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We perform binary evolution calculations on helium star—carbon–oxygen white dwarf (CO WD) binaries, using the stellar evolution code MESA. This single degenerate channel may contribute significantly to thermonuclear supernovae at short delay times. We examine the thermal-timescale mass transfer from a 1.1 to 2.0 helium star to a 0.90–1.05 CO WD for initial orbital periods in the range 0.05–1 day. Systems in this range may produce a thermonuclear supernova, helium novae, a helium star—oxygen–neon WD binary, or a detached double CO WD binary. Our time-dependent calculations that resolve the stellar structures of both binary components allow accurate distinction between the eventual formation of a thermonuclear supernova (via central ignition of carbon burning) and that of an ONe WD (in the case of off-center ignition). Furthermore, we investigate the effect of a slow WD wind, which implies a specific angular momentum loss from the binary that is larger than typically assumed. We find that this does not significantly alter the region of parameter space over which systems evolve toward thermonuclear supernovae. Our determination of the correspondence between initial binary parameters and the final outcome informs population synthesis studies of the contribution of the helium donor channel to thermonuclear supernovae. In addition, we constrain the orbital properties and observable stellar properties of the progenitor binaries of thermonuclear supernovae and helium novae. [ABSTRACT FROM AUTHOR]

Convective overshooting in super asymptotic giant branch stars has been suggested to lead to the formation of hybrid white dwarfs with carbon–oxygen cores and oxygen–neon mantles. As the white dwarf cools, this core–mantle configuration becomes convectively unstable and should mix. This mixing has been previously studied using stellar evolution calculations, but these made the approximation that convection did not affect the temperature profile of the mixed region. In this work, we perform direct numerical simulations of an idealized problem representing the core–mantle interface of the hybrid white dwarf. We demonstrate that, while the resulting structure within the convection zone is somewhat different than what is assumed in the stellar evolution calculations, the two approaches yield similar results for the size and growth of the mixed region. These hybrid white dwarfs have been invoked as progenitors of various peculiar thermonuclear supernovae. This lends further support to the idea that if these hybrid white dwarfs form, then they should be fully mixed by the time of explosion. These effects should be included in the progenitor evolution, in order to more accurately characterize the signatures of these events. [ABSTRACT FROM AUTHOR]

We explore the effects of the residual present in oxygen–neon white dwarfs (ONe WDs) on their evolution toward accretion-induced collapse (AIC). We produce a set of ONe WD models using Modules for Experiments in Stellar Astrophysics (MESA) and illustrate how the amount and location of the residual carbon depends on the initial mass of the star and assumptions about rotation and convective overshooting. We find a wide range of possible mass fractions roughly ranging from 0.1% to 10%. Convection and thermohaline mixing that occurs as the ONe WDs cool leads to nearly homogeneous interior compositions by the time that AIC would occur. We evolve these ONe WD models and some toy WD models toward AIC and find that regardless of the carbon fraction, the occurrence of Urca-process cooling due to implies that the models are unlikely to reach carbon ignition before electron captures on occur. Difficulties associated with modeling electron-capture-driven convective regions in these ONe WDs prevent us from evolving our MESA models all the way to thermonuclear oxygen ignition and the onset of collapse. Thus, firm conclusions about the effect of carbon on the final fates of these objects await improved modeling. However, it is clear that the inclusion of residual carbon can shift the evolution from that previously described in the literature and should be included in future models. [ABSTRACT FROM AUTHOR]

Type Iax supernovae may arise from failed explosions of white dwarfs (WDs) that leave behind a bound remnant (i.e., a “postgenitor” star) that could be identified in wide field surveys. To understand their observational signatures, we simulate these WD postgenitors from shortly after explosion until they move back down the WD cooling track, and we consider several possible WD masses and explosion energies. To predict the peculiar surface abundances of the WD postgenitors, our models take into account gravitational settling and radiative levitation. We find that radiative levitation is significant at temperatures above a mass-dependent critical temperature, typically in the range Teff ≈ (50–100) × 103 K, significantly increasing surface abundances of iron group elements. Due to enhanced iron group opacity compared to normal WDs, the postgenitor peak luminosity and cooling timescale depend sensitively on mass, with more massive WDs becoming brighter but cooling much faster. We discuss our results in light of recently discovered hypervelocity WDs with peculiar surface compositions, finding that our low-mass postgenitor models match many of their observational characteristics. Finally, we explore the effects of thermohaline diffusion, tentatively finding that it strongly suppresses abundance enhancements created by radiative levitation, but more realistic modeling is required to reach a firm conclusion. [ABSTRACT FROM AUTHOR]

The neutron excess at the time of explosion provides a powerful discriminant among models of Type Ia supernovae. Recent calculations of the carbon simmering phase in single degenerate progenitors have disagreed about the final neutron excess. We find that the treatment of mixing in convection zones likely contributes to the difference. We demonstrate that in Modules for Experiments in Stellar Astrophysics models, heating from exothermic weak reactions plays a significant role in raising the temperature of the white dwarf. This emphasizes the important role that the convective Urca process plays during simmering. We briefly summarize the shortcomings of current models during this phase. Ultimately, we do not pinpoint the difference between the results reported in the literature, but show that the results are consistent with different net energetics of the convective Urca process. This problem serves as an important motivation for the development of models of the convective Urca process suitable for incorporation into stellar evolution codes. [ABSTRACT FROM AUTHOR]

