Dissertations / Theses on the topic 'Stars of low metallicity'
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Masters, Craig Eugene Ashman Keith M. "The formation of low metallicity globular clusters." Diss., UMK access, 2007.
Abstract:
Thesis (Ph. D.)--Dept. of Physics and Dept. of Mathematics. University of Missouri--Kansas City, 2007."A dissertation in physics and mathematics." Advisor: Keith M. Ashman. Typescript. Vita. Title from "catalog record" of the print edition Description based on contents viewed Nov. 13, 2007. Includes bibliographical references (leaves 67-70). Online version of the print edition.
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Szécsi, Dorottya [Verfasser]. "The evolution of low-metallicity massive stars / Dorottya Szécsi." Bonn : Universitäts- und Landesbibliothek Bonn, 2016. http://d-nb.info/1113688262/34.
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Fukushima, Hajime. "Radiative feedback from massive stars in low-metallicity environments." Kyoto University, 2019. http://hdl.handle.net/2433/242603.
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Krawchuk, Curtis A. P. "The determination of metallicity and temperature of low-mass stars using broad-band photometry." Thesis, National Library of Canada = Bibliothèque nationale du Canada, 1997. http://www.collectionscanada.ca/obj/s4/f2/dsk2/tape15/PQDD_0035/MQ27361.pdf.
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Lindgren, Sara. "Metallicity determination of M dwarfs." Doctoral thesis, Uppsala universitet, Observationell astrofysik, 2017. http://urn.kb.se/resolve?urn=urn:nbn:se:uu:diva-332102.
Abstract:
M dwarfs constitute around 70% of all stars in the local Galaxy. Their multitude together with their long main-sequence lifetimes make them important for studies of global properties of the Galaxy such as the initial mass function or the structure and kinematics of stellar populations. In addition, the exoplanet community is showing an increasing interest for those small, cold stars. However, very few M dwarfs are well characterized, and in the case of exoplanetary systems the stellar parameters have a direct influence on the derived planet properties. Stellar parameters of M dwarfs are difficult to determine because of their low surface temperatures that result in an optical spectrum dominated by molecular lines. Most previous works have therefore relied on empirical calibrations. High-resolution spectrographs operating in the infrared, a wavelength region less affected by molecular lines, have recently opened up a new window for the investigation of M dwarfs. In the two first papers of this thesis we have shown that we can determine the metallicity, and in some cases the effective temperature, using synthetic spectral fitting with improved accuracy. This method is time consuming and therefore not practical or even feasible for studies of large samples of M dwarfs. When comparing our results from the high-resolution studies with available photometric calibrations we find systematic differences. In the third paper we therefore used our sample to determine a new photometric metallicity calibration. Compared to previous calibrations our new photometric calibration shows improved statistical characteristics, and our calibration gives similar results as spectroscopic calibrations. In a comparison with theoretical calculations we find a good agreement of the shapes and slopes of iso-metallicity lines with our empirical relation. Applying the photometric calibration to a sample of M dwarfs with confirmed exoplanets we find a possible giant planet-metallicity correlation for M dwarfs.
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Verdugo, Salgado Celia Anahi. "Star formation in low gas density and low metallicity environments." Observatoire de Paris, 2015. https://hal.science/tel-02095302.
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Dans les galaxies proches, la relation empirique de Kennicutt-Schmidt (KS) a été établie entre la densité surfacique de formation d’étoiles et la densité surfacique de gaz. Elle est forte et presque linéaire quand on considère le gaz moléculaire (H2) mais devient moins valable lorsque l’hydrogène n’est qu’atomique (HI). Ces régions de densité de gaz faible sont d’une importance cruciale dans le domaine de la formation d’étoiles, car elles possèdent les mêmes conditions que dans les galaxies de l’univers jeune, de faible métallicité. Cette thèse compile les données observationnelles obtenues au télescope de 30 m. De l’IRAM sur deux types de régions distinctes : les galaxies à disque ayant une émission ultra-violet étendue (XUV), et le milieu interstellaire déplacé dans le milieu intra-amas (ICM) chaud sous l’effet du balayage de la pression dynamique dans l’amas de la Vierge. Pour déterminer la présence des molécules H2 et afin d’étudier la relation KS dans ces régions, des observations de CO ont été faites dans les parties externes des galaxies XUV, et nous avons à la fois des détections et des limites supérieures. Les relations KS ont montré une loi de puissance discontinue pour des densités de gaz faibles, en dessous du seuil de la transition de phase HI-H2. Dans l’amas de la Vierge, des observations similaires de Co ont été faites le long des bras marée qui relient NGC4388 et M86, où aucune présence de H2 n’est attendue. Nous avons effectué des détections, montrant une efficacité de formation d’étoiles très faible, et montrant de nouveau une discontinuité de la relation KS pour des densités de gaz faibles, prouvant que les processus bien connus de consommation du gaz à forte densité ne peuvent être généralisés aux densités plus faibles. D’autre part, nos résultats montrent que le gaz H2 peut survivre un temps long que prévu dans l’ICM hostileIn nearby galaxies, an empirical relation has been established between star formation and gas surface densities, the Kennicutt-Schmidt (KS). The relation is nearly linear when molecular gas (H2) is considered, while is less tight with atomic hydrogen (HI). These low gas density regions are of a key importance in the field of star formation, since the are also low metallicity environments, resembling the conditions of a younger universe. This thesis summarizes the observational work done with the IRAM 30MT telescope in two kinds of such regions : disk galaxies with extended ultra-violet emission (XUV), and the interstellar medium going into the hot intra-cluster medium (ICM) under ram-pressure stripping in the Virgo cluster. The galex telescope has unveiled in far ultra-violet (FUV) star formation in the outer parts of some disk galaxies that was not traced by HA. To determine the presence of H2 and analyse the K-S relation in these regions, CO observations where done in the outskirts of several XUV disk galaxies, finding both detections and upper limits. These K-S relations showed a broken power law at low gas densities, below the HI-H2 threshold. In the Virgo cluster, similar CO observations were done along the HI tidal arm connecting NGC4388 and M86, where no H2 is expected. Two detections were found, showing very low star formation efficiencies (depleting less than 0,1 % of the gas reservoir per 10 [exposant] 8 yr), and showing again a disconuity of the K-S relation at low gas densities, probing that the process of a gas consumption into stars well known at high densities cannot be extrapolated to lower densities, and that H2 can survive a certain time in the hostile ICM
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Rajpurohit, Arvind Singh. "Low-mass stars as tracers of the milky way populations : investigating the effects of metallicity in cool atmosphere." Thesis, Besançon, 2013. http://www.theses.fr/2013BESA2023/document.
Abstract:
Les étoiles de petite masse (naines M) forment la composante stellaire dominante de notre galaxie et contribuent à la majeure partie de la matière baryonique dans la galaxie. Notre compréhension de la galaxie repose donc sur la connaissance de cette composante peu lumineuse. Par ailleurs, un nombre grandissant d’exoplanètes sont découverte autour de naines M, y compris des super-terres. L’étude des naines M a ainsi des implications importantes sur la physique stellaire, permettant de comprendre les processus en jeu dans ces atmosphères froides. Ces astres restent cependant peu connus du fait de leur faible luminosité intrinsèque. La description de ces étoiles nécessite une validation empirique fort, notamment en ce qui concerne les effets de l’abondance chimique sur la physique des atmosphères froides.Le but de cette thèse est de déterminer les paramètres fondamentaux des naines M et de tester les modèles d’atmosphères d’étoiles froides.Dans la première partie nous comparons des spectres calibrés dans le visible et l’infrarouge des composantes de système triple LHS 1070 avec des spectres synthétiques. Cette étude permet d’améliorer notre compréhension de la formation de la poussière dans les atmosphères très froides. Ce travail est étendu à un échantillon de naines M et permet de déterminer l’échelle de température des naines M. La seconde partie présente l’analyse des spectres à haute résolution de 21 naines M de faible métallicité (sous naines). Nous analysons en détail les signature moléculaires et atomiques dans le spectre. La comparaison avec des spectres synthétiques permet de mesurer les paramètres stellaires des étoiles et de déterminer avec précision leur métallicité. Ce travail doit initier des recherches futures. En particulier nous voulons étendre cette étude dans le domaine proche-infrarouge pour s’assurer que les résultats obtenus sont cohérents du domaine optique jusqu’à l’infrarouge procheVery Low-Mass stars (M dwarfs) are an important source of information for probing the lowmass end of the main sequence, down to the hydrogen burning limit. They are the dominantstellar component of the Galaxy and make up the majority of baryonic matter in the Galaxy.Moreover, an increasing number of M dwarfs are now known to host exoplanets, includingsuper-Earth exoplanets. The determination of accurate fundamental parameters for M dwarfshas therefore relevant implications for both stellar and Galactic astronomy as well as planetology.Despite their large number in the Galaxy, M dwarfs remain elusive objects and themodelling of their photosphere has long remained a challenge (molecular opacities, dust cloudformation). The description of these stars therefore need a strong empirical basis, or validation.In particular, the effect of metallicity on the physics of cool atmospheres are still poorly known,even for early-type M-dwarfs.[...]
