The Core Mass Growth and Stellar Lifetime of Thermally Pulsing Asymptotic Giant Branch Stars

We establish new constraints on the intermediate-mass range of the initial-final mass relation by studying white dwarfs in four young star clusters, and apply the results to study the evolution of stars on the thermally pulsing asymptotic giant branch (TP-AGB). We show that the stellar core mass on the AGB grows rapidly from 10% to 30% for stars with $M_{\rm initial}$ = 1.6 to 2.0 $M_\odot$. At larger masses, the core-mass growth decreases steadily to $\sim$10% at $M_{\rm initial}$ = 3.4 $M_\odot$. These observations are in excellent agreement with predictions from the latest TP-AGB evolutionary models in Marigo et al. (2013). We also compare to models with varying efficiencies of the third dredge-up and mass loss, and demonstrate that the process governing the growth of the core is largely the stellar wind, while the third dredge-up plays a secondary, but non-negligible role. Based on the new white dwarf measurements, we perform an exploratory calibration of the most popular mass-loss prescriptions in the literature. Finally, we estimate the lifetime and the integrated luminosity of stars on the TP-AGB to peak at $t$ $\sim$ 3 Myr and $E$ = 1.2 $\times$ 10$^{10}$ $L_\odot$ yr for $M_{\rm initial}$ $\sim$ 2 $M_\odot$ ($t$ $\sim$ 2 Myr for luminosities brighter than the RGB tip at $\log(L/L_{\odot})$ $>$ 3.4), decreasing to $t$ = 0.4 Myr and $E$ = 6.1 $\times$ 10$^{9}$ $L_\odot$ yr for stars with $M_{\rm initial}$ $\sim$ 3.5 $M_\odot$. The implications of these results are discussed with respect to general population synthesis studies that require correct modeling of the TP-AGB phase of stellar evolution.Comment: 14 pages, 7 figures, 4 tables. Accepted for publication in Ap

Analyse spectroscopique d'étoiles naines blanches riches en hydrogène(DA) : vers des modèles d'atmosphère améliorés sans paramètres libres

Le but de cette thèse est de raffiner et de mieux comprendre l'utilisation de la méthode spectroscopique, qui compare des spectres visibles de naines blanches à atmosphère riche en hydrogène (DA) à des spectres synthétiques pour en déterminer les paramètres atmosphériques (température effective et gravité de surface). Notre approche repose principalement sur le développement de modèles de spectres améliorés, qui proviennent eux-mêmes de modèles d'atmosphère de naines blanches de type DA. Nous présentons une nouvelle grille de spectres synthétiques de DA avec la première implémentation cohérente de la théorie du gaz non-idéal de Hummer & Mihalas et de la théorie unifiée de l'élargissement Stark de Vidal, Cooper & Smith. Cela permet un traitement adéquat du chevauchement des raies de la série de Balmer, sans la nécessité d'un paramètre libre. Nous montrons que ces spectres améliorés prédisent des gravités de surface qui sont plus stables en fonction de la température effective. Nous étudions ensuite le problème de longue date des gravités élevées pour les DA froides. L'hypothèse de Bergeron et al., selon laquelle les atmosphères sont contaminées par de l'hélium, est confrontée aux observations. À l'aide de spectres haute résolution récoltés au télescope Keck à Hawaii, nous trouvons des limites supérieures sur la quantité d'hélium dans les atmosphères de près de 10 fois moindres que celles requises par le scénario de Bergeron et al. La grille de spectres conçue dans ces travaux est ensuite appliquée à une nouvelle analyse spectroscopique de l'échantillon de DA du SDSS. Notre approche minutieuse permet de définir un échantillon plus propre et d'identifier un nombre important de naines blanches binaires. Nous déterminons qu'une coupure à un rapport signal-sur-bruit S/N > 15 optimise la grandeur et la qualité de l'échantillon pour calculer la masse moyenne, pour laquelle nous trouvons une valeur de 0.613 masse solaire. Finalement, huit nouveaux modèles 3D de naines blanches utilisant un traitement d'hydrodynamique radiative de la convection sont présentés. Nous avons également calculé des modèles avec la même physique, mais avec une traitement standard 1D de la convection avec la théorie de la longueur de mélange. Un analyse différentielle entre ces deux séries de modèles montre que les modèles 3D prédisent des gravités considérablement plus basses. Nous concluons que le problème des gravités élevées dans les naines blanches DA froides est fort probablement causé par une faiblesse dans la théorie de la longueur de mélange.The goal of this thesis is to refine and to understand better the spectroscopic method, which compares optical spectra of hydrogen-atmosphere white dwarfs (DA) with synthetic spectra to determine the atmospheric parameters (effective temperature and surface gravity). Our approach rests mainly on the development of improved model spectra, which come themselves from DA model atmospheres. We present a new grid of DA synthetic spectra with the first consistent implementation of the non-ideal gas theory of Hummer & Mihalas and the unified theory of Stark broadening from Vidal, Cooper & Smith. This allows for an adequate treatment of the quenching effects in Balmer lines, without the need of a free parameter. We show that these improved spectra predict surface gravities that are much more stable as a function of the effective temperature. We then study the long-standing problem that surface gravities in cool DA stars are significantly higher than those found in hotter DA white dwarfs. The hypothesis of Bergeron et al., according to which the atmospheres are contaminated by small amounts of helium, is constrained with observations. Using high-resolution spectra collected at the Keck in Hawaii, we find superior limits on the helium abundances in the atmospheres that are nearly 10 times lower than those required to sustain the Bergeron et al. scenario. The grid of spectra calculated in this work is then applied to a new spectroscopic analysis of the DA in the SDSS. Our careful analysis allows us to define a cleaner sample and to identify a large number of double degenerates. We find that a cutoff at a signal-to-noise ratio S/N > 15 optimizes the size and quality of the sample for computing the average mass, for which we find a value of 0.613 solar mass. Finally, eight new 3D white dwarf models with a radiative-hydrodynamics treatment of the convection are presented. We also calculated models with the same physics, except for a treatment of the convection with the standard mixing-length theory. A differential analysis between these two sets of models shows that the 3D models predict considerably lower surface gravities. We conclude that the high-log g problem in cool DA white dwarfs is caused by a weakness in the mixing-length theory

