White dwarfs (WDs) that accrete helium at rates ∼10{sup −8} M{sub ⊙} yr{sup −1}, such as those in close binaries with sdB stars, can accumulate large (≳0.1 M{sub ⊙}) helium envelopes, which are ...likely to detonate. We perform binary stellar evolution calculations of sdB+WD binary systems with MESA, incorporating the important reaction chain {sup 14}N(e{sup −},ν){sup 14}C(α,γ){sup 18}O (NCO), including a recent measurement for the {sup 14}C(α,γ){sup 18}O rate. In large accreted helium shells, the NCO reaction chain leads to ignitions at the dense base of the freshly accreted envelope, in contrast to 3α ignitions, which occur away from the base of the shell. In addition, at these accretion rates, the shells accumulate on a timescale comparable to their thermal time, leading to an enhanced sensitivity of the outcome on the accretion rate history. Hence, time dependent accretion rates from binary stellar evolution are necessary to determine the helium layer mass at ignition. We model the observed sdB+WD system CD −30{sup ∘}11223 and find that the inclusion of these effects predicts ignition of a 0.153 M{sub ⊙} helium shell, nearly a factor of two larger than previous predictions. A shell with this mass will ignite dynamically, a necessary condition for a helium shell detonation.
White dwarfs (WDs) that accrete helium at rates \(\sim {10}^{-8}\,{M}_{\odot }\,{\mathrm{yr}}^{-1}\), such as those in close binaries with sdB stars, can accumulate large (\(\gtrsim 0.1\,{M}_{\odot ...}\)) helium envelopes, which are likely to detonate. We perform binary stellar evolution calculations of sdB+WD binary systems with MESA, incorporating the important reaction chain \({}^{14}{\rm{N}}{({e}^{-},\nu )}^{14}{\rm{C}}{(\alpha ,\gamma )}^{18}{\rm{O}}\) (NCO), including a recent measurement for the \({}^{14}{\rm{C}}{(\alpha ,\gamma )}^{18}{\rm{O}}\) rate. In large accreted helium shells, the NCO reaction chain leads to ignitions at the dense base of the freshly accreted envelope, in contrast to \(3\alpha \) ignitions, which occur away from the base of the shell. In addition, at these accretion rates, the shells accumulate on a timescale comparable to their thermal time, leading to an enhanced sensitivity of the outcome on the accretion rate history. Hence, time dependent accretion rates from binary stellar evolution are necessary to determine the helium layer mass at ignition. We model the observed sdB+WD system CD \(-30^\circ 11223\) and find that the inclusion of these effects predicts ignition of a \(0.153\,{M}_{\odot }\) helium shell, nearly a factor of two larger than previous predictions. A shell with this mass will ignite dynamically, a necessary condition for a helium shell detonation.
White dwarfs (WDs) that accrete helium at rates , such as those in close binaries with sdB stars, can accumulate large ( ) helium envelopes, which are likely to detonate. We perform binary stellar ...evolution calculations of sdB+WD binary systems with MESA, incorporating the important reaction chain (NCO), including a recent measurement for the rate. In large accreted helium shells, the NCO reaction chain leads to ignitions at the dense base of the freshly accreted envelope, in contrast to ignitions, which occur away from the base of the shell. In addition, at these accretion rates, the shells accumulate on a timescale comparable to their thermal time, leading to an enhanced sensitivity of the outcome on the accretion rate history. Hence, time dependent accretion rates from binary stellar evolution are necessary to determine the helium layer mass at ignition. We model the observed sdB+WD system CD and find that the inclusion of these effects predicts ignition of a helium shell, nearly a factor of two larger than previous predictions. A shell with this mass will ignite dynamically, a necessary condition for a helium shell detonation.
Abstract
We explore changes in the adiabatic low-order g-mode pulsation periods of 0.526, 0.560, and 0.729
M
⊙
carbon–oxygen white dwarf models with helium-dominated envelopes due to the presence, ...absence, and enhancement of
22
Ne in the interior. The observed g-mode pulsation periods of such white dwarfs are typically given to 6−7 significant figures of precision. Usually white dwarf models without
22
Ne are fit to the observed periods and other properties. The rms residuals to the ≃150−400 s low-order g-mode periods are typically in the range of
σ
rms
≲ 0.3 s, for a fit precision of
σ
rms
/
P
≲ 0.3%. We find average relative period shifts of Δ
P
/
P
≃ ±0.5% for the low-order dipole and quadrupole g-mode pulsations within the observed effective temperature window, with the range of Δ
P
/
P
depending on the specific g-mode, abundance of
22
Ne, effective temperature, and the mass of the white dwarf model. This finding suggests a systematic offset may be present in the fitting process of specific white dwarfs when
22
Ne is absent. As part of the fitting processes involves adjusting the composition profiles of a white dwarf model, our study on the impact of
22
Ne can provide new inferences on the derived interior mass fraction profiles. We encourage routinely including
22
Ne mass fraction profiles, informed by stellar evolution models, to future generations of white dwarf model-fitting processes.