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Ramachandran, Varsha [Verfasser], and Wolf-Rainer [Akademischer Betreuer] Hamann. "Massive star evolution, star formation, and feedback at low metallicity : quantitative spectroscopy of OB stars in the Magellanic Clouds / Varsha Ramachandran ; Betreuer: Wolf-Rainer Hamann." Potsdam : Universität Potsdam, 2019. http://d-nb.info/1218405058/34.
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Kunawicz, Nadya. "Any old iron? : astrochemical modelling of star-forming regions at low metallicity." Thesis, University of Manchester, 2010. https://www.research.manchester.ac.uk/portal/en/theses/any-old-iron--astrochemical-modelling-of-starforming-regions-at-low-metallicity(2a356b1a-429c-40b5-b9c0-2b4cdab12ea8).html.
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Two pseudo-time-dependent chemical models have been utilised - one of a dark cloud,and one of a hot core - in order to model these clouds in low metallicity environments, such as other galaxies. The dark cloud model uses gas-phase chemistry, whereas the hot core model includes both gas-phase and surface chemistry. The simulations have been calculated with varying initial elemental abundances of C, O, N, S and the heavy metals Fe, Mg and Na (henceforth, M). These initial abundances are taken from observations of HII regions in the Galaxy, the Large Magellanic Cloud and the Small Magellanic Cloud. The results have been used to identify species which potentially trace the underlying metallicity in dark clouds and hot cores. In the dark cloud models, the most useful tracers are ratios of two species, notably CO/OH and HCO+/CO, which trace the underlying C and M abundances respectively. In the hot core models, the most useful metallicity tracer species are HNC and NH3. The HNC abundance traces an underlying change in metallicity, independent of any changes to the dust/gas ratio. The NH3 abundance traces the underlying N abundance. The hot core model output abundances were used with RATRAN, a non-LTE radiative transfer code, to predict the integrated intensity as a function of hot core radius for various species. The RATRAN results are more directly comparable with observations than the results from the chemical models. Less common isotopes have been used to limit the optical depth of the species modelled. The results show that the extent of the emission may not reflect the size of the hot core. HN13C and NH3 are confirmed as the most useful metallicity tracer species in hot cores.
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Lombardo, Linda. "Explorer l'histoire de la Galaxie grâce à la spectroscopie stellaire." Electronic Thesis or Diss., Université Paris sciences et lettres, 2022. http://www.theses.fr/2022UPSLO011.
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Ce travail de thèse présente plusieurs études qui utilisent la spectroscopie à haute résolution pour déterminer les propriétés chimiques des différentes populations stellaires de la Voie Lactée. Le document est structuré comme suit : Le premier chapitre de la thèse, divisée en 3 sections, est une introduction générale à la structure de la Voie lactée et à ses populations stellaires, suivie d'une partie décrivant les différentes méthodes utilisées pour mesurer les abondances chimiques des étoiles. La première section décrit les différents scénarios concernant la structure et la formation de la Voie Lactée, en présentant en particulier les découvertes les plus récentes. La deuxième section introduit les concepts physiques de base nécessaires et les objectifs des études présentes dans ce travail de thèse. La troisième section décrit les méthodes utilisées dans l'analyse des données spectroscopiques.Le deuxième chapitre présente les travaux effectués dans la cadre du projet MINCE. La première étude concerne l'analyse de la composition chimique d'un échantillon d'étoiles géantes jeunes qui ont été découvertes par hasard au cours des premières missions d'observations du projet MINCE. J'ai déterminé les paramètres stellaires, analysé les spectres, mesuré les vitesses de rotation de ces étoiles et comparé les résultats aux modèles théoriques, en reportant tous ces résultats dans un article. La deuxième étude présente les résultats obtenus par l'analyse du premier échantillon d'étoiles propres au projet MINCE. J'ai contribué à l'analyse d'une partie des spectres stellaires de ce tout premier jeu de spectres MINCE.Le troisième chapitre porte sur les résultats d'une analyse faite dans le contexte du projet CERES. La première partie de ce travail présente une détermination détaillée de la composition chimique de l'étoile RAVE J110842.1-715300, dont le but est de savoir si cette étoile provient de l'amas globulaire Omega Centauri. Ma contribution porte sur la détermination des paramètres stellaires de cette étoile. La deuxième étude menée dans le contexte de ce projet CERES est constituée de l'analyse d'un échantillon d'étoiles. J'ai déterminé les paramètres stellaires, calculé les modèles d'atmosphère et les abondances chimiques, et écrit l'article. Le quatrième chapitre porte sur un travail fait dans le contexte du projet "High-speed stars" s'intéressant aux étoiles à grande vitesse transversales héliocentriques (>= 500 km/s). La première étude porte sur le suivi spectroscopique à haute résolution de deux étoiles jeunes et pauvres en métaux de l'échantillon de Caffau et al. (2020), afin de déterminer si ces étoiles sont des "blue stragglers". J'ai obtenu et analysé les spectres UVES de ces deux étoiles. Les résultats ne sont pas encore publiés. La deuxième étude concerne une analyse détaillée de deux étoiles à grande vitesse observées avec le spectrographe HDS au télescope Subaru. Pour cette étude, j'ai déterminé l'abondance du Carbone.Le cinquième chapitre présente les résultats obtenus à partir de l'analyse d'un échantillon d'étoiles sélectionnées au moyen de la photométrie de PRISTINE. La première étude porte sur la détermination de la composition chimique d'un échantillon d'étoiles qui ont pu être enrichies par les éjectae de l'explosion de supernovae à instabilité de paires. Mon travail a consisté à sélectionner les candidats les plus intéressants, puis d'effectuer une mission d' observation avec le spectrographe SOPHIE à l'Observatoire de le Haute Provence (OHP). La deuxième étude présente les résultats préliminaires de la détermination de la composition chimique d'un échantillon d'étoiles Pristine sélectionnées comme extrêmement pauvres en métaux. Mon travail a porté sur la détermination des paramètres stellaires et le calcul des abondances chimiques. Un article est en préparation.Le sixième chapitre présente les conclusions de ce travail de thèse et apporte quelques reflexions sur les projets à venirThis thesis project presents several studies that are focused on the investigation of the chemical properties of different stellar populations in the Milky Way by means of high-resolution spectroscopy.The thesis is structured as follows:The first chapter is an introduction to the thesis project, and is divided into three sections. The first section describes the structure and formation scenarios of the Milky Way, in particular by referring to the most recent discoveries. The second section introduces the basic concepts and objectives of the studies presented in this thesis work. The third section describes the methods used to analyse the spectroscopic data.The second chapter presents the studies carried out in the context of the MINCE project. The first study is devoted to the chemical analysis of a sample of young giant stars that was serendipitously discovered during the first MINCE observations. My contribution in this work was to derive the stellar parameters, analyse the spectroscopic data, measure the rotational velocities, compare the results with theoretical models and write the paper. The second study presents the results obtained from the analysis of the first sample of MINCE stars. In this work, I contributed to the analysis of some of the stars in the sample.The third chapter presents the results obtained in the context of the CERES project. The first study presents a detailed chemical analysis of the star RAVE J110842.1-715300, with the aim of understanding whether or not it originated in the Omega Centauri globular cluster. My contribution in this study was to derive the stellar parameters of the star. The second study presents the results obtained for the CERES star sample. My contribution was to derive the parameters, compute model atmospheres, measure the chemical abundances, and write the paper.The fourth chapter presents the results obtained in the context of the High-speed stars project. The first study reports the results obtained from the high-resolution follow-up of two young and metal-poor stars in the sample of Caffau et al. (2020), to check whether they are blue stragglers or not. My contribution in this study was to obtain the high-resolution observations with UVES and to analyse the data. These results have not been published yet. The second study presents a detailed analysis of two high-speed stars observed with Subaru. In this study I was involved in the C abundance determination.The fifth chapter presents the results obtained from the chemical analysis of samples of stars selected using the Pristine photometry. The first study presents the chemical analysis of a sample of metal-poor stars that may have been enriched by the explosion of pair instability supernovae. My contribution was to select promising candidates and observe them with the SOPHIE spectrographat Observatoire de le Haute Provence (OHP)in visitor mode. The second study presents the preliminary results obtained from the chemical analysis of a sample of Pristine extremely metal-poor candidates. My contribution in this study was to derive the stellar parameters and the chemical abundances. The paper is in preparation.The sixth chapter concludes the thesis and gathers final reflections and future projects
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Polles, Fiorella Lucia. "Properties of the interstellar medium of the star-forming galaxy, IC10, at various spatial scales." Thesis, Université Paris-Saclay (ComUE), 2017. http://www.theses.fr/2017SACLS276/document.
