Search

Cette thèse présente une étude empirique et théorique approfondie de l'évolution spectrale des étoiles naines blanches, avec un accent particulier sur les naines blanches chaudes. La composition atmosphérique (et donc l'apparence spectrale) de ces cadavres stellaires peut changer drastiquement avec le temps à mesure qu'ils se refroidissent. Ce phénomène est généralement interprété comme le résultat d'une compétition entre divers mécanismes de transport des éléments dans l'enveloppe stellaire (tels que la diffusion, la convection, les vents et l'accrétion), mais demeure mal compris à plusieurs égards. Il est impératif de remédier à cette situation pour être en mesure d'exploiter le potentiel immense des naines blanches pour notre compréhension du passé de la Galaxie. Pour mieux caractériser l'incidence de l'évolution spectrale, nous effectuons tout d'abord une analyse spectroscopique exhaustive de près de 2000 naines blanches chaudes (Teff > 30,000 K) observées par le relevé SDSS. Nous déterminons les propriétés atmosphériques (notamment la température effective et la composition de surface) de ces objets à l'aide d'un nouvel ensemble de modèles d'atmosphère calculé spécifiquement à cet effet. En examinant la fréquence relative des étoiles riches en hydrogène et riches en hélium en fonction de la température, nous obtenons pour la première fois un portrait empirique détaillé de l'évolution spectrale des naines blanches chaudes. Plus spécifiquement, nous déduisons (1) qu'environ une étoile sur quatre arrive sur la séquence de refroidissement avec une atmosphère d'hélium, et (2) qu'environ deux tiers de ces objets développent ultérieurement une atmosphère d'hydrogène. En outre, nous accordons une attention particulière aux naines blanches hybrides (qui montrent à la fois des traces d'hydrogène et d'hélium) de notre échantillon et à ce que ces objets distinctifs nous apprennent sur l'évolution spectrale. Nous étudions ensuite l'évolution spectrale d'un point de vue théorique en modélisant les transformations chimiques qui s'opèrent dans les naines blanches. Pour ce faire, nous utilisons le code d'évolution stellaire STELUM, qui inclut un traitement cohérent et réaliste du transport des éléments et nous permet donc de réaliser les simulations numériques d'évolution spectrale les plus sophistiquées à ce jour. Nous modélisons la diffusion de l'hydrogène résiduel dans une enveloppe d'hélium à haute température, qui mène ultimement à la formation d'une atmosphère d'hydrogène. Nous simulons également le mélange convectif de cette couche superficielle d'hydrogène avec la couche sous-jacente d'hélium à basse température, qui produit à nouveau une surface dominée par l'hélium. En outre, nous étudions le transport du carbone dans les étoiles riches en hélium, incluant à la fois le tri gravitationnel à haute température et le dragage convectif à basse température. Ces calculs donnent lieu à plusieurs résultats astrophysiques d'intérêt. Nous obtenons notamment une contrainte inédite sur la quantité d'hydrogène résiduel contenue dans les naines blanches chaudes dominées par l'hélium. Nous démontrons aussi que la bifurcation observée dans le diagramme couleur-magnitude des naines blanches découvertes par le satellite Gaia est une signature du processus de mélange convectif à basse température. Par ailleurs, nos modèles fournissent de précieuses informations sur les propriétés des étoiles polluées par le carbone, en particulier sur leur masse et leur zone convective. Enfin, le résultat le plus important de cette thèse est la résolution définitive du problème le plus sérieux de la théorie de l'évolution spectrale, soit le problème de l'origine de l'hydrogène à la surface des naines blanches de type DBA., This thesis presents an in-depth empirical and theoretical study of the spectral evolution of white dwarf stars, with a particular focus on hot white dwarfs. The atmospheric composition (and thus the spectral appearance) of these stellar remnants can change drastically over time as they cool. This phenomenon is generally interpreted as the result of an interplay between various element transport mechanisms in the stellar envelope (such as diffusion, convection, winds, and accretion), but remains poorly understood in several respects. It is imperative to remedy this situation to be able to exploit the immense potential of white dwarfs for our understanding of the past of the Galaxy. To better characterize the incidence of spectral evolution, we first carry out an exhaustive spectroscopic analysis of nearly 2000 hot white dwarfs (Teff > 30,000 K) observed by the SDSS survey. We determine the atmospheric properties (in particular the effective temperature and surface composition) of these objects using a new set of model atmospheres calculated specifically for this purpose. By examining the relative frequency of hydrogen-rich and helium-rich stars as a function of temperature, we obtain for the first time a detailed empirical picture of the spectral evolution of hot white dwarfs. More specifically, we infer (1) that about one in four stars enters the cooling sequence with a helium atmosphere, and (2) that about two-thirds of these objects eventually develop a hydrogen atmosphere. Furthermore, we pay special attention to the hybrid white dwarfs (which exhibit traces of both hydrogen and helium) in our sample and to what can be learned about spectral evolution from these distinctive objects. We then study spectral evolution from a theoretical point of view by modeling the chemical transformations that take place in white dwarfs. To do this, we use the stellar evolution code STELUM, which includes a consistent and realistic treatment of element transport and therefore allows us to perform the most sophisticated numerical simulations of spectral evolution to date. We model the diffusion of residual hydrogen in a helium envelope at high temperature, which ultimately leads to the formation of a hydrogen atmosphere. We also simulate the convective mixing of this superficial hydrogen layer with the underlying helium layer at low temperature, which once again produces a helium-dominated surface. Furthermore, we study the transport of carbon in helium-rich stars, including both gravitational settling at high temperature and convective dredge-up at low temperature. These calculations give rise to several astrophysical results of interest. In particular, we obtain an unprecedented constraint on the amount of residual hydrogen contained within hot helium-dominated white dwarfs. We also demonstrate that the bifurcation observed in the color-magnitude diagram of white dwarfs discovered by the Gaia satellite is a signature of the convective mixing process at low temperature. Furthermore, our models provide valuable information on the properties of carbon-polluted stars, in particular on their mass and convective zone. Finally, the most important result of this thesis is the definitive resolution of the most serious problem of the theory of spectral evolution, namely the problem of the origin of hydrogen at the surface of DBA-type white dwarfs.

