In this thesis, I focus on the physical and chemical evolution at the earliest stages of low-mass star formation. I report results from the Spitzer Space Telescope and molecular line observations of 9 species toward the dark cloud L43, a survey of 10 Class 0 and 6 Class I protostars with 8 molecular lines, and a survey of 9 Very Low Luminosity Objects (VeLLOs) with 11 molecular lines. From the observational results, CO depletion is extensively observed with C¹⁸O(2-1) maps. A general evolutionary trend is also seen toward the Class 0 and I samples: higher deuterium fractionation at higher CO depletion. For the VeLLO candidates and starless cores with N₂D⁺(3-2) detection, we found the deuterium ratio of N₂D⁺/N₂H⁺ is higher comparing with the Class 0 and I samples. We use DCO⁺(3-2) maps to trace the velocity structures. Also, HCO⁺(3-2) blue profiles are seen toward the VeLLO candidate L328, indicating possible infall. To test theoretical models and to interpret the observations, we adopt a modeling sequence with self-consistent calculations of dust radiative transfer, gas energetics, chemistry, and line radiative transfer. In the L43 region described in Chapter 2, a starless core and a Class I protostar are evolving in the same environment. We modeled both sources with the same initial conditions to test the chemical characteristics with and without protostellar heating. The physical model consists of a series of Bonner-Ebert spheres describing the pre-protostellar (PPC) stages following by standard inside-out collapse (Shu 1977). The model best matches the observed lines suggests a longer total timescale at the PPC stage, with faster evolution at the later steps with higher densities. In Chapter 3, we modeled the entire group of Class 0 and I protostars. The trend of decreasing deuterium ratio can be seen after the temperature is high enough for CO to evaporate. After the evaporation, the history of heavy depletion (e.g, from longer PPC timescales or different grain surface properties) no longer affects the line intensities of gas-phase CO. The HCO⁺ blue profiles, which are used as infall indicators, are predicted to be observed when infall is beyond the CO evaporation front. The low luminosity of VeLLOs cannot be explained by standard models with steady accretion, and we tested an evolutionary model incorporating episodic accretion to investigate the thermal history and chemical behaviors. We tested a few chemical parameters to compare with the observations and the results from Chapter 2 and 3. The modeling results from episodic accretion models show that CO and N₂ evaporate from grain mantle surfaces at the accretion bursts and can freeze back onto grain surfaces during the long periods of quiescent phases. Deuterated species, such as N₂D⁺ and H₂D⁺, are most sensitive to the temperature. Possible good tracers for the thermal history include the line intensities of gas-phase N₂H+ relative to CO, as well as CO₂ and CO ice features. / text

We report on a search for short -period binary systems composed of pairs of evolved stars. The search is being carried out concurrently with a program to characterize the kinematical properties of two different samples of stars. Each sample has produced one close binary candidate for which further spectroscopic observations are planned. We also recapitulate the discovery of a close detached binary system composed of two cool DA white dwarfs, and we discuss the null results of Ha observations of the suspected white dwarf /brown dwarf system G 29-38.