Abstract:
Les propriétés du milieu interstellaire (MIS) influencent fortement l’environnement et les processus menant à la formation d’étoiles qui, à son tour, dicte l’évolution d’une galaxie. Les galaxies naines du Groupe Local sont de parfaits laboratoires pour comprendre comment le contenu en métaux (ou métallicité) du MIS affecte l’interaction entre le gaz, la poussière et les étoiles. Mon travail de thèse porte sur les propriétés physiques des régions HII et du gaz diffus ionisé de la galaxie naine IC10, de métallicité 1/3 solaire. La proximité de cette galaxie (d=700kpc) permet l’analyse du MIS à différentes échelles spatiales: des nuages brillants compacts (25pc) au corps entier de la galaxie formant des étoiles (650pc). Afin de mesurer les propriétés physiques du MIS, j’ai modélisé les raies d’émission en infrarouge observées avec Spitzer et Herschel grâce à des modèles de photoionisation et de photodissociation. Je présente une exploration complète de plusieurs méthodes pour déterminer, de manière la plus fiable et selon les contraintes disponibles, les propriétés du MIS à diverses échelles. J’ai contraint les propriétés des nuages compacts les plus brillants dans IC10 et montré que l’émission à plus grande échelle (300pc) est dominée par celle de ces nuages. Enfin, je démontre le besoin d’un modèle à plusieurs composantes pour reproduire les observations dans leur ensembleThe properties of the Interstellar Medium (ISM) strongly influence the environment and processes that lead to star-formation, which in turn, drives the evolution of a galaxy. Dwarf galaxies in the Local Group are perfect laboratories to investigate how the metal-poor ISM affects the interplay between gas, dust and stars. In this thesis, I investigate the properties of the HII regions and the diffuse ionized gas of the nearby dwarf galaxy IC10, which has a metallicity of 1/3 solar. Its proximity (d=700 kpc) enables the analysis on different spatial scales: from the compact clumps (~25 pc) to the whole star-forming body of the galaxy (~650pc). In order to measure the physical properties of the ISM, I model the infrared emission lines observed with Spitzer and Herschel with photoionization and photodissociation models. I present an extensive exploration of different methods to determine the most reliable ISM properties, based on the available constraints. I determined the properties of the brightest star-forming clumps within the galaxy and show that the emission at large scales (~300 pc) is dominated by that of the compact, bright clumps that lie within the region. I further demonstrate the need for a multi-component model to fully reproduce the observations
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Lau, Ho Bun. "Evolution and nucleosynthesis of zero-metallicity AGB stars." Thesis, University of Cambridge, 2008. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.612023.
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PACE, GIANCARLO. "AGE, METALLICITY AND ACTIVITY IN SOLAR-TYPE STARS." Doctoral thesis, Università degli studi di Trieste, 2005. http://thesis2.sba.units.it/store/handle/item/13164.
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García, Pérez Ana E., Melissa Ness, Annie C. Robin, Inma Martinez-Valpuesta, Jennifer Sobeck, Gail Zasowski, Steven R. Majewski, et al. "The Bulge Metallicity Distribution from the APOGEE Survey." IOP PUBLISHING LTD, 2018. http://hdl.handle.net/10150/626538.
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The Apache Point Observatory Galactic Evolution Experiment (APOGEE) provides spectroscopic information of regions of the inner Milky Way, which are inaccessible to optical surveys. We present the first large study of the metallicity distribution of the innermost Galactic regions based on high-quality measurements for 7545 red giant stars within 4.5 kpc of the Galactic center, with the goal to shed light on the structure and origin of the Galactic bulge. Stellar metallicities are found, through multiple Gaussian decompositions, to be distributed in several components, which is indicative of the presence of various stellar populations such as the bar or the thin and the thick disks. Super-solar ([Fe/H] = +0.32) and solar ([Fe/H] = +0.00) metallicity components, tentatively associated with the thin disk and the Galactic bar, respectively, seem to be major contributors near the midplane. A solar-metallicity component extends outwards in the midplane but is not observed in the innermost regions. The central regions (within 3 kpc of the Galactic center) reveal, on the other hand, the presence of a significant metal-poor population ([Fe/H] = -0.46), tentatively associated with the thick disk, which becomes the dominant component far from the midplane (vertical bar Z vertical bar >= +0.75 kpc). Varying contributions from these different components produce a transition region at +0.5 kpc <= vertical bar Z vertical bar <= +1.0 kpc, characterized by a significant vertical metallicity gradient.
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Wisniewski, John Patrick. "The effect of age and metallicity on Be circumstellar disk formation /." See Full Text at OhnioLINK ETD Center (Requires Adobe Acrobat Reader for viewing), 2005. http://www.ohiolink.edu/etd/view.cgi?acc%5Fnum=toledo1124412024.
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Thesis (Ph.D.)--University of Toledo, 2005.Typescript. "A dissertation [submitted] as partial fulfillment of the requirements of the Doctor of Philosophy degree in Physics." Bibliography: leaves 328-333.
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Covarrubias, Ricardo Alberto. "Does the metallicity affect the fate of massive stars? /." Thesis, Connect to this title online; UW restricted, 2007. http://hdl.handle.net/1773/5442.
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Alves, Cruz Monique. "Nucleosynthesis in extremely metal-poor and zero metallicity stars." Diss., lmu, 2012. http://nbn-resolving.de/urn:nbn:de:bvb:19-153061.
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Masters, Craig Eugene Ashman Keith M. "The low metallicity globular clusters." Diss., UMK access, 2005.
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Thesis (M.S.)--Dept. of Physics. University of Missouri--Kansas City, 2005."A thesis in physics." Typescript. Advisor: K. M. Ashman Vita. Title from "catalog record" of the print edition Description based on contents viewed June 26, 2006. Includes bibliographical references (leaves 54-56). Online version of the print edition.
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Mulders, Gijs D., Ilaria Pascucci, Dániel Apai, Antonio Frasca, and Joanna Molenda-Żakowicz. "A SUPER-SOLAR METALLICITY FOR STARS WITH HOT ROCKY EXOPLANETS." IOP PUBLISHING LTD, 2016. http://hdl.handle.net/10150/622435.
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Host star metallicity provides a measure of the conditions in protoplanetary disks at the time of planet formation. Using a sample of over 20,000 Kepler stars with spectroscopic metallicities from the LAMOST survey, we explore how the exoplanet population depends on host star metallicity as a function of orbital period and planet size. We find that exoplanets with orbital periods less than 10 days are preferentially found around metal-rich stars ([Fe/H] similar or equal to 0.15 +/- 0.05 dex). The occurrence rates of these hot exoplanets increases to similar to 30% for super-solar metallicity stars from similar to 10% for stars with a sub-solar metallicity. Cooler exoplanets, which reside at longer orbital periods and constitute the bulk of the exoplanet population with an occurrence rate of greater than or similar to 90%, have host star metallicities consistent with solar. At short orbital periods, P < 10 days, the difference in host star metallicity is largest for hot rocky planets (< 1.7 R-circle plus), where the metallicity difference is [Fe/H] similar or equal to 0.25 +/- 0.07 dex. The excess of hot rocky planets around metal-rich stars implies they either share a formation mechanism with hot Jupiters, or trace a planet trap at the protoplanetary disk inner edge, which is metallicity dependent. We do not find statistically significant evidence for a previously identified trend that small planets toward the habitable zone are preferentially found around low-metallicity stars. Refuting or confirming this trend requires a larger sample of spectroscopic metallicities.
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Badenes, Carles, Christine Mazzola, Todd A. Thompson, Kevin Covey, Peter E. Freeman, Matthew G. Walker, Maxwell Moe, et al. "Stellar Multiplicity Meets Stellar Evolution and Metallicity: The APOGEE View." IOP PUBLISHING LTD, 2018. http://hdl.handle.net/10150/627094.