We study the evolution and final outcome of long-lived ( years) remnants from the merger of an He white dwarf (WD) with a more massive C/O or O/Ne WD. Using Modules for Experiments in Stellar Astrophysics (), we show that these remnants have a red giant configuration supported by steady helium burning, adding mass to the WD core until it reaches . At that point, the base of the surface convection zone extends into the burning layer, mixing the helium-burning products (primarily carbon and magnesium) throughout the convective envelope. Further evolution depletes the convective envelope of helium and dramatically slows the mass increase of the underlying WD core. The WD core mass growth re-initiates after helium depletion, as then an uncoupled carbon-burning shell is ignited and proceeds to burn the fuel from the remaining metal-rich extended envelope. For large enough initial total merger masses, O/Ne WD cores would experience electron-capture triggered collapse to neutron stars (NSs) after growing to near Chandrasekhar mass (). Massive C/O WD cores could suffer the same fate after a carbon-burning flame converts them to ONe. The NS formation would release erg into the remaining extended low mass envelope. Using the STELLA radiative transfer code, we predict the resulting optical light curves from these exploded envelopes. Reaching absolute magnitudes of , these transients are bright for about one week and have many features of the class of luminous, rapidly evolving transients studied by Drout and collaborators. [ABSTRACT FROM AUTHOR]

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 FROM AUTHOR]

Accretion-induced collapse (AIC) occurs when an O/Ne white dwarf (WD) grows to nearly the Chandrasekhar mass (), reaching central densities that trigger electron captures in the core. Using Modules for Experiments in Stellar Astrophysics (MESA), we present the first true binary simulations of He star + O/Ne WD binaries, focusing on a He star in a 3 hr orbital period with O/Ne WDs. The helium star fills its Roche lobe after core helium burning is completed and donates helium on its thermal timescale to the WD, , which is a rate high enough that the accreting helium burns stably on the WD. The accumulated carbon/oxygen ashes from the helium burning undergo an unstable shell flash that initiates an inwardly moving, carbon burning flame. This flame is only quenched when it runs out of carbon at the surface of the original O/Ne core. Subsequent accumulation of fresh carbon/oxygen layers also undergo thermal instabilities, but no mass loss is triggered, which allows , and then triggers the onset of AIC. We also discuss the scenario of accreting C/O WDs that experience shell carbon ignitions to become O/Ne WDs, and then, under continuing mass transfer, lead to AIC. Studies of the AIC event rate using binary population synthesis should include all of these channels, especially this latter channel, which has been previously neglected but might dominate the rate. [ABSTRACT FROM AUTHOR]

In most astrophysical situations, the radioactive decay of to occurs via electron capture with a fixed half-life of 6.1 days. However, this decay rate is significantly slowed when the nuclei are fully ionized because K-shell electrons are unavailable for capture. In this paper, we explore the effect of these delayed decays on white dwarfs (WDs) that may survive Type Ia and Type Iax supernovae (SNe Ia and SNe Iax). The energy released by the delayed radioactive decays of and drives a persistent wind from the surviving WD’s surface that contributes to the late-time appearance of these SNe after emission from the bulk of the SN ejecta has faded. We use the stellar evolution code MESA to calculate the hydrodynamic evolution and resulting light curves of these winds. Our post-SN Ia models conflict with late-time observations of SN 2011fe, but uncertainties in our initial conditions prevent us from ruling out the existence of surviving WD donors. Much better agreement with observations is achieved with our models of post-SN Iax bound remnants, providing evidence that these explosions are due to deflagrations in accreting WDs that fail to completely unbind the WDs. Future radiative transfer calculations and wind models utilizing simulations of explosions for more accurate initial conditions will extend our study of radioactively powered winds from post-SN surviving WDs and enable their use as powerful discriminants among the various SN Ia and SN Iax progenitor scenarios. [ABSTRACT FROM AUTHOR]

It has been proposed that mixing induced by convective overshoot can disrupt the inward propagation of carbon deflagrations in super-asymptotic giant branch stars. To test this theory, we study an idealized model of convectively bounded carbon flames with 3D hydrodynamic simulations of the Boussinesq equations using the pseudo-spectral code Dedalus. Because the flame propagation timescale is much longer than the convection timescale, we approximate the flame as fixed in space, and only consider its effects on the buoyancy of the fluid. By evolving a passive scalar field, we derive a turbulent chemical diffusivity produced by the convection as a function of height, . Convection can stall a flame if the chemical mixing timescale, set by the turbulent chemical diffusivity, , is shorter than the flame propagation timescale, set by the thermal diffusivity, κ, i.e., when . However, we find for most of the flame because convective plumes are not dense enough to penetrate into the flame. Extrapolating to realistic stellar conditions, this implies that convective mixing cannot stall a carbon flame and that “hybrid carbon–oxygen–neon” white dwarfs are not a typical product of stellar evolution. [ABSTRACT FROM AUTHOR]

White dwarfs accreting from helium stars can stably burn at the accreted rate and avoid the challenge of mass loss associated with unstable helium burning that is a concern for many SNe Ia scenarios. We study binaries with helium stars of mass , which have lost their hydrogen rich envelopes in an earlier common envelope event and now orbit with periods () of several hours with non-rotating 0.84 and C/O WDs. The helium stars fill their Roche lobes after exhaustion of central helium and donate helium on their thermal timescales ( years). As shown by others, these mass transfer rates coincide with the steady helium burning range for WDs, and grow the WD core up to near the Chandrasekhar mass () and a core carbon ignition. We show here, however, that many of these scenarios lead to an ignition of hot carbon ashes near the outer edge of the WD and an inward going carbon flame that does not cause an explosive outcome. For hr, C/O WDs with donor masses experience a shell carbon ignition, while will fall below the steady helium burning range and undergo helium flashes before reaching core C ignition. Those with will experience a core C ignition. We also calculate the retention fraction of accreted helium when the accretion rate leads to recurrent weak helium flashes. [ABSTRACT FROM AUTHOR]