Les nébuleuses planétaires (NP) sont le produit de l'évolution d'étoiles de masses intermédiaires après leur expansion sphérique à la fin de leurs vies. Il a été estimé observationnellement que 80 % des NP ont des formes non-sphériques. Une fraction si élevée est déroutante et a mobilisé la communauté de recherche sur les NP pendant plus de trente ans. Un scénario qui permettrait de justifier les formes observées serait que les étoiles progénitrices de noyaux de NP (NNP) ne sont pas simples, mais possèdent un compagnon. Les formes des nébuleuses seraient ainsi le résultat de l'interaction avec le compagnon. La fraction si élevée de NP non-sphériques impliquerait donc une fraction élevée de NNP binaires, faisant de la parité stellaire un canal de formation privilégié pour les NP. Après avoir présenté l'état de connaissance actuelle concernant la formation et la mise en forme des NP, je présente mes travaux visant à détecter un excès infrarouge qui serait la signature de la présence d'un compagnon orbitant le NNP. La première partie de ce projet consiste en l'analyse de données et photométrie acquises par moi-même. Dans la deuxième partie je présente une tentative d'utilisation de jeux de données d'archives : la campagne optique Sloan Digital Sky Survey Data Release 7 et la version étendue de la base de données assemblée par Frew (2008). Je présente aussi les résultats d'une analyse de vitesses radiales de spectres VLT/UVES pour 14 NNP dans le but de détecter des compagnons spectroscopiques. Finalement j'expose les détails d'une analyse de photométrie de données optiques dans le but de détecter des compagnons orbitant autour de NNP en utilisant la technique de variabilité photométrique. Le résultat principal de cette thèse réside dans les analyses d'excès infrarouge proche que je combine avec des données publiées précédemment. Je conclus que si la fraction détectée d'excès infrarouge proche est attribuée à la présence de compagnons stellaires, alors la fraction binaire de NNP est plus grande que celle attendue en se basant sur la population binaire de progéniteurs de la séquence principale et ainsi conclus que la multiplicité stellaire est un canal de formation privilégié pour la formation des NP. Je clos en soulignant la nécessité d'un échantillon d'étude d'environ 150 objets pour réduire l'incertitude sur la fraction binaire et appuyer les conclusions statistiques de ce résultat. / Planetary nebulae (PNe) are the products of the evolution of intermediate mass stars that have expanded spherically at the end of their lives. Observationally, it has been estimated that 80% of them have non-spherical shapes. Such a high fraction is puzzling and has occupied the PN community for more than 30 years. One scenario that would allow to justify the observed shapes is that a comparable fraction of the progenitors of central stars of PN (CSPN) are not single, but possess a companion. The shape of the nebulae would then be the result of an interaction with this companion. The high fraction of non-spherical PNe would thus imply a high fraction of binary CSPN, making binarity a preferred channel for PN formation. After presenting the current state of knowledge regarding PN formation and shaping and reviewing the diverse efforts to find binaries in PNe, I present my work to detect a near-infrared excess that would be the signature of the presence of cool companions. The first part of the project consists in the analysis of data and photometry acquired and conducted by myself. The second part details an attempt to make use of archived datasets: the Sloan Digital Sky Survey Data Release 7 optical survey and the extended database assembled by Frew (2008). I also present results from a radial velocity analysis of VLT/UVES spectra for 14 objects aiming to the detection of spectroscopic companions. Finally I give details of the analysis of optical photometry data from our observations associated to the detection of companions around CSPN using the photometric variability technique. The main result of this thesis is from the near-infrared excess studies which I combine with previously-published data. I conclude that the if the detected red and NIR flux excess is indicative of a stellar companion then the binary fraction is larger than what we may expect based on the main-sequence progenitor population binary fraction and therefore conclude that binarity is a preferential channel for the formation of PN. I finish by underlining the need for a sample size of ∼ 150 objects to decrease the uncertainty on the PN population binary fraction and increase the statistical significance of this result.

