The evolution of ultra-massive carbon oxygen white dwarfs (original) (raw)
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Astronomy & Astrophysics
Context. The existence of ultra-massive white dwarf stars, MWD ≳ 1.05 M⊙, has been reported in several studies. These white dwarfs are relevant for the role they play in type Ia supernova explosions, the occurrence of physical processes in the asymptotic giant-branch phase, the existence of high-field magnetic white dwarfs, and the occurrence of double-white-dwarf mergers. Aims. We aim to explore the formation of ultra-massive, carbon-oxygen core white dwarfs resulting from single stellar evolution. We also intend to study their evolutionary and pulsational properties and compare them with those of the ultra-massive white dwarfs with oxygen-neon cores resulting from carbon burning in single progenitor stars, and with binary merger predictions. The aim is to provide a theoretical basis that can eventually help to discern the core composition of ultra-massive white dwarfs and the circumstances of their formation. Methods. We considered two single-star evolution scenarios for the forma...
The evolution of ultra-massive white dwarfs
Astronomy & Astrophysics, 2019
Ultra-massive white dwarfs are powerful tools used to study various physical processes in the asymptotic giant branch (AGB), type Ia supernova explosions, and the theory of crystallization through white dwarf asteroseismology. Despite the interest in these white dwarfs, there are few evolutionary studies in the literature devoted to them. Here we present new ultra-massive white dwarf evolutionary sequences that constitute an improvement over previous ones. In these new sequences we take into account for the first time the process of phase separation expected during the crystallization stage of these white dwarfs by relying on the most up-to-date phase diagram of dense oxygen/neon mixtures. Realistic chemical profiles resulting from the full computation of progenitor evolution during the semidegenerate carbon burning along the super-AGB phase are also considered in our sequences. Outer boundary conditions for our evolving models are provided by detailed non-gray white dwarf model atm...
Formation and evolution of hybrid He–CO white dwarfs and their properties
Monthly Notices of the Royal Astronomical Society, 2018
White dwarfs (WDs) are the stellar core remnants of low mass (8 M) stars. They are typically divided into three main composition groups: oxygen-neon (ONe), carbon-oxygen (CO), and helium (He) WDs. The evolution of binary systems can significantly change the evolution of the binary stellar components. In particular, striping the envelope of an evolved star can give rise to a core remnant, which can later evolve into a WD with significantly different composition. Here we focus on the formation and evolution of hybrid HeCO WDs. We follow the formation and stellar evolution of such WDs for a range of initial conditions and provide their detailed structure, mass-radius relation and luminosity-temperature evolution. We confirm that both low-mass WDs (<0.45 M , typically thought to be He-WDs) and intermediate-mass WDs (0.45 < M WD ≤ 0.7, typically thought to be CO-WDs) could in fact be hybrid HeCO WDs, with 5-25 (75-95) per cent of their mass in He (CO). We use population synthesis calculations to infer the birth rate and properties of such WDs. We find that hybrid HeCO-WD comprise the majority of young (<2 Gyr) WDs in binaries, but are rarer among older WDs in binaries. The high frequency and large He content of such WDs could have an important role in WD-WD mergers, and may give rise to sub-Chandrasekhar thermonuclear supernova explosions.
The composition of massive white dwarfs and their dependence on C-burning modeling
Astronomy & Astrophysics, 2022
Context. Recent computations of the interior composition of ultra-massive white dwarfs (WDs) have suggested that some WDs could be composed of neon (Ne)-dominated cores. This result is at variance with our previous understanding of the chemical structure of massive WDs, where oxygen is the predominant element. In addition, it is not clear whether some hybrid carbon (C) oxygen (O)-Ne WDs might form when convective boundary mixing is accounted for during the propagation of the C-flame in the C-burning stage. Both the Ne-dominated and hybrid CO-Ne core would have measurable consequences for asteroseismological studies based on evolutionary models. Aims. In this work, we explore in detail to which extent differences in the adopted micro- and macro-physics can explain the different final WD compositions that have been found by different authors. Additionally, we explore the impact of such differences on the cooling times, crystallization, and pulsational properties of pulsating WDs. Meth...
The evolution of white dwarfs resulting from helium-enhanced, low-metallicity progenitor stars
Astronomy & Astrophysics, 2017
Context. Some globular clusters host multiple stellar populations with different chemical abundance patterns. This is particularly true for ω Centauri, which shows clear evidence of a helium-enriched subpopulation characterized by a helium abundance as high as Y = 0.4 Aims. We present a whole and consistent set of evolutionary tracks from the ZAMS to the white dwarf stage that is appropriate for the study of the formation and evolution of white dwarfs resulting from the evolution of helium-rich progenitors. Methods. We derived white dwarf sequences from progenitors with stellar mass ranging from 0.60 to 2.0 M and for an initial helium abundance of Y = 0.4. We adopted two values of metallicity: Z = 0.001 and Z = 0.0005. Results. We explored different issues of white dwarf evolution and their helium-rich progenitors. In particular, the final mass of the remnants, the role of overshooting during the thermally pulsing phase, and the cooling of the resulting white dwarfs differ markedly from the evolutionary predictions of progenitor stars with the standard initial helium abundance. Finally, the pulsational properties of the resulting white dwarfs are also explored. Conclusions. We find that, for the range of initial masses explored in this paper, the final mass of the helium-rich progenitors is markedly higher than the final mass expected from progenitors with the usual helium abundance. We also find that progenitors with initial mass lower than M 0.65 M evolve directly into helium-core white dwarfs in less than 14 Gyr, and that, for larger progenitor masses, the evolution of the resulting low-mass carbon-oxygen white dwarfs is dominated by residual nuclear burning. For heliumcore white dwarfs, we find that they evolve markedly faster than their counterparts coming from standard progenitors. Also, in contrast with what occurs for white dwarfs resulting from progenitors with the standard helium abundance, the impact of residual burning on the cooling time of white dwarfs is not affected by the occurrence of overshooting during the thermally pulsing phase of progenitor stars.