Davis, S., et al., We present a study of the optical and near-infrared (NIR) spectra of SN 2013ai along with its light curves. These data range from discovery until 380 days after explosion. SN 2013ai is a fast declining Type II supernova (SN II) with an unusually long rise time, 18.9 2.7 days in the V-band, and a bright V-band peak absolute magnitude of -18.7 0.06 mag. The spectra are dominated by hydrogen features in the optical and NIR. The spectral features of SN 2013ai are unique in their expansion velocities, which, when compared to large samples of SNe II, are more than 1,000 km s-1 faster at 50 days past explosion. In addition, the long rise time of the light curve more closely resembles SNe IIb rather than SNe II. If SN 2013ai is coeval with a nearby compact cluster, we infer a progenitor zero-age main-sequence mass of ∼17 M o˙. After performing light-curve modeling, we find that SN 2013ai could be the result of the explosion of a star with little hydrogen mass, a large amount of synthesized 56Ni, 0.3-0.4 M o˙, and an explosion energy of 2.5-3.0 1051 erg. The density structure and expansion velocities of SN 2013ai are similar to those of the prototypical SN IIb, SN 1993J. However, SN 2013ai shows no strong helium features in the optical, likely due to the presence of a dense core that prevents the majority of γ-rays from escaping to excite helium. Our analysis suggests that SN 2013ai could be a link between SNe II and stripped-envelope SNe., The work of the CSP-II has been generously supported by the National Science Foundation under grants AST-1008343, AST1613426, AST-1613455, and AST1613472. The CSP-II was also supported in part by the Danish Agency for Science and Technology and Innovation through a Sapere Aude Level 2 grant. M.F. is supported by a Royal Society—Science Foundation Ireland University Research Fellowship. P.H. acknowledges support by grants of the NSF AST-1008962 and NASA’s ATP1909476. L.G. was funded by the European Union’s Horizon 2020 research and innovation program under the Marie Skłodowska-Curie grant agreement No. 839090. This work has been partially supported by the Spanish grant PGC2018-095317- B-C21 within the European Funds for Regional Development (FEDER). T.W.C. acknowledges the EU Funding under Marie Skłodowska-Curie grant agreement No 842471. M.S. is supported by generous grants from Villum FONDEN (13261, 28021) and by a project grant (8021-00170B) awarded by the Independent Research Fund Denmark.