The discovery of superluminous supernovae in the past decade challenged our understanding of explosive stellar death. Subsequent extensive observations of superluminous supernova light curves and spectra has provided some insight for the nature of these events. We present observations of one of the most luminous self-interacting supernovae ever observed, the hydrogen-rich SN 2008am discovered by the Robotic Optical Transient Search Experiment Supernova Verification Project with the ROTSE-IIIb telescope located in the McDonald Observatory. We provide theoretical modeling of superluminous supernova light curves and fit the models to a number of observed events and similar transients in order to understand the mechanism that is responsible for the vast amounts of energy emitted by these explosions. The models we investigate include deposition of energy due to the radioactive decays of massive amounts of nickel-56, interaction of supernova ejecta with a dense circumstellar medium and magnetar spin-down. To probe the nature of superluminous supernovae progenitor stars we study the evolution of massive stars, including important effects such as rotation and magnetic fields, and perform multi-dimensional hydrodynamics simulations of the resulting explosions. The effects of rotational mixing are also studied in solar-type secondary stars in cataclysmic variable binary star systems in order to provide an explanation for some carbon-depleted examples of this class. We find that most superluminous supernovae can be explained by violent interaction of the SN ejecta with >1 Msun dense circumstellar shells ejected by the progenitor stars in the decades preceding the SN explosion. / text

Open clusters are important tools for studying of the Galactic disk and for understanding stellar evolution. Stars of clusters have constant age, composition and distance. This makes them to be excellent laboratories for investigations of stellar and Galactic chemical evolution. The main aim of the study is the analysis of mixing tracers and extra mixing evidences in photospheres of evolved stars. Extra-mixing processes may modify the surface abundances of 12^C/13^C and 12^C/14^N ratios. This study aims to test theoretical models. The secondary goal is a contribution to the study of present properties of the Galactic disc. The 12^C/13^C ratios in helium-core-burning clump stars for the clusters with turn-off masses lower than 2 solar masses are in agreement with the Cool-bottom processing (CBP) model and the Thermohaline mixing (TH) model. The observed 12^C/13^C ratios of the helium-core-burning stars in the open cluster NGC 6134 support evidences of larger then theoretically predicted extra-mixing in stars heavier than 2.5 solar masses. In two open clusters the 12^C/13^C ratios in core-helium-burning stars are lower than in the first ascent giants, located above the RGB luminosity bump. This could be caused by the material mixing during a very violent helium flash event. Almost all element to iron ratios show essentially flat radial abundance distributions across Galactic disk. The radial distributions of [O/Fe] and [Mg/Fe] have a tendency to increase towards larger... [to full text] / Padrikieji spiečiai naudojami astrofizikoje, tiriant Galaktikos disko cheminę raidą bei žvaigždžių evoliuciją. Spiečiaus žvaigždžių amžius, atstumas nuo Saulės, pirminė cheminė sudėtis yra apytiksliai vienodi, tad juos galima laikyti kosminėmis laboratorijomis. Pagrindinis disertacijos tikslas skirtas tirti maišymosi procesus evoliucionavusiose žvaigždėse. Siekiama įvertinti kaip maišymosi procesai paveikia 12^C/13^C ir 12^C/14^N gausų santykius ir palyginti su teoriniais maišymosi modeliais. Kitas tikslas skirtas prisidėti prie Galaktikos disko cheminės sudėties bei evoliucijos tyrimo. 