Étude photométrique des étoiles naines blanches dans le domaine infrarouge

Mémoire numérisé par la Direction des bibliothèques de l'Université de Montréal

Carbon dredge-up required to explain the Gaia white dwarf colour-magnitude bifurcation

The Gaia colour--magnitude diagram reveals a striking separation between hydrogen-atmosphere white dwarfs and their helium-atmosphere counterparts throughout a significant portion of the white dwarf cooling track. However, pure-helium atmospheres have Gaia magnitudes that are too close to the pure-hydrogen case to explain this bifurcation. To reproduce the observed split in the cooling sequence, it has been shown that trace amounts of hydrogen and/or metals must be present in the helium-dominated atmospheres of hydrogen-deficient white dwarfs. Yet, a complete explanation of the Gaia bifurcation that takes into account known constraints on the spectral evolution of white dwarfs has thus far not been proposed. In this work, we attempt to provide such a holistic explanation by performing population synthesis simulations coupled with state-of-the-art model atmospheres and evolutionary calculations that account for element transport in the envelopes of white dwarfs. By relying on empirically grounded assumptions, these simulations successfully reproduce the bifurcation. We show that the convective dredge-up of optically undetectable traces of carbon from the deep interior is crucial to account for the observations. Neither the convective dilution/mixing of residual hydrogen nor the accretion of hydrogen or metals can be the dominant drivers of the bifurcation. Finally, we emphasize the importance of improving theoretical models for the average ionization level of carbon in warm dense helium, which governs the shape of the diffusive tail of carbon and in turn the predicted amount of dredged-up carbon.Comment: Accepted for publication in MNRAS, minor changes following reports from reviewer

