afin d'améliorer les performances scientifiques et de diminuer la complexité des instruments interférométriques, je propose d'utiliser l'optique intégrée planaire pour la fonction de recombinaison. Je présente les résultats des caractérisations d'un recombinateur réalisé dans une technologie d'échanges d'ions sur substrat de verre. Cette "puce optique" comporte deux entrées et trois sorties : une voie interférométrique et deux voies de calibration photométrique. Elle est optimisée pour la bande H et assure le maintien de la polarisation. Les caractérisations montrent des pertes photoniques réduites, un contraste laser de ~ 90% et un contraste en bande large maximum de ~ 30%. Les origines de cette chute de contraste sont identifiées et des méthodes de caractérisation plus poussées sont proposées. Plusieurs perspectives de recombinaison multi-télescopes se dégagent à la suite de ce travail. Enfin, à la lumière de ces résultats, je présente le concept d'un instrument recombinateur portable baptisé IONIC, qui pourrait être le précurseur de l'instrumentation de deuxième génération des interféromètres monomodes au sol et dans l'espace. Ses principaux atouts sont la précision de mesures sur les visibilités, la compacité et la stabilité.
50
Lyo, A.-Ran Physical Environmental & Mathematical Sciences Australian Defence Force Academy UNSW. "The nearby young [special character] Chamaeleontis cluster as a laboratory for star formation and evolution." Awarded by:University of New South Wales - Australian Defence Force Academy. School of Physical, Environmental and Mathematical Sciences, 2004. http://handle.unsw.edu.au/1959.4/38707.
Full textAbstract:
[Special characters cannot be displayed. Please see the pdf version of the Abstract for an accurate reproduction.] We studied the circumstellar discs, the initial mass function (IMF), mass distribution, binarity and the fundamental properties of the [special character] 9 Myr-old pre-main sequence (PMS) [special character] Chamaeleontis cluster. Using JHKL colour-colour and colour-excess diagrams, we found the circumstellar disc fraction to be [special character] 0.60 among the late-type members. Four stars with [special character] (K - L) > 0.4 were identified as experiencing ongoing accretion which was later confirmed by high-resolution spectroscopic study. Quantitative analysis of the H[special character] profiles found accretion in these four stars at rates comparable to that of two members of the similarly-aged TW Hydrae Association (TWA); rates 1 - 3 orders of magnitude lower than in younger classical T Tauri stars. Together these results suggest that, while the mass accretion rate decreases with age, PMS stars can retain their inner discs for [special character] 10 Myr. An optical photometric survey spanning 1.3 ?? 1.3 pc added two low-mass stars to the cluster inventory. Together with other recent surveys the population is likely to be significantly complete for primaries with masses M > 0.15M[special character]. The cluster now consists of 18 primaries and 9 confirmed and candidate secondaries, with [special character] 2-4 times higher multiplicity than seen in field dwarfs. The cluster IMF is consistent with that of rich young clusters and field stars. By extending the IMF to lower masses, we predict 20-29 low-mass stars and brown dwarfs may remain undiscovered. From study of the cluster???s spatial and mass distribution, we find the [special character] Cha cluster has significant mass segregation, with > 50 per cent of the stellar mass residing within the central 0.17 pc. Lastly we classified members of the cluster with low-resolution spectra, providing information about the fundamental properties of the PMS stars by comparison to standard dwarfs. Broadband VRI colours and pseudocontinuum indices derived for the cluster stars are indistinguishable from dwarfs at visual and red wavelengths. This suggests the temperature sequence for the PMS [special character] Cha cluster is similar to that of the dwarf sequence. Narrow-band spectral indices for the [special character] Cha cluster possibly indicate higher metallicity and strongly indicate lower surface gravity than the dwarf indices.