12^C/13^C ir 12^C/14^N gausų santykiai nustatyti penkių spiečių evoliucionavusių žvaigždžių atmosferose. Helį centre deginančiose žvaigždėse kurių žvaigždžių posūkio iš pagrindinės sekos masės (M_toff) yra mažesnės nei 2 saulės masės, 12^C/13^C santykiai gerai sutampa su termohalinio (TH) bei šaltojo žemutinių sluoksnių (CBP) papildomo maišymosi modeliais, o tokių spiečių kurių M_toff yra didesnės nei 2,5 saulės masių papildomas maišymasis yra didesnis nei numato teoriniai modeliai. Dviejuose padrikuosiuose spiečiuose anglies izotopų gausų santykiai helį centre deginančių žvaigždžių atmosferose yra mažesni nei žvaigždėse, perkopusiose šviesio mazgą raudonųjų milžinių sekoje. Tai galėtų būti dėl galimo papildomo medžiagos maišymosi helio žybsnio metu. Beveik visų tirtų elementų radialinis pasiskirstymas Galaktikoje yra iš esmės plokščias, bet [O/Fe] ir [Mg/Fe] gausos turi tendenciją augti... [toliau žr. visą tekstą]

Padrikieji spiečiai naudojami astrofizikoje, tiriant Galaktikos disko cheminę raidą bei žvaigždžių evoliuciją. Spiečiaus žvaigždžių amžius, atstumas nuo Saulės, pirminė cheminė sudėtis yra apytiksliai vienodi, tad juos galima laikyti kosminėmis laboratorijomis. Pagrindinis disertacijos tikslas skirtas tirti maišymosi procesus evoliucionavusiose žvaigždėse. Siekiama įvertinti kaip maišymosi procesai paveikia 12^C/13^C ir 12^C/14^N gausų santykius ir palyginti su teoriniais maišymosi modeliais. Kitas tikslas skirtas prisidėti prie Galaktikos disko cheminės sudėties bei evoliucijos tyrimo. 12^C/13^C ir 12^C/14^N gausų santykiai nustatyti penkių spiečių evoliucionavusių žvaigždžių atmosferose. Helį centre deginančiose žvaigždėse kurių žvaigždžių posūkio iš pagrindinės sekos masės (M_toff) yra mažesnės nei 2 saulės masės, 12^C/13^C santykiai gerai sutampa su termohalinio (TH) bei šaltojo žemutinių sluoksnių (CBP) papildomo maišymosi modeliais, o tokių spiečių kurių M_toff yra didesnės nei 2,5 saulės masių papildomas maišymasis yra didesnis nei numato teoriniai modeliai. Dviejuose padrikuosiuose spiečiuose anglies izotopų gausų santykiai helį centre deginančių žvaigždžių atmosferose yra mažesni nei žvaigždėse, perkopusiose šviesio mazgą raudonųjų milžinių sekoje. Tai galėtų būti dėl galimo papildomo medžiagos maišymosi helio žybsnio metu. Beveik visų tirtų elementų radialinis pasiskirstymas Galaktikoje yra iš esmės plokščias, bet [O/Fe] ir [Mg/Fe] gausos turi tendenciją augti... [toliau žr. visą tekstą] / Open clusters are important tools for studying of the Galactic disk and for understanding stellar evolution. Stars of clusters have constant age, composition and distance. This makes them to be excellent laboratories for investigations of stellar and Galactic chemical evolution. The main aim of the study is the analysis of mixing tracers and extra mixing evidences in photospheres of evolved stars. Extra-mixing processes may modify the surface abundances of 12^C/13^C and 12^C/14^N ratios. This study aims to test theoretical models. The secondary goal is a contribution to the study of present properties of the Galactic disc. The 12^C/13^C ratios in helium-core-burning clump stars for the clusters with turn-off masses lower than 2 solar masses are in agreement with the Cool-bottom processing (CBP) model and the Thermohaline mixing (TH) model. The observed 12^C/13^C ratios of the helium-core-burning stars in the open cluster NGC 6134 support evidences of larger then theoretically predicted extra-mixing in stars heavier than 2.5 solar masses. In two open clusters the 12^C/13^C ratios in core-helium-burning stars are lower than in the first ascent giants, located above the RGB luminosity bump. This could be caused by the material mixing during a very violent helium flash event. Almost all element to iron ratios show essentially flat radial abundance distributions across Galactic disk. The radial distributions of [O/Fe] and [Mg/Fe] have a tendency to increase towards larger... [to full text]

In this work the effect of rotation on oscillation frequencies have been inves- tigated . Rotation has been treated as a perturbation and detailed calculations were done on the infuence of second order rotation . We used an evolutionary model of DELTA-Scuti star V1162 Ori with a mass of 1:8 solar mass. The eigenfrequencies were calculated in two cases , the slow rotation case with vsini = 46km/s and the fast rotation case with vsini = 61.9km/s. Calculation were carried out by a modifed oscillation program and results were compared to observations .