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We use the multi-epoch radial velocities acquired by the Apache Point Observatory Galactic Evolution Experiment (APOGEE) survey to perform a large-scale statistical study of stellar multiplicity for field stars in the Milky Way, spanning the evolutionary phases between the main sequence (MS) and the red clump. We show that the distribution of maximum radial velocity shifts (Delta RVmax) for APOGEE targets is a strong function of log g, with MS stars showing Delta RVmax as high as similar to 300 km s(-1), and steadily dropping down to similar to 30 km s(-1) for log g similar to 0, as stars climb up the red giant branch (RGB). Red clump stars show a distribution of Delta RVmax values comparable to that of stars at the tip of the RGB, implying they have similar multiplicity characteristics. The observed attrition of high Delta RVmax systems in the RGB is consistent with a lognormal period distribution in the MS and a multiplicity fraction of 0.35, which is truncated at an increasing period as stars become physically larger and undergo mass transfer after Roche Lobe overflow during H-shell burning. The Delta RVmax distributions also show that the multiplicity characteristics of field stars are metallicity-dependent, with metal-poor ([Fe/H] less than or similar to -0.5) stars having a multiplicity fraction a factor of 2-3 higher than metal-rich ([Fe/H] less than or similar to 0.0) stars. This has profound implications for the formation rates of interacting binaries observed by astronomical transient surveys and gravitational wave detectors, as well as the habitability of circumbinary planets.
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Gáspár, András, George H. Rieke, and Nicholas Ballering. "THE CORRELATION BETWEEN METALLICITY AND DEBRIS DISK MASS." IOP PUBLISHING LTD, 2016. http://hdl.handle.net/10150/621391.
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We find that the initial dust masses in planetary debris disks are correlated with the metallicities of their central stars. We compiled a large sample of systems, including Spitzer, the Herschel DUNES and DEBRIS surveys, and WISE debris disk candidates. We also merged 33 metallicity catalogs to provide homogeneous [Fe/H] and sigma([Fe/H]) values. We analyzed this merged sample, including 222 detected disks (74 warm and 148 cold) around a total of 187 systems (some with multiple components) and 440 disks with only upper limits (125 warm and 315 cold) around a total of 360 systems. The disk dust masses at a common early evolutionary point in time were determined using our numerical disk evolutionary code, evolving a unique model for each of the 662 disks backward to an age of 1 Myr. We find that disk-bearing stars seldom have metallicities less than [Fe/H] = -0.2 and that the distribution of warm component masses lacks examples with large mass around stars of low metallicity ([Fe/H] < -0.085). Previous efforts to find a correlation have been largely unsuccessful; the primary improvements supporting our result are (1) basing the study on dust masses, not just infrared excess detections; (2) including upper limits on dust mass in a quantitative way; (3) accounting for the evolution of debris disk excesses as systems age; (4) accounting fully for the range of uncertainties in metallicity measurements; and (5) having a statistically large enough sample.
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Bolatto, Pereira Alberto D. "The interstellar medium in low metallicity environments." Thesis, Boston University, 2001. https://hdl.handle.net/2144/36768.
Abstract:
Thesis (Ph.D.)--Boston UniversityPLEASE NOTE: Boston University Libraries did not receive an Authorization To Manage form for this thesis or dissertation. It is therefore not openly accessible, though it may be available by request. If you are the author or principal advisor of this work and would like to request open access for it, please contact us at open-help@bu.edu. Thank you.
This dissertation studies the interstellar medium (ISM) in dwarf galaxies. Dwarf galaxies are important because: 1) they constitute the largest fraction of extragalactic systems, and 2) they provide templates for primordial galaxies. Indeed, local active dwarf galaxies resemble primitive systems, since they are poor in dust and heavy elements and they are profusely forming massive stars. Because dwarf galaxies are nearby, however, they can be observed in much greater detail than distant primordial systems. Therefore studies of the ISM in nearby dwarf galaxies can be used to understand the processes at work in primitive galaxies. This work focuses on the effects of low heavy element abundances (i.e., low metallicities) on the star-forming ISM. Low metallicities are known to drastically affect the ISM. With decreasing metallicity, an increasingly large fraction of the molecular ISM is photodissociated into atoms and ions. We modeled and observed the emission of a sample of low metallicity dwarf galaxies in the millimeter, submillimeter, and far-infrared wavebands. The submillimeter waveband allows us to observe the mid-J rotational transitions of carbon monoxide (CO), the usual tracer of the molecular ISM, and the fine structure transit ions of neutral carbon ([C I]), a tracer of translucent and photodissociated material. We studied regions in the Large and Small Magellanic Clouds and the Northern Hemisphere dwarf galaxy IC 10. We find that the preponderant mechanism producing neutral carbon inside molecular clouds is photodissociation. We observe a moderate increase in the ratio of [C I] to CO emission for decreasing metallicity. Our models of clumpy, unresolved photo dissociation regions explain these observations as the natural result of an augmented fraction of photo dissociated material. Finally, our observations of the submillimeter thermal dust continuum in IC 10 find an abnormally low emissivity exponent for its graybody emission. We conclude that the unusual dust continuum is caused by the selective destruction of small grains, brought about by the combined effects of low metallicities and high radiation fields.
2031-01-01
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Spalding, Eckhart. "A New Set of Spectroscopic Metallicity Calibrations for RR Lyrae Variable Stars." UKnowledge, 2014. http://uknowledge.uky.edu/physastron_etds/22.
Abstract:
RR Lyrae stars are old, iron-poor, Helium-burning variable stars. RR Lyraes are extremely useful for tracing phase-space structures and metallicities within the galaxy because they are easy to identify, have consistent luminosities, and are found in large numbers in the galactic disk, bulge, and halo. Here we present a new set of spectroscopic metallicity calibrations that use the equivalent widths of the Ca II K, Hγ, and Hδ lines to calculate metallicity values. Applied to spectroscopic survey data, these calibrations will help shed light on the evolution of the Milky Way and other galaxies.
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Santucci, Rafael Miloni. "Identificação de subestruturas no halo galáctico através de estrelas azuis tardias." Universidade de São Paulo, 2016. http://www.teses.usp.br/teses/disponiveis/14/14131/tde-12062018-113847/.
Abstract:
Tudo que vive por muito tempo está apto a contribuir com boas histórias sobre o passado. Isto não é diferente com as estrelas azuis tardias que são encontradas em todos os ambientes estelares. Essas estrelas velhas mostram-se aparentemente muito jovens, e talvez por isso, nunca tenham sido ouvidas em um contexto maior que suas próprias vidas. Este trabalho interpreta a história que elas contam sobre a natureza do halo galáctico, através de seus parâmetros físicos fundamentais: coordenadas, temperaturas, gravidades superficiais, metalicidades, cores, distâncias e idades. Este trabalho utiliza dados do Sloan Digital Sky Survey para reunir candidatas a estrelas azuis tardias (BSSs) através de critérios espectrofotométricos. Ao todo, 8001 candidatas a BSSs sobreviveram aos diversos métodos de seleção aplicados, constituindo a base de dados deste estudo. Essa amostra permitiu estimar a frequência média de BSSs no halo em relação ao número de estrelas azuis do ramo horizontal (BHBs) em 2.15±0.13 BSS/BHB, valor similar ao encontrado em galáxias anãs próximas (~2.24±0.17). Verificou-se também que as BSSs apresentam um gradiente de cor em função da distância ao centro Galáctico, aparentemente independente da metalicidade. À variação de cor foi atribuída uma variação de idade, que forneceu um gradiente médio de -0.034±0.002 Ganos/kpc no halo. Esse resultado mostra que as regiões mais velhas se concentram preferencialmente no centro da Galáxia, e ficam cada vez mais jovens para distâncias maiores. O gradiente de cor das BSSs possibilitou a construção de mapas de idade do halo galáctico, que foram superpostos às posições centrais de uma coleção de subestruturas encontrada na literatura. Aproximadamente 60% delas tem posições que concordam com as flutuações de cor observadas nos mapas, além de apresentarem propriedades cinemáticas e químicas similares às BSSs nessas regiões (em 2).Everything that lives long enough is able to contribute with good stories about the past. This statement also applies to the blue straggler stars (BSSs), which can be found in all stellar environments. These old stars appear to be very young and perhaps because of this have never been properly addressed in a context larger than their own lives. This work interprets the story they tell about the nature of the Galactic halo, through their fundamental physical parameters: coordinates, temperatures, surface gravities, metallicities, colors, distances and ages. This work uses the Sloan Digital Sky Survey (SDSS) database to select BSS candidates through photometric and spectroscopic criteria. Altogether, 8001 BSS candidates survived the various selection methods applied and were used as the database for this study. This large sample allowed the determination of the average frequency of BSSs in the halo, compared to the number of blue horizontal branch stars (BHBs). The average frequency of BSS/BHB found in the galactic halo by this work is 2.15±0.13, very similar to this ratio in nearby dwarf galaxies (~2.24±0.17). In addition, this work verified that the BSSs show a color gradient as a function of distance to the galactic center, which appears to be independent from metallicity. To this color variation was assigned an age variation, yielding an average gradient of -0.034±0.002 Gyr/kpc in the halo. This result shows that the older regions preferentially occur in the center of the Galaxy and get younger for larger distances. The BSSs color gradient allowed the construction of galactic halo age maps. These maps were superimposed to the central positions of a collection of substructures found in the literature. Approximately 60% of them have positions that agree with the color fluctuations observed in the maps, as well as having similar kinematic and chemical properties to the BSSs in those regions (within 2).