Initial-final mass relation from white dwarfs within 40 pc

We present an initial-final mass relation derived from the spectroscopically-complete volume-limited 40 pc sample of white dwarfs. The relation is modelled using population synthesis methods to derive an initial stellar population which can be fit to the observed mass distribution of white dwarfs. The population synthesis accounts for binary evolution, where higher-mass white dwarfs are more likely to be merger products than their lower-mass counterparts. Uncertainties are accounted from the initial mass function, stellar metallicity and age of the Galactic disc. We also consider biases induced by the spectral type of the white dwarf where pure-hydrogen atmosphere white dwarfs are likely to have more accurate masses, whilst the full white dwarf sample will have fewer biases arising from spectral evolution. We provide a four-piece segmented linear regression using Monte Carlo methods to sample the 1-$\sigma$ range of uncertainty on the initial stellar population. The derived initial-final mass relation provides a self-consistent determination of the progenitor mass for white dwarfs in the Solar neighbourhood which will be useful to study the local stellar formation history.Comment: Eleven pages. Accepted for publication in MNRA

The Onset of Convective Coupling and Freezing in the White Dwarfs of 47 Tucanae

Using images from the Hubble Space Telescope Advanced Camera for Surveys, we measure the rate of cooling of white dwarfs in the globular cluster 47 Tucanae and compare it to modelled cooling curves. We examine the effects of the outer convective envelope reaching the nearly isothermal degenerate core and the release of latent heat during core crystallisation on the white dwarf cooling rates. For white dwarfs typical of 47 Tuc, the onset of these effects occur at similar times. The latent heat released during crystallisation is a small heat source. In contrast, the heat reservoir of the degenerate core is substantially larger. When the convective envelope reaches the nearly isothermal interior of the white dwarf, the star becomes brighter than it would be in the absence of this effect. Our modelled cooling curves that include this convective coupling closely match the observed luminosity function of the white dwarfs in 47 Tuc.Comment: 6 pages, 5 figures. Submitted to MNRA

Gaia white dwarfs within 40 pc : I. Spectroscopic observations of new candidates

We present a spectroscopic survey of 230 white dwarf candidates within 40 pc of the Sun from the William Herschel Telescope and Gran Telescopio Canarias. All candidates were selected from Gaia Data Release 2 (DR2) and in almost all cases, had no prior spectroscopic classifications. We find a total of 191 confirmed white dwarfs and 39 main-sequence star contaminants. The majority of stellar remnants in the sample are relatively cool (〈Teff〉 = 6200 K), showing either hydrogen Balmer lines or a featureless spectrum, corresponding to 89 DA and 76 DC white dwarfs, respectively. We also recover two DBA white dwarfs and 9–10 magnetic remnants. We find two carbon-bearing DQ stars and 14 new metal-rich white dwarfs. This includes the possible detection of the first ultra-cool white dwarf with metal lines. We describe three DZ stars for which we find at least four different metal species, including one that is strongly Fe- and Ni-rich, indicative of the accretion of a planetesimal with core-Earth composition. We find one extremely massive (1.31 ± 0.01 M⊙) DA white dwarf showing weak Balmer lines, possibly indicating stellar magnetism. Another white dwarf shows strong Balmer line emission but no infrared excess, suggesting a low-mass sub-stellar companion. A high spectroscopic completeness (>99 per cent) has now been reached for Gaia DR2 sources within 40-pc sample, in the Northern hemisphere (δ > 0°) and located on the white dwarf cooling track in the Hertzsprung–Russell diagram. A statistical study of the full northern sample is presented in a companion paper

The ubiquity of carbon dredge-up in hydrogen-deficient white dwarfs as revealed by GALEX

The convective dredge-up of carbon from the interiors of hydrogen-deficient white dwarfs has long been invoked to explain the presence of carbon absorption features in the spectra of cool DQ stars ($T_{\rm eff} < 10{,}000\,{\rm K}$). It has been hypothesized that this transport process is not limited to DQ white dwarfs and also operates, albeit less efficiently, in non-DQ hydrogen-deficient white dwarfs within the same temperature range. This non-DQ population is predominantly composed of DC white dwarfs, which exhibit featureless optical spectra. However, no direct observational evidence of ubiquitous carbon pollution in DC stars has thus far been uncovered. In this Letter, we analyze data from the Galaxy Evolution Explorer (GALEX) to reveal the photometric signature of ultraviolet carbon lines in most DC white dwarfs in the $8500\,{\rm K} \leq T_{\rm eff} \leq 10{,}500\,{\rm K}$ temperature range. Our results show that the vast majority of hydrogen-deficient white dwarfs experience carbon dredge-up at some point in their evolution.Comment: Accepted for publication in MNRAS Letter