La diffusion atomique doit être prise en compte dans les modèles d'évolution stellaire car il s'agit d'une conséquence directe du fait que les étoiles sont des sphères auto-gravitantes composées d'un mélange de différents gaz (les éléments chimiques). L'équilibre des étoiles conduit à des gradients internes de pression, de densité et de température ainsi qu'à un transfert radiatif, l'ensemble produisant un effet sélectif sur les éléments (dans la plupart des cas dominé par la compétition entre le triage gravitationnel et les accélérations radiatives).Les interactions entre la diffusion atomique et les processus hydrodynamiques tels que la convection dynamique et la perte de masse sont étudiées depuis longtemps. Un processus important a cependant été oublié dans les modèles. Il s'agit de la convection double diffusive (ou thermohaline) induite par un gradient de μ instable, qui peut être produite par une accumulation locale d'éléments lourds à l'intérieur des étoiles due aux accélérations radiatives. Contrairement aux autres processus de mélange, il s'agit d'une conséquence directe de la diffusion atomique. Un effet similaire se produit à la base de la zone convective de surface en cas d'accrétion d’éléments lourds à la surface de l'étoile.Nous avons étudié la convection thermohaline induite par l'accrétion dans le cas du système 16 Cygni et les propriétés de ces deux étoiles avec le code TGEC. Nous avons inclus la prescription de Brown et al. 2013 pour la convection thermohaline. Nous avons ensuite calculé les fréquences d'oscillations de ces modèles à l'aide du code PULSE pour les comparer aux fréquences observées par Kepler. A partir de ces modèles, nous avons pu montrer qu'en accrétant 2/3 de la masse terrestre au début de la séquence principale (sur le modèle 16 Cyg B), la convection thermohaline induite par l'accrétion mélangeait l'étoile suffisamment profondément pour atteindre la zone de destruction du lithium et ainsi obtenir des abondances de lithium cohérentes avec les observations de 16 Cyg A et B.Nous avons étudié l'accumulation d’éléments lourds et l'effet de la convection thermohaline dans le cas des étoiles de type A. Dans ces étoiles, des abondances "particulières" (par rapport au soleil) ont été observées. Ceci est dû aux effets de la diffusion atomique qui sont très importants dans ces étoiles. Cependant, la diffusion atomique seule produit des variations d'abondances trop importantes et un moyen de reproduire les observations est de mélanger l'étoile assez profondément. Nous avons ensuite calculé des modèles incluant la diffusion atomique et la convection thermohaline en utilisant le code TGEC. Nous avons montré que ce processus pouvait modifier la structure interne de ces étoiles, et aussi les abondances de surface. Nous avons aussi inclus la convection thermohaline et l'accrétion dans le code de Montréal/Montpellier. Nous avons modifié plusieurs parties de ce code afin de pouvoir faire des comparaisons avec le TGEC pour comparer les résultats. Les résultats obtenus sont très similaires.Nous avons aussi déterminé les paramètres de l'étoile 94 Ceti à partir d'observations obtenues avec un instrument au sol. Cette étoile à une masse de 1.44 MΘ et est une bonne cible pour étudier l'effet des accélérations radiatives (qui ont un effet non négligeable pour des masses supérieures à 1.2 MΘ). Nous avons aussi comparé des modèles incluant des atmosphères complets afin d'en déterminer l'impact sur les fréquences.Nous avons travaillé sur les étoiles du halo pauvres en métaux pour lesquelles est observé une dispersion inexpliquée des abondances de lithium pour les métallicité très faible. Nous avons étudié la possibilité d'une accrétion sur ces étoiles qui pourrait produire de la convection thermohaline et détruire du lithium. / Atomic diffusion must be taken into account in the computations of stellar structure and evolution as it is a direct consequence of the fact that stars are self-gravitating spheres composed of a mixture of different gases (the chemical elements). The stellar equilibrium leads to internal gradients of pressure, density and temperature as well as an upward radiative transfer which produces a selective effect on the elements (in most cases dominated by the competition between gravitational settling and radiative acceleration).The interactions between atomic diffusion and well-known hydrodynamical processes like dynamical convection and mass loss have been studied for a long time. An important process was however forgotten in these computations. This is the double-diffusive (or fingering or thermohaline) convection induced by unstable μ-gradients, which can be produced by the local accumulation of heavy elements inside stars due to radiative acceleration. Contrary to the other hydrodynamical processes, fingering convection is not arbitrarily added in the computations. It is directly induced by atomic diffusion itself and cannot be avoided. It is thus very important to add this hydrodynamical process in stellar evolution modelling, which has never been done before our work. A similar effect occurs below the convective zone in case of accretion of heavy matter onto a star.We studied the accretion-induced fingering convection in the case of the stellar system 16 Cygni. We studied the properties of these two stars by computing models with the Toulouse Geneva Evolution Code (TGEC). We included the Brown et al. 2013 prescription for the computations of fingering convection in the code. We computed oscillation frequencies of these models using the PULSE code to compare it with Kepler observations. We found that if 2/3 of Earth mass is accreted at the beginning of the main sequence (on 16 Cyg B model), the accretion-induced fingering convection mixes the star deep enough to destroy the lithium and obtain the observed difference between 16 Cyg A and B.We studied the heavy element accumulation and the induced fingering convection in the case of Am stars. In these stars, peculiar surface abundances are observed (compared to the sun). This peculiarity is related to the effect of atomic diffusion, very important in these types of stars. However, atomic diffusion alone leads to abundance variations which are too large and one way to reproduce the observed abundance quantitatively is to assume mixing deep enough inside the star. We computed models including atomic diffusion (with radiative acceleration) and fingering convection with this prescription using the TGEC code. We find than this process may change the internal structure of the stars, and also the surface abundances. We also included fingering convection and the accretion process in the Montreal/Montpellier code. We modified some parts of this code (e.g. turbulence profiles) to compare the results obtained with the two codes. We computed some models and I found that the results are quite similar.We determined the stellar parameters of the star 94 Ceti (by using similar seismic computations as for 16 Cyg A and B) using ground-based observations. This star has a mass of 1.44 MΘ and is a good target to study the effect of radiative accelerations (which occur for masses larger than 1.2-1.3 MΘ). We also compared models with full atmosphere with the observations to determine the impact on oscillation frequencies.We worked on metal poor halo stars for which a dispersion of lithium surface abundance is observed for very small metallicities. We studied the possibility of an accretion of matter that can trigger fingering convection and destroy lithium.