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Alves, Cruz Monique [Verfasser], and Achim [Akademischer Betreuer] Weiss. "Nucleosynthesis in extremely metal-poor and zero metallicity stars / Monique Alves Cruz. Betreuer: Achim Weiss." München : Universitätsbibliothek der Ludwig-Maximilians-Universität, 2012. http://d-nb.info/1032131225/34.
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de, Koter A., J. S. Vink, and L. Muijres. "Constraints on wind clumping from the empirical mass-loss vs. metallicity relation for early-type stars." Universität Potsdam, 2007. http://opus.kobv.de/ubp/volltexte/2008/1773/.
Abstract:
We present the latest results on the observational dependence of the mass-loss rate in stellar winds of O and early-B stars on the metal content of their atmospheres, and compare these with predictions. Absolute empirical rates for the mass loss of stars brighter than 10$^{5.2} L_{odot}$, based on H$alpha$ and ultraviolet (UV) wind lines, are found to be about a factor of two higher than predictions. If this difference is attributed to inhomogeneities in the wind this would imply that luminous O and early-B stars have clumping factors in their H$alpha$ and UV line forming regime of about a factor of 3--5. The investigated stars cover a metallicity range $Z$ from 0.2 to 1 $Z_{odot}$. We find a hint towards smaller clumping factors for lower $Z$. The derived clumping factors, however, presuppose that clumping does not impact the predictions of the mass-loss rate. We discuss this assumption and explain how we intend to investigate its validity in more detail.
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Santucci, Rafael Miloni. "Caracterização de estrelas azuis tardias no campo galáctico." Universidade de São Paulo, 2012. http://www.teses.usp.br/teses/disponiveis/14/14131/tde-20062013-155903/.
Abstract:
As estrelas azuis tardias (blue straggler stars ou estrelas BS) são estrelas de sequência principal que apresentam um aparente atraso evolutivo em relação às suas vizinhanças. Elas foram identificadas inicialmente na sequência principal de aglomerados globulares acima do ponto de turnoff no diagrama HR. Desde então, têm sido encontradas em todos os ambientes estelares: aglomerados abertos e globulares, galáxias anãs próximas e entre as estrelas de campo na Galáxia. O principal objetivo deste trabalho é construir uma grande amostra de estrelas BS no campo galáctico. Para tanto, métodos de separação desses objetos de estrelas BHB foram comparados com critérios de seleção que envolvem parâmetros atmosféricos estimados pelo SEGUE Stellar Parameter Pipeline (SSPP). Tal procedimento permitiu incluir também estrelas com magnitudes g maiores que 18, antes excluídas pelos métodos tradicionais para objetos com razão sinal-ruído menores que 9. Os métodos apresentados neste trabalho permitiram a seleção de uma amostra de 8001 candidatas a estrelas BS que foram analisadas cinematicamente, através de suas velocidades radiais. Verificou-se que aproximadamente um quinto dessa amostra (cerca de 1500 objetos) possui características que a associa à corrente de Sagitário, sugerindo uma origem extragaláctica para tais objetos.Blue straggler stars (BSSs) are main sequence stars that exhibit an apparent evolutionary delay over the region where they are found in the HR diagram. They were initially identified in the main sequence of globular clusters above the turnoff point. Since then, they have been found in many different stellar environments: globular and open clusters, near dwarf galaxies, and also in the field of the Galaxy. The main goal of this work is to build a large sample of BSSs in the galactic field region. In order to accomplish this task, classical methods of separation of BSS from BHB stars were compared with proposed restrictions based on their atmospheric parameters, which are estimated by the SEGUE Stellar Parameter Pipeline (SSPP). The latter procedure allowed to include stars with magnitudes g greater than 18, not handled by traditional methods when signal-noise ratio is smaller than 9. The selection methods gathered a sample of 8001 BS stars. These stars were kinematically analyzed through their radial velocities. The results suggest that many of them (about 1500) can have extragalactic origin, associated with Sagittarius stream.
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Goldman, Steve. "The metallicity dependence of maser emission and mass loss from red supergiants and asymptotic giant branch stars." Thesis, Keele University, 2017. http://eprints.keele.ac.uk/4258/.
Abstract:
Asymptotic Giant Branch (AGB) stars and red supergiants (RSGs) are some of the largest contributors of dust and material back to the Universe. While a substantial amount of RSG dust will be destroyed by the subsequent supernova, the dust from AGB stars will be injected into the Interstellar Medium. Understanding the contribution of these stars and how it is affected by changes in stellar parameters is critically important to determining stellar lifetimes, constraining stellar models, and understanding the evolutionary paths of core-collapse progenitors and their subsequent supernova. This study has discovered four new circumstellar masers in the Large Magellanic Cloud (LMC), and increased the number of reliable wind speeds found outside of our galaxy from 5 to 13. Results have been used to develop a relation for the wind speed as a function of luminosity and metallicity. A further analysis of the spectral energy distributions of these and Galactic OH/IR stars has led to the development of a robust empirical mass-loss prescription that uses luminosity, pulsation period and gas-to-dust ratio, a measure of metallicity. These results suggest that mass loss in this phase is (nearly) independent of metallicity between a half and twice solar metallicity. A radio survey of evolved stars in the Small Magellanic Cloud (SMC) has found that none of the most luminous sources expected to show OH maser emission, do so. While sources may lie just below both the detection threshold and/or the required OH column density, a number of sources should still exhibit OH maser emission. It is possible that the OH masing phase of lower metallicity massive AGB stars and RSGs is cut short. New observations in the radio, optical and infra-red have been used to constrain the location and mass-loss rate of the prominent dust producing LMC RSGs, IRAS 05280−6910 and IRAS 05346−6949. These results suggest that these sources may exhibit a dusty torus geometry, but confirmation will require further high angular resolution studies. The results of this work have made progress in understanding the wind driving and mass loss mechanism, and have set the stage for much larger upcoming surveys with the SKA and its pathfinders ASKAP and MeerKAT.
29
Young, D. R. "Core-collapse supernovae in low-metallicity galaxies and all-sky transient surveys." Thesis, Queen's University Belfast, 2010. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.517021.
30
Cormier, Diane. "The physical properties in the interstellar medium of low-metallicity dwarf galaxies." Paris 7, 2012. http://www.theses.fr/2012PA077193.
