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Register a new userThe pulsational properties of ultra-massive DB white dwarfs with carbon-oxygen cores coming from single-star evolution
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Added by
Alejandro C\\'orsico
Publication date
2020
fields
Physics
and research's language is
English
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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 ultra-massive white dwarfs and the scenario of their formation through asteroseismology, anticipating the possible future detection of pulsations in this type of stars. We find that, given that the white dwarf models coming from the three scenarios considered are characterized by distinct core chemical profiles, their pulsation properties are also different, thus leading to distinctive signatures in the period-spacing and mode-trapping properties. Our results indicate that, in case of an eventual detection of pulsating ultra-massive helium-rich white dwarfs, it would be possible to derive valuable information encrypted in the core of these stars in connection with the origin of such exotic objects. The detection of pulsations in these stars has many chances to be achieved soon through observations collected with ongoing space missions.
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(Abridged abstract) We explore the formation of ultra-massive (M_{rm WD} gtrsim 1.05 M_sun$), carbon-oxygen core white dwarfs resulting from single stellar evolution. We also 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. We consider two single-star evolution scenarios for the formation of ultra-massive carbon-oxygen core white dwarfs that involve rotation of the degenerate core after core helium burning and reduced mass-loss rates in massive asymptotic giant-branch stars. We compare our findings with the predictions from ultra-massive white dwarfs resulting from the merger of two equal-mass carbon-oxygen core white dwarfs, by assuming complete mixing between them and a carbon-oxygen core for the merged remnant. The resulting ultra-massive carbon-oxygen core white dwarfs evolve markedly slower than their oxygen-neon counterparts. Our study strongly suggests the formation of ultra-massive white dwarfs with carbon-oxygen core from single stellar evolution. We find that both the evolutionary and pulsation properties of these white dwarfs are markedly different from those of their oxygen-neon core counterparts and from those white dwarfs with carbon-oxygen core that might result from double degenerate mergers. This can eventually be used to discern the core composition of ultra-massive white dwarfs and their formation scenario.
Ultra-massive white dwarfs are powerful tools 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 atmospheres for hydrogen and helium composition. We assessed the impact of all these improvements on the evolutionary properties of ultra-massive white dwarfs, providing up-dated evolutionary sequences for these stars. We conclude that crystallization is expected to affect the majority of the massive white dwarfs observed with effective temperatures below $40,000, rm K$. Moreover, the calculation of the phase separation process induced by crystallization is necessary to accurately determine the cooling age and the mass-radius relation of massive white dwarfs. We also provide colors in the GAIA photometric bands for our H-rich white dwarf evolutionary sequences on the basis of new models atmospheres. Finally, these new white dwarf sequences provide a new theoretical frame to perform asteroseismological studies on the recently detected ultra-massive pulsating white dwarfs.
We investigate properties of carbon-oxygen white dwarfs with respect to the composite uncertainties in the reaction rates using the stellar evolution toolkit, Modules for Experiments in Stellar Astrophysics (MESA) and the probability density functions in the reaction rate library STARLIB. These are the first Monte Carlo stellar evolution studies that use complete stellar models. Focusing on 3 M$_{odot}$ models evolved from the pre main-sequence to the first thermal pulse, we survey the remnant core mass, composition, and structure properties as a function of 26 STARLIB reaction rates covering hydrogen and helium burning using a Principal Component Analysis and Spearman Rank-Order Correlation. Relative to the arithmetic mean value, we find the width of the 95% confidence interval to be $Delta M_{{rm 1TP}}$ $approx$ 0.019 M$_{odot}$ for the core mass at the first thermal pulse, $Delta$$t_{rm{1TP}}$ $approx$ 12.50 Myr for the age, $Delta log(T_{{rm c}}/{rm K}) approx$ 0.013 for the central temperature, $Delta log(rho_{{rm c}}/{rm g cm}^{-3}) approx$ 0.060 for the central density, $Delta Y_{rm{e,c}} approx$ 2.6$times$10$^{-5}$ for the central electron fraction, $Delta X_{rm c}(^{22}rm{Ne}) approx$ 5.8$times$10$^{-4}$, $Delta X_{rm c}(^{12}rm{C}) approx$ 0.392, and $Delta X_{rm c}(^{16}rm{O}) approx$ 0.392. Uncertainties in the experimental $^{12}$C($alpha,gamma)^{16}rm{O}$, triple-$alpha$, and $^{14}$N($p,gamma)^{15}rm{O}$ reaction rates dominate these variations. We also consider a grid of 1 to 6 M$_{odot}$ models evolved from the pre main-sequence to the final white dwarf to probe the sensitivity of the initial-final mass relation to experimental uncertainties in the hydrogen and helium reaction rates.
The present work is designed to explore the evolutionary and pulsational properties of low-mass white dwarfs with carbon/oxygen cores. In particular, we follow the evolution of a 0.33 Msun white dwarf remnant in a self-consistent way with the predictions of nuclear burning, element diffusion and the history of the white dwarf progenitor. Attention is focused on the occurrence of hydrogen shell flashes induced by diffusion processes during cooling phases. The evolutionary stages prior to the white dwarf formation are also fully accounted for by computing the conservative binary evolution of an initially 2.5-Msun Pop. I star with a 1.25 Msun companion, and period P_i= 3 days. Evolution is followed down to the domain of the ZZ Ceti stars on the white dwarf cooling branch. We find that chemical diffusion induces the occurrence of an additional hydrogen thermonuclear flash which leads to stellar models with thin hydrogen envelopes. As a result, a fast cooling is encountered at advanced stages of evolution. In addition, we explore the adiabatic pulsational properties of the resulting white dwarf models. As compared with their helium-core counterparts, low-mass oxygen-core white dwarfs are characterized by a pulsational spectrum much more featured, an aspect which could eventually be used for distinguishing both types of stars if low-mass white dwarfs were in fact found to pulsate as ZZ Ceti-type variables. Finally, we perform a non-adiabatic pulsational analysis on the resulting carbon/oxygen low-mass white dwarf models.
Ultra-massive DA WD stars are expected to harbor ONe cores resulting from the progenitor evolution through the Super-AGB phase. As evolution proceeds during the WD cooling phase, a crystallization process resulting from Coulomb interactions in very dense plasmas is expected to occur, leading to the formation of a highly crystallized core. Pulsating ultra-massive WDs offer a unique opportunity to infer and test the occurrence of crystallization in WD interiors as well as physical processes related with dense plasmas. We aim to assess the adiabatic pulsation properties of ultra-massive DA WD with ONe cores. We studied the pulsation properties of ultra-massive DA WD stars with ONe cores. We employed a new set of ultra-massive WD evolutionary sequences of models with stellar masses in the range 1.10 $leq M_{star}/M_{sun} leq$ 1.29 computed by taking into account the complete evolution of the progenitor stars and the WD stage. When crystallization set on in our models, we took into account latent heat release and also the expected changes in the core chemical composition that are due to phase separation according to a phase diagram suitable for O and Ne plasmas. We computed nonradial pulsation g-modes of our sequences of models at the ZZ Ceti phase by taking into account a solid core. We explored the impact of crystallization on their pulsation properties, in particular, the structure of the period spectrum and the distribution of the period spacings. We find that it would be possible, in principle, to discern whether a WD has a nucleus made of CO or a nucleus of ONe by studying the spacing between periods. The features found in the period-spacing diagrams could be used as a seismological tool to discern the core composition of ultra-massive ZZ Ceti stars, something that should be complemented with detailed asteroseismic analysis using the individual observed periods.
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