2020
Ultra-massive white dwarfs are relevant for their role as type Ia Supernova progenitors, the occurrence of physical processes in the asymptotic giant-branch phase, the existence of high-field magnetic white dwarfs, and the occurrence of double white dwarf mergers. Some hydrogen-rich ultra-massive white dwarfs are pulsating stars, and as such, they offer the possibility of studying their interiors through asteroseismology. On the other hand, pulsating helium-rich ultra-massive white dwarfs could be even more attractive objects for asteroseismology if they were found, as they should be hotter and less crystallized than pulsating hydrogen-rich white dwarfs, something that would pave the way for probing their deep interiors. We explore the pulsational properties of ultra-massive helium-rich white dwarfs with carbon-oxygen and oxygen-neon cores resulting from single stellar evolution. Our goal is to provide a theoretical basis that could eventually help to discern the core composition of...
The evolution of iron-core white dwarfs
Monthly Notices of the Royal Astronomical Society, 2000
) present observational evidence supporting the existence of some white dwarf (WD) stars with iron -rich, core composition. In this connection, the present paper is aimed at exploring the structure and evolution of iron -core WDs by means of a detailed and updated evolutionary code. In particular, we examined the evolution of the central conditions, neutrino luminosity, surface gravity, crystallization, internal luminosity profile and ages. We find that the evolution of iron -rich WDs is markedly different from that of their carbon -oxygen counterparts. In particular, cooling is strongly accelerated (up to a factor of five for models with pure iron composition) as compared with the standard case. Thus, if iron WDs were very numerous, some of them would have had time enough to evolve at lower luminosities than that corresponding to the fall -off in the observed WD luminosity function.
Full evolution of low-mass white dwarfs with helium and oxygen cores
Monthly Notices of the Royal Astronomical Society, 2007
We study the full evolution of low-mass white dwarfs with helium and oxygen cores. We revisit the age dichotomy observed in many white dwarf companions to millisecond pulsar on the basis of white dwarf configurations derived from binary evolution computations. We evolve 11 dwarf sequences for helium cores with final masses of 0. 1604, 0.1869, 0.2026, 0.2495, 0.3056, 0.3333, 0.3515, 0.3844, 0.3986, 0.4160 and 0.4481 M . In addition, we compute the evolution of five sequences for oxygen cores with final masses of 0.3515, 0.3844, 0.3986, 0.4160 and 0.4481 M . A metallicity of Z = 0.02 is assumed. Gravitational settling, chemical and thermal diffusion are accounted for during the white dwarf regime. Our study reinforces the result that diffusion processes are a key ingredient in explaining the observed age and envelope dichotomy in low-mass helium-core white dwarfs, a conclusion we arrived at earlier on the basis of a simplified treatment for the binary evolution of progenitor stars. We determine the mass threshold where the age dichotomy occurs. For the oxygen white dwarf sequences, we report the occurrence of diffusion-induced, hydrogen-shell flashes, which, as in the case of their helium counterparts, strongly influence the late stages of white dwarf cooling. Finally, we present our results as a set of white dwarf mass-radius relations for helium and oxygen cores.
White dwarf evolutionary sequences for low-metallicity progenitors: The impact of third dredge-up
Astronomy and Astrophysics
We present new white dwarf evolutionary sequences for low-metallicity progenitors. White dwarf sequences have been derived from full evolutionary calculations that take into account the entire history of progenitor stars, including the thermally-pulsing and the post-asymptotic giant branch phases. We show that for progenitor metallicities in the range 0.00003--0.001, and in the absence of carbon enrichment due to the occurrence of a third dredge-up episode, the resulting H envelope of the low-mass white dwarfs is thick enough to make stable H burning the most important energy source even at low luminosities. This has a significant impact on white dwarf cooling times. This result is independent of the adopted mass-loss rate during the thermally-pulsing and post-AGB phases, and the planetary nebulae stage. We conclude that in the absence of third dredge-up episodes, a significant part of the evolution of low-mass white dwarfs resulting from low-metallicity progenitors is dominated by ...
On the formation of oxygen-neon white dwarfs in close binary systems
Astronomy and Astrophysics, 2001
The evolution of a star of initial mass 10 M , and metallicity Z = 0.02 in a Close Binary System (CBS) is followed from its main sequence until an ONe degenerate remnant forms. Restrictions have been made on the characteristics of the companion as well as on the initial orbital parameters in order to avoid the occurrence of reversal mass transfer before carbon is ignited in the core. The system undergoes three mass loss episodes. The first and second ones are a consequence of a case B Roche lobe overflow. During the third mass loss episode stellar winds may play a role comparable to, or even more important than Roche lobe overflow. In this paper, we extend the previously existing calculations of stars of intermediate mass belonging to close binary systems by following carefully the carbon burning phase of the primary component. We also propose different possible outcomes for our scenario and discuss the relevance of our findings. In particular, our main result is that the resulting white dwarf component of mass 1.1 M more likely has a core composed of oxygen and neon, surrounded by a mantle of carbonoxygen rich material. The average abundances of the oxygen-neon rich core are X(O 16 ) = 0.55, X(Ne 20 ) = 0.28, X(Na 23 ) = 0.06 and X(Mg 24 ) = 0.05. This result has important consequences for the Accretion Induced Collapse scenario. The average abundances of the carbon-oxygen rich mantle are X(O 16 ) = 0.55, and X(C 12 ) = 0.43. The existence of this mantle could also play a significant role in our understanding of cataclysmic variables.