Abstract:
Les galaxies naines locales à formation d'étoiles sont des laboratoires idéaux pour analyser les processus de formation d'étoiles à faible métallicité et le rôle de l'enrichissement en métaux sur les conditions physiques et, plus généralement, l'évolution des galaxies. Mon travail de thèse concerne l'étude des propriétés du gaz dans les galaxies naines du "The Herschel Dwarf Galaxy Survey", combinant observations et modélisation. J'ai examiné le rôle des traceurs les plus importants du milieu interstellaire (MIS) multi phases, de l'infrarouge moyen au submillimétrique. Je me suis particulièrement intéressée aux agents de refroidissement du gaz ionisé et du gaz neutre grâce à des observations du télescope spatial Herschel (e. G. Raies à [OIII] 88, [OI] 63, [CII] 157 micron), et de la moléculeCO provenant de la phase moléculaire, grâce à des observations au sol complémentaires. Les données sont interprétées avec des modèles de transfert radiatif afin d'en extraire les informations physiques (densité, champ de radiation, facteurs de remplissage). Ce travail a aidé à mieux comprendre la structure et les conditions physiques du MIS à faible métallicité. Il met en avant le caractère poreux du MIS des galaxies naines, où les photons ultraviolets provenant des régions de formation d'étoiles massives peuvent exciter le gaz sur de grandes distances. Cela entraine la présence d'un milieu diffus ionisé/neutre remplissant une grande partie du volume de la galaxie, des régions de photodissociation fragmentées, et peu de gaz moléculaire observé, le processus de photodissociation agissant à grande échelleIn the framework of galaxy evolution, local star-forming dwarf galaxies are ideal laboratories to study star formation processes at low metallicity and the role of metal enrichment on the physical conditions. My thesis has focused on the study of the gas properties of the dwarf galaxies from "The Herschel Dwarf Galaxy Survey", combining observations and modeling efforts. I have investigated the role of the most important tracers of the multi-phase the interstellar medium (ISM), in the mid-infrared to submillimeter range. Particular attention was paid to the ionized and neutral gas coolants observed with Herschel Space Observatory (e. G. [OIII] 88, [OI]63, [CII] 157 micron lines), and to the CO molecule, probing the molecular phase, with complementary ground-based observations. The data are interpreted in physical terms (density, radiation field, filling factors) with radiative transfer models. This work has helped elucidate the structure and conditions of the low-metallicity ISM. It highlights the porosity of the ISM of dwarf galaxies, with ultraviolet photons from the massive star-forming regions excitinig the gas out to large distances. This results in the presence of large volume filling factor diffuse ionized/neutral gas, clumpy photodissociation regions, and little observed molecular gas due to large-scale photodissociation
31
Chen, Yang. "Evolution of Very Low Mass Stars and Very Massive Stars in PARSEC." Doctoral thesis, SISSA, 2015. http://hdl.handle.net/20.500.11767/4879.
Abstract:
During my Ph.D. study, I have been concentrating on the evolutionary tracks and atmosphere models of very low mass stars (VLMSs; ∼ 0.1 − 0.6 Msun) and very massive stars (VMSs; ∼ 12−350 Msun), based on the PAdova-TRieste Stellar Evolution Code (PARSEC). For the very low mass models, it appeared that the previous models computed with PARSEC were unable to correctly predict some basic observational relations. These relations include the mass–radius relation and the colour-magnitude relations of cool dwarfs. We replace the approximate boundary conditions used in PARSEC with those provided by more realistic atmosphere modelling. We implement the T – tau relations from Phoenix/BT-Settl model atmospheres as the outer boundary conditions in the PARSEC code, finding that this change alone reduces the discrepancy in the mass–radius relation from 8 to 5 per cent. Furthermore, we convert the theoretical quantities to the magnitudes and colors with the stellar spectral libraries from Phoenix/BT-Settl. We compare the models with multi–band photometries of clusters Praesepe, M 67, 47 Tuc and NGC 6397, showing that the use of T – tau relations clearly improves the description of the optical colours and magnitudes. However, using both Kurucz and Phoenix model spectra, the models are still systematically fainter and bluer than the observations. We then apply a shift to the above T – tau relations, increasing from 0 at Teff = 4730 K to ∼14 per cent at Teff = 3160 K, to reproduce the observed mass–radius relation of dwarf stars. Taking this experiment as a calibration of the T – tau relations, we can reproduce the optical and near-infrared CMDs of low mass stars in the old metal–poor globular clusters NGC 6397 and 47 Tuc, and in the intermediate–age and young Solar–metallicity open clusters M 67 and Praesepe. Thus, we extend PARSEC models using this calibration, providing VLMS models that are more suitable for the lower main sequence stars over a wide range of metallicities and wavelengths. For the very massive stars, the Padova models were computed more than 20 years ago and were not distributed to the community because suitable bolometric corrections for these models were not yet implemented. In this project, we complement the PARSEC data base with the stellar evolutionary tracks of massive stars, from the pre-main sequence phase to the central Carbon ignition. We consider a broad range of metallicities, 0.0001≤ Z ≤ 0.04 and initial masses up to Mini =350 Msun. The main difference with respect to our previous models of massive stars is the adoption of a recent formalism accounting for the mass-loss enhancement when the ratio of the stellar luminosity to the Eddington luminosity, Γe , approaches unity. With this new formalism, the models are able to reproduce the Humphreys-Davidson limit observed in the Galactic and Large Magellanic Cloud colour-magnitude diagrams, without an ad hoc mass-loss enhancement. We also follow the predictions of recent wind models indicating that the metallicity dependence of the mass-loss rates becomes shallower when Γ e approaches unity. We thus find that massive stars may suffer from substantial mass-loss even at low metallicity. We also predict that the Humphreys-Davidson limit should become brighter at decreasing metallicity. We supplement the evolutionary tracks with new theoretical bolometric correction tables, useful for comparing tracks and isochrones with the observations. For this purpose, we homogenize existing stellar atmosphere libraries of hot and cool stars (PoWR, ATLAS9 and Phoenix) and add, where needed, new atmosphere models computed with WM-basic. The model grids are fully adequate in mass, age and metallicity for performing detailed investigations of the properties of very young stellar systems in both local and distant galaxies. The new tracks supersede the previous old Padova models of massive stars. Therefore, my work together with the already updated PARSEC models of the other masses could depict the full evolution of stars across the mass range from ∼ 0.08 Msun to 350 Msun and over a wide range of metallicity from super-Solar (Z = 0.04) to extreme metal-poor (Z = 0.0001). Consequently, they would provide paramount information for studies not only on stars or star clusters but also on galaxy formation and evolution.
32
Schlesinger, Katharine J. "The Metallicity Distribution Functions of Cool Stars in the SEGUE Survey: Clues to Understanding Milky Way Formation and Evolution." The Ohio State University, 2011. http://rave.ohiolink.edu/etdc/view?acc_num=osu1312924463.
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Pineda, John Sebastian. "Activity and kinematics of low mass stars." Thesis, Massachusetts Institute of Technology, 2010. http://hdl.handle.net/1721.1/61260.
Abstract:
Thesis (S.B.)--Massachusetts Institute of Technology, Dept. of Physics, 2010.Cataloged from PDF version of thesis.
Includes bibliographical references (p. 95-96).
We present an analysis of the magnetic activity, photometry and kinematics of approximately 70000 M dwarfs from the Sloan digital Sky Survey (SDSS) Data Release 7. This new analysis explores the spatial distribution of these M dwarf properties as a function of vertical distance from the Galactic plane (Z) and distance from the Galactic center (R). We confirm the established trends of decreasing magnetic activity, as measured by Ha emission, with increasing distance from the mid-plane of the disk but also observe a new trend in Galactocentric radius, apparent in the analysis of spectral types M3 and M4 of a small increase in activity with increasing R. Examining the color indices r - z, r - i and g - r from the SDSS ugriz photometry reveals noticeable gradients in the vertical direction but not in the radial direction. To analyze the kinematics we develop a new technique utilizing probability distributions and a pseudo-montecarlo data fitting scheme to determine the parameters (o- 1, pi, 0-2, 12) and normalization of the underlying Gaussians making up the kinematic distributions of the stellar population. We analyze each of the spatial velocities VR, Vz , and Ve defined in a Galactocentric cylindrical coordinate system. The kinematic analysis reproduced previous trends of increasing dispersion with increasing distance from the mid-plane, but with much greater accuracy and reliability and to distances farther out away from the mid-plane. The analysis did not reveal any significant kinematic trends in the radial domain.
by John Sebastian Pineda.
S.B.
34
Rainger, Paul P. "Studies of low-mass interacting binary stars." Thesis, University of St Andrews, 1990. http://hdl.handle.net/10023/14369.