Estrelas anãs brancas são o último estágio evolutivo observável de mais de 95% das estrelas e também um resultado comum na evolução de estrelas binárias. O estudo de anãs brancas é, portanto, uma ferramenta poderosa na compreensão da evolução de estrelas simples e binárias, da função de massa inicial local, e da perda de massa após a sequência principal, levando-nos a uma melhor compreensão do histórico de formação e evolução estelar de diferentes populações. Para que esses estudos sejam possíveis, é necessária uma amostra grande e preferencialmente completa de anãs brancas, cobrindo todo o intervalo de parâmetros físicos. A maneira mais simples de obter isso é utilizando dados de grandes projetos de mapeamento. O Sloan Digital Sky Survey já permitiu o aumento do número de anãs brancas conhecidas em cinco vezes até o data release 10. Neste trabalho, estendemos a busca por anãs brancas aos novos objetos no data release 12, descobrindo 3 157 novas anãs brancas e 1 349 novas subanãs. Pela primeira vez, estendemos essa busca para log g < 6.5, correspondente a M < 0.3 M⊙. Anãs brancas abaixo desse limite de massa não podem formar-se em um tempo de Hubble; contudo, se a estrela é parte de um sistema de binárias próximas, a perda de massa pode ser tão intensa que a anã branca resultante tem massa inferior ao limite por evolução simples. Esses objetos são chamados anãs brancas de massa extremamente-baixa (ELMs, do inglês extremely-low mass white dwarfs) Elas têm Teff < 20 000 K e 5.0 . log g . 6.5 e espectros muito similares a estrelas A de sequência principal. Menos de cem são conhecidas, e a maioria dos objetos foi descoberta tendo em conta um critério de seleção tendencioso, que excluiu ELMs frias (Teff < 9 000 K) e com mais baixa massa (M . 0.15 M⊙), tornando difícil verificar modelos e compreender as propriedades das ELMs como classe. Nós identificamos milhares de objetos cujas propriedades físicas, temperatura efetiva e log g, estão no intervalo de produtos de evolução binária, como as ELMs. Nós os chamamos de sdAs, referindo-nos ao seu log g que as coloca abaixo da sequência principal e seu espectro dominado por hidrogênio. As sdAs parecem conter populações estelares sobrepostas, e encontramos que 7% são mais compatíveis com (pre-)ELMs do que com objetos de sequência principal. Obtivemos espectroscopia resolvida temporalmente para 26 objetos e pudemos confirmar que 15 estão em binárias. Um objeto faz parte de um sistema eclipsante, enquanto outro é pulsante — o oitavo membro da classe de ELMs pulsantes. Outros seis objetos também apresentaram pulsações em nossas observações, cinco desses estão na vizinhança da faixa de instabilidade das ELMs. Com estes resultados, aumentamos a população de ELMs por um fator de 20%, elevando a fração de ELMs de 4 para 20%, um valor que é consistente com as previsões de modelos evolucionários. / White dwarf stars are the final observable evolutionary state of over 95% of stars and also a common outcome in binary evolution. Therefore, studying white dwarfs is a powerful tool to understand both single and binary stellar evolution, local initial mass function, and post-main sequence mass loss, leading us to a better comprehension of the history of stellar formation and evolution of different stellar populations. In order to make this type of studies possible, a large and preferably complete sample of white dwarf stars, covering the whole range of physical parameters, is required. The simplest way to achieve that is to take advantage of data provided by large surveys. The Sloan Digital Sky Survey has already allowed the increase of the number of known white dwarf stars fivefold up to its data release 10. In this work, we extended the search for white dwarfs to the new objects in the data release 12, discovering 3 157 new white dwarfs and 1 349 new subdwarfs. For the first time, we have extended this search to log g < 6.5, corresponding to M < 0.3 M⊙. White dwarfs below this mass limit cannot be formed through single evolution within a Hubble time; however, if the star is part of a close binary system, the mass loss of the system may be so intense that the resulting white dwarf has mass below the single evolution limit. These objects are known as extremely-low mass white dwarfs (ELMs) They show Teff < 20 000 K and 5.0 . log g . 