A New Generation of Cool White Dwarf Atmosphere Models Using Ab Initio Calculations

Due to their high photospheric density, cool helium-rich white dwarfs (particularly DZ, DQpec and ultracool) are often poorly described by current atmosphere models. As part of our ongoing efforts to design atmosphere models suitable for all cool white dwarfs, we investigate how the ionization ratio of heavy elements and the H$_2$-He collision-induced absorption (CIA) spectrum are altered under fluid-like densities. For the conditions encountered at the photosphere of cool helium-rich white dwarfs, our ab initio calculations show that the ionization of most metals is inhibited and that the H$_2$-He CIA spectrum is significantly distorted for densities higher than 0.1 g/cm$^3$.Comment: 4 pages, 2 figures, submitted for publication in the proceedings of the 20th European Workshop on White Dwarf

Pressure Distortion of the H2_2-He Collision-Induced Absorption at the Photosphere of Cool White Dwarf Stars

Collision-induced absorption (CIA) from molecular hydrogen is a dominant opacity source in the atmosphere of cool white dwarfs. It results in a significant flux depletion in the near-IR and IR parts of their spectra. Because of the extreme conditions of helium-rich atmospheres (where the density can be as high as a few g/cm$^3$), this opacity source is expected to undergo strong pressure distortion and the currently used opacities have not been validated at such extreme conditions. To check the distortion of the CIA opacity we applied state-of-the-art ab initio methods of computational quantum chemistry to simulate the CIA opacity at high densities. The results show that the CIA profiles are significantly distorted above densities of $0.1\,{\rm g/cm}^3$ in a way that is not captured by the existing models. The roto-translational band is enhanced and shifted to higher frequencies as an effect of the decrease of the interatomic separation of the H$_2$ molecule. The vibrational band is blueward shifted and split into $Q_R$ and $Q_P$ branches, separated by a pronounced interference dip. Its intensity is also substantially reduced. The distortions result in a shift of the maximum of the absorption from $2.3\,\mu{\rm m}$ to $3-7 \mu{\rm m}$, which could potentially explain the spectra of some very cool, helium-rich white dwarfs.Comment: 12 pages, 13 figures. Accepted for publication in The Astrophysical Journa

La liberté dans son rapport à l'autorité chez Soren Kierkegaard

Ce mémoire se propose d'étudier la conception de la liberté chez Soren Kierkegaard. On y voit sa conception du paradoxe, de la qualité et du saut qualitatif, lesquels constituent la dialectique de la liberté correspondant à son concept d'existence. L'existence est une première base de la liberté. L'autre base est l'intériorité. En l'homme, le christianisme voit un combat intérieur pour devenir meilleur. Notre nature doit faire l'objet d'une perpétuelle réaffirmation, laquelle comporte effort et souffrance. Nous manifestons notre liberté à partir de cette intériorité en devenir et dans une angoisse. L'angoisse est l'état, concrètement envisagé, de l'homme libre devant prendre une décision. L'angoisse vient de tous ces possibles entre lesquels il faudra choisir. Nous examinons également quel est le statut de cet idéal auquel le chrétien doit se rapporter. Nous développons la notion du péché, et nous faisons ressortir quelles sont les caractéristiques du premier choix libre chrétien, c'est-à-dire la chute. Il ressort de notre analyse que la liberté se rapporte à l'autorité par son intériorité. L'autorité a pour elle l'être de l'idéal. De même, toute autorité se doit, de près ou de loin, de correspondre à l'idéal de l'assujetti, sinon il y a contrainte, donc autoritarisme. ______________________________________________________________________________ MOTS-CLÉS DE L’AUTEUR : Kierkegaard, Existence, Péché, Liberté, Autorité, Théologie, Philosophie, Angoisse, Intériorité, Dieu, Péché originel, Transcendance, Paradoxe, Individu, Désespoir

Étude de la relation structure-fonction d'un riborégulateur lysine

Une imposante portion du génome d'un organisme ne sera jamais exprimée sous forme de protéines. Cet acide désoxyribonucléique (ADN) non codant se trouve cependant à être, én bonne partie, transcrit en acide ribonucléique _(ARN). Depuis plusieurs années, les études menées sur ces ARN non codants (ARNnc) démontrent qu'ils seraient impliqués dans différents modes de régulation génétique. Les riborégulateurs sont un exemple de telles voies de régulation nouvellement élucidées. La nouveauté du mécanisme réside dans le fait qu'en principe, ces ARNnc modulent l'expression génétique sans avoir recours aux protéines. C'est la liaison directe d'un métabolite à l'ARN messager (ARNm) qui, par l'induction de changements allostériques, influence l'expression du gène situé en aval du riborégulateur. La bactérie Bacillus subtilis possède une grande variété de riborégulateurs qui régulent l'expression de plus de 2% de ses gènes. Le présent travail porte sur le riborégulateur lysine lysC de. B. subtilis et a comme objectif de comprendre l'influence de la structure du riborégulateur sur sa fonction de régulation de la transcription. Dans un premier temps, mes travaux ont démontré la présence et l'importance fonctionnelle d'une interaction boucle-boucle entre les tiges P1 et P2, et d'un motif de type kink turn (K-turn ) dans la tige P2. L'effet de ces motifs structuraux a été évalué par des essais de transcription, où le riborégulateur est inactivé lorsque des mutations sont introduites dans ces motifs. Des essais de fluorescence montrent aussi les répercussions de la formation de cette interaction sur la conformation du site de liaison. Finalement, nos résultats démontrent que l'interaction boucle-boucle dépend de la présence du motif K-turn , d'où l'importance de ce dernier. Dans une seconde publication, nous avons démontré qu'il y avait un effet synergique entre le repliement de l'interaction boucle-boucle et celui d'une deuxième interaction tertiaire de type hélice-boucle, entre la tige P2 et la boucle de la tige P4. L'influence de cette dernière sur l'organisation du site de liaison et sur la terminaison prématurée de la transcription a aussi été établie, entre autres avec l'utilisation d'un vecteur FRET impliquant les tiges P1 et P5 qui s'est avéré très informatif sur le repliement du coeur. Cet article fait également la démonstration de la contribution des autres éléments du riborégulateur qui ne sont pas impliqués dans des interactions tertiaires, notamment la tige P5 de l'aptamère et une tige nommée P6 située dans la plate-forme d'expression, dont les longueurs' sont variables d'une séquence à l'autre. Les résultats obtenus au cours de mon doctorat nous ont aidé à mieux comprendre comment les changements de structure du riborégulateur lysine affectent sa capacité à réguler l'expression d'un gène. De telles connaissances pourraient s'avérer précieuses pour une éventuelle utilisation des riborégulateurs comme outils biologiques ou comme cibles pour de nouveaux antibiotiques. J'ai d'ailleurs participé A l'écriture d'une revue qui traite des mécanismes de régulation des riborégulateurs et leur. potentiel comme outils biologiques. Cette revue a été publiée dans le journal ChemBioChem et est jointe en annexe