Abstract:
Spectroscopic and photometric observations of eight contact/near-contact binaries are presented and analysed. Spectroscopic observations were obtained at 4200 Å (radial velocity spectra) and 6563 Å (hydrogen-alpha line profiles). New photometric observations were obtained at visual and infrared wavelengths, and other previously published light curves are also re-analysed. Absolute dimensions have been obtained for five systems; TY Boo, VW Boo, BX And, SS Ari and AG Vir, and their evolutionary positions discussed. Four of the systems are found to be in marginal but poor thermal contact, exhibiting regions of apparent "excess luminosity" in their light curves. A qualitative analysis of these "hot spot" regions has been attempted for the first time using spot models now incorporated into a light curve synthesis programme. Substantial time for this project was awarded on telescopes funded by the United Kingdom Science and Engineering Research Council (SERC), comprising 14 nights at the Issac Newton Telescope (INT) on La Palma, and 4 nights at the United Kingdom Infrared Telescope (UKIRT) on Mauna Kea. Additional observations were made during an 8 night commissioning run on the Jacobus Kapteyn Telescope (JKT) on La Palma, and extensive observations were made with the Twin Photometric Telescope (TPT) at St Andrews University Observatory between 1985 and 1989. These resulted in over 100 spectra at 4200 Å and over 50 spectra at 6563 Å (INT and JKT observations), over 300 infrared photometric observations (UKIRT), and over 3500 visual photometric observations (TPT). Of the five systems analysed in detail in this work, TY Boo appears to be a normal shallow-contact W-type system. Both VW Boo and BX And exhibit regions of "excess luminosity" around the ingress and egress of secondary minimum which are well modelled by a warm spot on the cooler component sitting symmetrically around the neck joining the pair. Such a phenomenon may be expected to arise naturally in systems which have come into contact but are not yet/currently in thermal contact, exhibiting a temperature difference between the components. BXAnd like other B-type systems seems to be reaching this contact state for the first time, but the position of VW Boo is uncertain, and whilst evidence that it could be in the "broken contact" state predicted by the TRO Theory is far from conclusive, its lower orbital angular momentum clearly marks the system as worthy of further study. SS Ari and AG Vir exhibit light curves with unequal quadrature heights. Attempts to treat the higher quadrature as a region of "excess luminosity" due to an energy transfer "warm spot" does not however provide a good model of this phenomenon. Since invoking a dark starspot model also does not provide a good explanation for such systems, it may be that this form of light curve distortion is due to an entirely different form of distorting surface phenomenon. Like BX And, AG Vir appears to be just reaching contact for the first time, but like VW Boo, the slightly lower angular momentum of SS Ari warrants further study.
35
Mendes, Luiz Themystokliz Sanctos. "Evolutionary models of rotating low mass stars." Universidade Federal de Minas Gerais, 1999. http://hdl.handle.net/1843/BUOS-9GGP5S.
Abstract:
We have investigated the combined effects of rotation and internal ngular momentum redistribution on the structure and evolution of low mass tars, from the pre-main sequence to the main sequence phase. As a tool for that study, the ATON stellar evolutionary code (Mazzitelli 1989; Ventura et al. 1998) has been modified in order to include those effects. Rotation as implemented according to the equipotential technique developed by Kippenhahn and Thomas (1970) and later improved by Endal and Sofia (1976). Angular momentum redistribution in radiative regions was modeled through an advection-diffusion partial differential equation based on the framework originally introduced by Chaboyer and Zahn (1992), which is based on the sole assumption of stronger turbulent transport in the horizontal direction than in the vertical one. The diffusion coefficient of this equation is obtained from characteristic lengths and velocities of typical rotation-induced hydrodynamical instabilities. This improved code was used to compute a series of rotating low mass stellar models (with masses ranging from 1.2 Msun down to 0.6 Msun). Regarding the structural (hydrostatic) effects of rotation, the general features of these models show that rotating stars behave as if they were non-rotating stars of slightly lower masses, in accordance with previous results by other researchers. A study of this mass-lowering effect for the considered range of masses shows that rotation decreases lithium depletion while the star is fully convective but increases it as soon as the star develops a radiative core. The net effect is a enhanced lithiumepletion, in disagreement with observational data which suggest that faster rotators in young open clusters experience less lithium depletion. Angular momentum redistribution in the considered models is very effective in smoothing their internal angular velocity profile as soon as the star reaches the zero age main sequence, but fails to reproduce the flat solar rotation rate obtained from helioseismology, indicating that, in the Sun, angular momentum transport is more efficient than current models. The internal angular momentum transport also contributes to a still higher lithium depletion than the models computed with only the structural effects of rotation, thus suggesting that other physical phenomena must play a role regarding both lithium depletion and the rotation profile evolution of these stars.Neste trabalho nós investigamos os efeitos combinados da rotação e da redistribuição de momento angular em estrelas de baixa massa, desde a etapa pré-sequência principal até a sequência principal. Como uma ferramenta para tal estudo, o código evolutivo estelar ATON (Mazzitelli 1989; Ventura et al. 1998b) foi alterado de forma a incluir tais efeitos. A rotação foi implementada segundo a técnica de superfícies equipotenciais desenvolvida por Kippenhahn & Thomas (1970) e posteriormente aperfeiçoada por Endal & Sofia (1976). A redistribuição de momento angular foi modelada por intermédio de uma equação diferencial parcial do tipo difusivo-advectivo, com base na teoria originalmente desenvolvida por Chaboyer & Zahn (1992), a qual considera como único pressuposto que o transporte turbulento é muito mais forte na direção horizontal que na vertical. O coeficiente de difusão desta equação é obtido a partir de comprimentos e velocidades característicos de instabilidades hidrodinâmicas induzidas pela rotação. Este novo código foi empregado para calcular uma série de modelos com rotação de estrelas de baixa massa (na faixa de 0.6 M_ a 1.2 M_). As características gerais destes modelos, com relação aos efeitos estruturais (hidrostáticos) da rotação, mostram que estrelas com rotação comportam-se como se fossem estrelas sem rotação, porém de massa ligeiramente menor, estando de acordo com resultados anteriores por outros pesquisadores. Um estudo deste efeito de abaixamento de massa" para a faixa de massas considerada revela que a rotação diminui a queima de lítio enquanto a estrela é totalmente convectiva, mas aumenta a mesma tão logo a estrela desenvolve um núcleo radiativo. O efeito líquido é um aumento da queima de lítio, em desacordo com dados observacionais que mostram que as estrelas de aglomerados abertos jovens que giram mais rápido são as que apresentam menor queima de lítio. A redistribuição de momento angular nos modelos considerados torna-se muito eficaz na suavização do gradiente interno de velocidade angular tão logo a estrela atinja a idade zero na sequência principal, mas não é capaz de reproduzir a curva de rotação solar obtida da heliosismologia, indicando que o transporte de momento angular no Sol é mais eficiente do que o predito pelos atuais modelos. O transporte interno de momento angular também contribui para uma queima ainda maior de lítio com relação aos modelos calculados somente com os efeitos estruturais. Isto sugere que outros fenômenos físicos devem ser importantes tanto para a queima de lítio quanto para a evolução da curva de velocidade angular das estrelas.
36
Dopcke, Gustavo [Verfasser], and Ralf S. [Akademischer Betreuer] Klessen. "On the physics of the low metallicity IMF / Gustavo Dopcke ; Betreuer: Ralf S. Klessen." Heidelberg : Universitätsbibliothek Heidelberg, 2013. http://d-nb.info/1177148528/34.
37
Kumari, Nimisha. "Spatially-resolved studies of nearby star-forming galaxies." Thesis, University of Cambridge, 2018. https://www.repository.cam.ac.uk/handle/1810/283605.
Abstract:
Spatially-resolved studies of nearby star-forming galaxies are essential to understand various physical and chemical phenomena at play in the interstellar medium in the galaxies, and consequently to obtain a comprehensive picture of galaxy formation and evolution. In this thesis, I perform spatially-resolved analyses of chemical abundances and star-formation in nearby star-forming galaxies - blue compact dwarf galaxies (BCDs) and spiral galaxies. I map various properties of H II regions and the surrounding gas within three BCDs, using integral field spectroscopic (IFS) data from the Gemini Multi-Object Spectrograph-North. While answering questions related to chemical homogeneity, ionisation mechanisms and stellar populations within BCDs, I address more profound issues, which go beyond the characterisation of studied BCDs and aim to explain global phenomena with broader implications. The BCD NGC 4449 hosts a metal-poor central star-forming region, which I explain by various scenarios related to the interplay between star-formation, metal-distribution and gas dynamics within galaxies. The BCD NGC 4670 shows an unusual negative relationship between the nitrogen-to-oxygen ratio and oxygen abundance at spatially-resolved scales. I explore this relation with chemical evolution models and by comparison to other star-forming galaxies and suggest that nitrogen enrichment, variations in star-formation efficiency or hydrodynamical effects may be responsible for the observed relation. For another BCD, SBS 1415+437, the spatially-resolved abundances on average agree with the integrated abundance, implying that low-redshift spatially-resolved results may be directly compared with unresolved high-redshift results. I study spiral galaxies to address long-standing issues related to the reliability of metallicity calibrators and the Schmidt Law of star-formation. Using IFS data of twenty-four spiral galaxies taken with the Multi-Unit Spectroscopic Explorer, I find that the current strong-line metallicity calibrators for H II regions are unsuitable for regions dominated by diffuse ionised gas (DIG). I devise new recipes for estimating the metal-content of the DIG. For another set of nine spiral galaxies, I use multi-wavelength data to show that the spatially-resolved Schmidt relation is very sensitive to the consideration of diffuse background, which is a component unrelated to the current star-formation. Removal of this component from the SFR tracers and the atomic gas results in similar local and global Schmidt relation. To conclude, the spatially-resolved analyses presented in this thesis have led to discoveries and further questions, which I will address in my ongoing and future works.