Stars with unusual properties can provide a wealth of information about rare stages of stellar evolution and exotic physics. However, determining the true nature of peculiar stars is often difficult. ...In this work, we conduct a systematic search for cool and luminous stars in the Magellanic Clouds with extreme variability, motivated by the properties of the unusual Small Magellanic Cloud star and Thorne- ytkow Object (T O) candidate HV 2112. Using light curves from ASAS-SN, we identify 38 stars with surface temperatures T < 4800 K, luminosities (L/L ) > 4.3, variability periods >400 days, and variability amplitudes ΔV > 2.5 mag. Eleven of these stars possess the distinctive double-peaked light-curve morphology of HV 2112. We use the pulsation properties and derived occurrence rates for these 12 objects to constrain their nature. From comparisons to stellar populations and models, we find that one star may be a red supergiant with large-amplitude pulsations. For the other 11 stars, we derive current masses of ∼5-10 M , below the theoretical minimum mass of ∼15 M for T Os to be stable, casting doubt on this interpretation. Instead, we find that the temperatures, luminosities, mass-loss rates (MLRs), and periods of these stars are consistent with predictions for super-asymptotic giant branch (s-AGB) stars that have begun carbon burning but have not reached the superwind phase. We infer lifetimes in this phase of ∼(1-7) × 104 yr, also consistent with an s-AGB interpretation. If confirmed, these objects would represent the first identified population of s-AGB stars, illuminating the transition between low- and high-mass stellar evolution.
We enhance the treatment of crystallization for models of white dwarfs (WDs) in the stellar evolution software MESA by implementing carbon-oxygen (C/O) phase separation. The phase separation process ...during crystallization leads to transport of oxygen toward the center of WDs, resulting in a more compact structure that liberates gravitational energy as additional heating that modestly slows WD cooling timescales. We quantify this cooling delay in MESA C/O WD models over the mass range 0.5-1.0 \(M_\odot\), finding delays of 0.5-0.8 Gyr for typical C/O interior profiles. MESA WD cooling timescales including this effect are generally comparable to other WD evolution models that make similar assumptions about input physics. When considering phase separation alongside \(^{22}\)Ne sedimentation, however, we find that both MESA and BaSTI WD cooling models predict a more modest sedimentation delay than the latest LPCODE models, and this may therefore require a re-evaluation of previously proposed solutions to some WD cooling anomalies that were based on LPCODE models of \(^{22}\)Ne sedimentation. Our implementation of C/O phase separation in the open-source stellar evolution software MESA provides an important tool for building realistic grids of WD cooling models, as well as a framework for expanding on our implementation to explore additional physical processes related to phase transitions and associated fluid motions in WD interiors.
White dwarf stars reveal signatures of material accreted from their surroundings. Making quantitative inferences about the processes that supply this material requires theoretical models of white ...dwarf surface structure. In this dissertation, I examine methods for building evolutionary white dwarf models that include element diffusion, convection, and thermohaline instability. Each of these mixing processes that occur at white dwarf surfaces has important implications for observable signatures of accreted material. Models that account for all types of surface mixing allow for inferences about accretion rates and composition of bodies that supply the material. The picture that emerges from models presented in this work is one of planetary systems supplying rocky debris at higher rates and from larger mass reservoirs than previously thought.
The polluted white dwarf (WD) system SDSS J122859.93+104032.9 (SDSS J1228) shows variable emission features interpreted as originating from a solid core fragment held together against tidal forces by ...its own internal strength, orbiting within its surrounding debris disk. Estimating the size of this orbiting solid body requires modeling the accretion rate of the polluting material that is observed mixing into the WD surface. That material is supplied via sublimation from the surface of the orbiting solid body. The sublimation rate can be estimated as a simple function of the surface area of the solid body and the incident flux from the nearby hot WD. On the other hand, estimating the accretion rate requires detailed modeling of the surface structure and mixing in the accreting WD. In this work, we present MESA WD models for SDSS J1228 that account for thermohaline instability and mixing in addition to heavy element sedimentation to accurately constrain the sublimation and accretion rate necessary to supply the observed pollution. We derive a total accretion rate of \(\dot M_{\rm acc}=1.8\times 10^{11}\,\rm g\,s^{-1}\), several orders of magnitude higher than the \(\dot M_{\rm acc}=5.6\times 10^{8}\,\rm g\,s^{-1}\) estimate obtained in earlier efforts. The larger mass accretion rate implies that the minimum estimated radius of the orbiting solid body is r\(_{\rm{min}}\) = 72 km, which, although significantly larger than prior estimates, still lies within upper bounds (a few hundred km) for which the internal strength could no longer withstand tidal forces from the gravity of the WD.
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