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

The Q Branch Cooling Anomaly Can Be Explained by Mergers of White Dwarfs and Subgiant Stars

Gaia's exquisite parallax measurements allowed for the discovery and characterization of the Q branch in the Hertzsprung-Russell diagram, where massive C/O white dwarfs (WDs) pause their dimming due to energy released during crystallization. Interestingly, the fraction of old stars on the Q branch is significantly higher than in the population of WDs that will become Q branch stars or that were Q branch stars in the past. From this, Cheng et al. inferred that ~6% of WDs passing through the Q branch experience a much longer cooling delay than that of standard crystallizing WDs. Previous attempts to explain this cooling anomaly have invoked mechanisms involving super-solar initial metallicities. In this paper, we describe a novel scenario in which a standard composition WD merges with a subgiant star. The evolution of the resulting merger remnant leads to the creation of a large amount of 26Mg, which, along with the existing 22Ne, undergoes a distillation process that can release enough energy to explain the Q branch cooling problem without the need for atypical initial abundances. The anomalously high number of old stars on the Q branch may thus be evidence that mass transfer from subgiants to WDs leads to unstable mergers.Comment: Accepted for publication in ApJL. Added text and a figure to better motivate the initial conditions of the merger remnant evolution. Also amended text regarding the estimated numbers of WD + subgiant merger

Distillation of 56^{56}Fe in Ultramassive O-Ne White Dwarfs

When white dwarfs freeze the plasma mixtures inside them undergo separation processes which can produce radical changes in the composition profile of the star. The abundance of neutron rich elements, such as $^{22}$Ne or $^{56}$Fe, determines whether or not the first crystals are more or less dense than the surrounding fluid and thus whether they sink or float. These processes have now been studied for C-O-Ne and C-O-Fe mixtures, finding that distillation and precipitation processes are possible in white dwarfs. In this work, we calculate the phase diagram of more complicated O-Ne-Fe mixtures and make predictions for the internal structure of the separated white dwarf. There are two possible outcomes determined by a complicated interplay between the Ne abundance, the $^{22}$Ne fraction, and the $^{56}$Fe abundance. Either Fe distills to form an inner core because the first O-Ne solids are buoyant, or an O-Ne inner core forms and Fe accumulates in the liquid until Fe distillation begins and forms a Fe shell. In the case of an Fe shell, a Rayleigh-Taylor instability may arise and overturn the core. In either case, Fe distillation may only produce a cooling delay of order 0.1 Gyr as these processes occur early at high white dwarf luminosities. Fe inner cores and shells may be detectable through asteroseismology and could enhance the yield of neutron rich elements such as $^{55}$Mn and $^{58}$Ni in supernovae.Comment: 8 pages, 3 figures, accepted for publication in Ap

A Spectro-photometric Analysis of Cool White Dwarfs in the Gaia and Pan-STARRS Footprint

We present a spectro-photometric analysis of 2880 cool white dwarfs within 100 pc of the Sun and cooler than Teff = 10,000 K, with grizy Pan-STARRS photometry and Gaia trigonometric parallaxes available. We also supplement our data sets with near-infrared JHK photometry, when available, which is shown to be essential for interpreting the coolest white dwarfs in our sample. We perform a detailed analysis of each individual object using state-of-the-art model atmospheres appropriate for each spectral type including DA, DC, DQ, DZ, He-rich DA, and the so-called IR-faint white dwarfs. We discuss the temperature and mass distributions of each subsample, as well as revisit the spectral evolution of cool white dwarfs. We find little evidence in our sample for the transformation of a significant fraction of DA stars into He-atmosphere white dwarfs through the process of convective mixing between Teff = 10,000 K and 6500 K, although the situation changes drastically in the range Teff = 6500 - 5500 K where the fraction of He-atmosphere white dwarfs reaches 45%. However, we also provide strong evidence that at even cooler temperatures (Teff < 5200 K), most DC white dwarfs have H atmospheres. We discuss a possible mechanism to account for this sudden transformation from He- to H-atmosphere white dwarfs involving the onset of crystallization and the occurrence of magnetism. Finally, we also argue that DQ, DZ, and DC white dwarfs may form a more homogeneous population than previously believed.Comment: Accepted for publication in MNRAS (21 pages, 20 figures

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