6.5 and spectra very similar to main sequence A stars. Less than a hundred of them are known, and most objects were discovered relying on biased selection criteria, that excluded cool (Teff < 9 000 K), lowermass (M . 0.15 M⊙) ELMs, making it difficult to validate the models and comprehend the properties of the ELMs as a class. We have identified thousands of objects whose physical properties, effective temperature and surface gravity, place them in the range of by-products of binary interaction such as the ELMs. We have called them sdAs, referring to their sub-main sequence log g and hydrogen dominated spectra. They seem to be composed of overlapping stellar populations, and we found that at least 7% are more likely ELMs or their precursors, the pre-ELMs, than main sequence stars. Obtaining time-resolved spectroscopy for 26 objects, we could confirm 15 to be in close binaries. One of them is also an eclipsing system, while another is a pulsator — the eighth member of the pulsating ELM class. Other six new pulsators were found as part of our follow-up, five of them in the vicinity of the ELM instability strip. With these results, we increase the population of ELMs by 20%, raising the fraction of cool ELMs from 4 to 20%, which is consistent with the predictions from the evolutionary models.

abstract: I present a catalog of 1,794 stellar evolution models for solar-type and low-mass stars, which is intended to help characterize real host-stars of interest during the ongoing search for potentially habitable exoplanets. The main grid is composed of 904 tracks, for 0.5-1.2 M_sol at scaled metallicity values of 0.1-1.5 Z_sol and specific elemental abundance ratio values of 0.44-2.28 O/Fe_sol, 0.58-1.72 C/Fe_sol, 0.54-1.84 Mg/Fe_sol, and 0.5-2.0 Ne/Fe_sol. The catalog includes a small grid of late stage evolutionary tracks (25 models), as well as a grid of M-dwarf stars for 0.1-0.45 M_sol (856 models). The time-dependent habitable zone evolution is calculated for each track, and is strongly dependent on stellar mass, effective temperature, and luminosity parameterizations. I have also developed a subroutine for the stellar evolution code TYCHO that implements a minimalist coupled model for estimating changes in the stellar X-ray luminosity, mass loss, rotational velocity, and magnetic activity over time; to test the utility of the updated code, I created a small grid (9 models) for solar-mass stars, with variations in rotational velocity and scaled metallicity. Including this kind of information in the catalog will ultimately allow for a more robust consideration of the long-term conditions that orbiting planets may experience. In order to gauge the true habitability potential of a given planetary system, it is extremely important to characterize the host-star's mass, specific chemical composition, and thus the timescale over which the star will evolve. It is also necessary to assess the likelihood that a planet found in the "instantaneous" habitable zone has actually had sufficient time to become "detectably" habitable. This catalog provides accurate stellar evolution predictions for a large collection of theoretical host-stars; the models are of particular utility in that they represent the real variation in stellar parameters that have been observed in nearby stars. / Dissertation/Thesis / Doctoral Dissertation Astrophysics 2017