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Cossburn, Martin R. "The study of low-mass stars and brown dwarfs." Thesis, University of Leicester, 1999. http://hdl.handle.net/2381/30616.
Abstract:
This thesis describes the search for brown dwarfs in open clusters using optical and infrared photometry, optical spectroscopy and surveys using the Hubble Space Telescope (HST). It also includes an observational study of very low-mass stars (VLMS) using a filter combination (I and Z), unused previously in this field enabling the calibration of the colour (I--Z), for future survey work. Following a short introduction explaining the reasons for studying brown dwarfs this thesis reviews the theory of their formation and evolution and describes recent searches for brown dwarfs and their results. Modern CCD technology has led to larger and larger area cluster surveys and so chapter 3 reviews this technology and the data reduction software and techniques necessary for analysis. Chapters 4, 5, 6 and 7 present the observational results. Chapter 4 presents an essentially null result of a deep HST survey in the Pleiades. Chapter 5 presents the d ata used to calibrate the I--Z colour and chapter 6 describes the discovery of PIZ 1, a brown dwarf in the Pleiades of mass 0.048 M⊙ and effective temperature ~2200K. Chapter 7 presents the infrared follow-up results of a number of recent large area optical surveys. Chapters 8 and 9 use the results from the latest surveys to determine the mass and luminosity functions of both Praesepe and the Pleiades. In Praesepe the mass function is calculated down to the brown dwarf limit in the cluster and shows no sign of turning down. In the Pleiades the mass function appears to be flattening across the stellar sub-stellar boundary. Finally in Chapter 10 I summarise the important conclusions from each chapter and identify areas of future work.
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Pinfield, David J. "Low mass stars and brown dwarfs in open clusters." Thesis, University of Leicester, 1998. http://hdl.handle.net/2381/30612.
Abstract:
This thesis describes several open cluster searches for very low mass stars (VLM) and brown dwarfs (BD) (M/M? < 0.08). With recent developments in charged coupled devices, it has become possible to survey open clusters to well into the BD regime. The principal aim of this work is to employ these new instruments to conduct such open cluster surveys, to investigate the properties of VLM and BD populations. The thesis begins with a short introduction, which describes the fundamental properties of brown dwarfs, relates research in this field to other areas of astronomy, and summarises the project aims and thesis structure. The second chapter gives a review of the current theoretical and observational status of the field, and the third chapter describes the principles and methods used to carry out the photometry presented in this thesis. The results are presented in chapters (4), (5), (6) and (7). An RIZ survey of Praesepe produces a sample of 26 candidate VLM and BDs. K band photometric follow up of 11 of these highlights one as a binary system that almost certainly contains at least one brown dwarf. K photometry is still needed for the remaining 15. The RIZ survey also finds a possible steepening of Praesepe's mass function into the BD regime. The Praesepe infrared sequence compares very well with the latest non gray model predictions, and the binary fraction in Praesepe is found to be essentially the same as for The Pleiades. A catalogue of Pleiades stars is used to model the cluster. King profiles are plotted for 4 mass bins and these profiles suggest that the cluster is relaxed. Virial analysis is then used to show that the cluster contains not more than 131M of dark matter (BDs) to add to the observed mass of 735M. Further deep surveys in The Pleiades, Praesepe and the Hyades result in 88 new Pleiades BD candidates, 141 Praesepe VLM candidates and 33 Hyades BD candidates. These new candidate lists, as well as finder charts are given in the appendices.
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Kenyon, Michael John. "Low mass stars and brown dwarfs around σ Orionis." Thesis, Keele University, 2004. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.409546.
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Lehner, Nicolas. "Interstellar gas and hot stars in low density environments." Thesis, Queen's University Belfast, 2000. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.343034.
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Pokorny, Richard S. "A high proper motion search for low mass stars." Thesis, Liverpool John Moores University, 2004. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.419966.
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Brown, Daniel. "Impact of rotation on the evolutionof low-mass stars." Thesis, Liverpool John Moores University, 2007. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.446410.
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Quanz, Sascha Patrick. "Circumstellar disks and envelopes around young low-mass stars." [S.l. : s.n.], 2007. http://nbn-resolving.de/urn:nbn:de:bsz:16-opus-74785.
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Ziegerer, Eva [Verfasser], and Ulrich [Gutachter] Heber. "Hypervelocity Stars - Kinematic studies of low-mass stars and hot subdwarfs / Eva Ziegerer ; Gutachter: Ulrich Heber." Erlangen : Friedrich-Alexander-Universität Erlangen-Nürnberg (FAU), 2017. http://d-nb.info/1152438239/34.
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Birkby, Jayne Louise. "Observational constraints on low-mass stellar evolution and planet formation." Thesis, University of Cambridge, 2012. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.610381.
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Saylor, Dicy, Sebastien Lepine, Ian Crossfield, and Erik A. Petigura. "Light-curve Modulation of Low-mass Stars in K2. I. Identification of 481 Fast Rotators in the Solar Neighborhood." IOP PUBLISHING LTD, 2017. http://hdl.handle.net/10150/626405.
Abstract:
The K2 mission is targeting large numbers of nearby (d < 100 pc) GKM dwarfs selected from the SUPERBLINK proper motion survey (mu > 40 mas yr(-1), V < 20). Additionally, the mission is targeting low-mass, high proper motion stars associated with the local (d < 500 pc) Galactic halo population also selected from SUPERBLINK. K2 campaigns 0 through 8 monitored a total of 26,518 of these cool main-sequence stars. We used the auto-correlation function to search for fast rotators by identifying short-period photometric modulations in the K2 light curves. We identified 481 candidate fast rotators with rotation periods < 4 days that show light-curve modulations consistent with starspots. Their kinematics show low average transverse velocities, suggesting that they are part of the young disk population. A subset (13) of the fast rotators is found among those targets with colors and kinematics consistent with the local Galactic halo population and may represent stars spun up by tidal interactions in close binary systems. We further demonstrate that the M dwarf fast rotators selected from the K2 light curves are significantly more likely to have UV excess and discuss the potential of the K2 mission to identify new nearby young GKM dwarfs on the basis of their fast rotation rates. Finally, we discuss the possible use of local halo stars as fiducial, non-variable sources in the Kepler fields.
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Covey, Kevin R. "Dynamical properties of embedded protostars and the luminosity function of the galactic disk /." Thesis, Connect to this title online; UW restricted, 2006. http://hdl.handle.net/1773/5448.
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Szigeti, László, Szabolcs Mészáros, Verne V. Smith, Katia Cunha, Nadège Lagarde, Corinne Charbonnel, D. A. García-Hernández, et al. "12C/13C isotopic ratios in red-giant stars of the open cluster NGC 6791." OXFORD UNIV PRESS, 2018. http://hdl.handle.net/10150/627135.
Abstract:
Carbon isotope ratios, along with carbon and nitrogen abundances, are derived in a sample of 11 red-giant members of one of the most metal-rich clusters in the Milky Way, NGC 6791. The selected red-giants have a mean metallicity and standard deviation of [Fe/H] = +0.39 +/- 0.06 (Cunha et al. 2015). We used high-resolution H-band spectra obtained by the SDSS-IV Apache Point Observatory Galactic Evolution Experiment. The advantage of using high-resolution spectra in the H band is that lines of CO are well represented and their line profiles are sensitive to the variation of C-12/C-13. Values of the C-12/C-13 ratio were obtained from a spectrum synthesis analysis. The derived C-12/C-13 ratios varied between 6.3 and 10.6 in NGC 6791, in agreement with the final isotopic ratios from thermohaline-induced mixing models. The ratios derived here are combined with those obtained for more metal poor red-giants from the literature to examine the correlation between C-12/C-13, mass, metallicity, and evolutionary status.
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Cuadrado, Regina Aznar. "Binarity and the origin of subdwarf B stars." Thesis, Queen's University Belfast, 2001. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.483466.