Author(s): Strotjohann, NL; Ofek, EO; Gal-Yam, A; Bruch, R; Schulze, S; Shaviv, N; Sollerman, J; Filippenko, AV; Yaron, O; Fremling, C; Nordin, J; Kool, EC; Perley, DA; Ho, AYQ; Yang, Y; Yao, Y; Soumagnac, MT; Graham, ML; Barbarino, C; Tartaglia, L; De, K; Goldstein, DA; Cook, DO; Brink, TG; Taggart, K; Yan, L; Lunnan, R; Kasliwal, M; Kulkarni, SR; Nugent, PE; Masci, FJ; Rosnet, P; Adams, SM; Andreoni, I; Bagdasaryan, A; Bellm, EC; Burdge, K; Duev, DA; Dugas, A; Frederick, S; Goldwasser, S; Hankins, M; Irani, I; Karambelkar, V; Kupfer, T; Liang, J; Neill, JD; Porter, M; Riddle, RL; Sharma, Y; Short, P; Taddia, F; Tzanidakis, A; Van Roestel, J; Walters, R; Zhuang, Z | Abstract: Interaction-powered supernovae (SNe) explode within an optically thick circumstellar medium (CSM) that could be ejected during eruptive events. To identify and characterize such pre-explosion outbursts, we produce forcedphotometry light curves for 196 interacting SNe, mostly of Type IIn, detected by the Zwicky Transient Facility between early 2018 and 2020 June. Extensive tests demonstrate that we only expect a few false detections among the 70,000 analyzed pre-explosion images after applying quality cuts and bias corrections. We detect precursor eruptions prior to 18 Type IIn SNe and prior to the Type Ibn SN 2019uo. Precursors become brighter and more frequent in the last months before the SN and month-long outbursts brighter than magnitude -13 occur prior to 25% (5-69%, 95% confidence range) of all Type IIn SNe within the final three months before the explosion. With radiative energies of up to 1049erg, precursors could eject ~1M⊙of material. Nevertheless, SNe with detected precursors are not significantly more luminous than other SNe IIn, and the characteristic narrow hydrogen lines in their spectra typically originate from earlier, undetected mass-loss events. The long precursor durations require ongoing energy injection, and they could, for example, be powered by interaction or by a continuum-driven wind. Instabilities during the neon- and oxygen-burning phases are predicted to launch precursors in the final years to months before the explosion; however, the brightest precursor is 100 times more energetic than anticipated.

This is an Open Access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/4.0/), which permits unrestricted reuse, distribution, and reproduction in any medium, provided the original work is properly cited., Born-again stars allow probing stellar evolution in human timescales and provide the most promising path for the formation of hydrogen-deficient post-asymptotic giant branch objects, but their cold and molecular components remain poorly explored. Here we present ALMA observations of V 605 Aql that unveil for the first time the spatio-kinematic distribution of the molecular material associated with a born-again star. Both the continuum and molecular line emission exhibit a clumpy ring-like structure with a total extent of ≈1'' in diameter. The bulk of the molecular emission is interpreted as being produced in a radially expanding disk-like structure with an expansion velocity ${v}_{\exp }$ ∼ 90 km s−1 and an inclination i ≈ 60° with respect to the line of sight. The observations also reveal a compact high-velocity component, ${v}_{\exp }$ ∼ 280 km s−1, that is aligned perpendicularly to the expanding disk. This component is interpreted as a bipolar outflow with a kinematical age τ ≲ 20 yr, which could either be material that is currently being ejected from V 605 Aql, or is being dragged from the inner parts of the disk by a stellar wind. The dust mass of the disk is in the range Mdust ∼ 0.2–8 × 10−3 M⊙, depending on the dust absorption coefficient. The mass of the CO is MCO ≈ 1.1 × 10−5 M⊙, which is more than three orders of magnitude larger than the mass of the other detected molecules. We estimate a 12C/13C ratio of 5.6 ± 0.6, which is consistent with the single stellar evolution scenario in which the star experienced a very late thermal pulse instead of a nova-like event as previously suggested. © 2022. The Author(s). Published by the American Astronomical Society., This Letter makes use of the following ALMA data: ADS/ JAO.ALMA#2019.1.01408.S. ALMA is a partnership of ESO (representing its member states), NSF (USA), and NINS (Japan), together with NRC (Canada), MOST, and ASIAA (Taiwan), and KASI (Republic of Korea), in cooperation with the Republic of Chile. The Joint ALMA Observatory is operated by ESO, AUI/NRAO, and NAOJ. J.A.T. is grateful for funding by Fundación Marcos Moshinsky (México) and the DGAPA UNAM projects IA100720 and IA101622. J.B.R.G. thanks Consejo Nacional de Ciencias y Tecnología (CONACYT) Mexico for a research student grant. M.A.G. acknowledges support from the Spanish Ministerio de Ciencia, Innovación y Universidades (MCIU) grant PGC2018-102184-B-I00 and from the State Agency for Research of the Spanish MCIU through the "Center of Excellence Severo Ochoa" award to the Instituto de Astrofísica de Andalucía (SEV-2017-0709). L.A.Z. acknowledges financial support from CONACyT-280775 and UNAM-PAPIIT IN110618 grants, México. S.P.T.M. acknowledges the Chalmers Gender Initiative for Excellence (Genie). This work has made extensive use of NASA's Astrophysics Data System. The authors thank Theo Khouri for helpful discussions. The authors also thank the anonymous referee for constructive comments and suggestions that helped to improve the manuscript.

We have conducted a highly sensitive census of the evolved-star population in the metal-poor dwarf galaxy Leo P and detected four asymptotic giant branch (AGB) star candidates. Leo P is one of the best examples of a nearby analog of high-redshift galaxies because of its primitive metal content (2% of the solar value), proximity, and isolated nature, ensuring a less complicated history. Using medium-band optical photometry from the Hubble Space Telescope (HST), we have classified the AGB candidates by their chemical type. We have identified one oxygen-rich source which appears to be dusty in both the HST and Spitzer observations. Its brightness, however, suggests it may be a planetary nebula or post-AGB object. We have also identified three carbon-rich candidates, one of which may be dusty. Follow-up observations are needed to confirm the nature of these sources and to study the composition of any dust that they produce. If dust is confirmed, these stars would likely be among the most metal-poor examples of dust-producing stars known and will provide valuable insight into our understanding of dust formation at high redshift., Accepted for publication in ApJ

Common envelope evolution, the key orbital tightening phase of the traditional formation channel for close binaries, is a multistage process that presents many challenges to the establishment of a fully descriptive, predictive theoretical framework. In an approach complementary to global 3D hydrodynamical modeling, we explore the range of applicability for a simplified drag formalism that incorporates the results of local hydrodynamic "wind tunnel" simulations into a semi-analytical framework in the treatment of the common envelope dynamical inspiral phase using a library of realistic giant branch stellar models across the low, intermediate, and high mass regimes. In terms of a small number of key dimensionless parameters, we characterize a wide range of common envelope events, revealing the broad range of applicability of the drag formalism as well its self-similar nature across mass regimes and ages. Limitations arising from global binary properties and local structural quantities are discussed together with the opportunity for a general prescriptive application for this formalism., 16 pages, 12 figures

The compact binary radio pulsar system J0453+1559 (Martinez et al. 2015) consists of a recycled pulsar as primary component of $1.559(5)\,M_\odot$ and an unseen companion star of $1.174(4)\,M_\odot$. Because of the relatively large orbital eccentricity of $e = 0.1125$, it was argued that the companion is a neutron star, making it the neutron star with the lowest accurately determined mass to date. However, a direct observational determination of the nature of the companion is currently not feasible. Moreover, state-of-the-art stellar evolution and supernova modeling are contradictive concerning the possibility to produce such a low-mass neutron star remnant. Here we challenge the neutron star interpretation by reasoning that the lower-mass component could instead be a white dwarf born in a thermonuclear electron-capture supernova (tECSN) event, in which oxygen-neon deflagration in the degenerate stellar core of an ultra-stripped progenitor ejects several $0.1\,M_\odot$ of matter and leaves a bound ONeFe white dwarf as the second-formed compact remnant. We determine the ejecta mass and remnant kick needed in this scenario to explain the properties of PSR~J0453+1559 by a neutron star--white dwarf system. More work on tECSNe is needed to assess the viability of this scenario., Comment: ApJL, accepted for publication (6 pages, 4 figures)