Convection Destroys the Core/Mantle Structure in Hybrid C/O/Ne White Dwarfs

Rocha

2019

ApJ

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We explore the effects of the residual 12 C present in oxygen-neon white dwarfs (ONe WDs) on their evolution towards accretion-induced collapse (AIC). We produce a set of ONe WD models using 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 12 C mass fractions roughly ranging from 0.1 to 10 per cent. 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 towards AIC and find that regardless of the carbon fraction, the occurrence of Urca-process cooling due to 23 Na implies that the models are unlikely to reach carbon ignition before electron captures on 24 Mg 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.

Section: Series Model Initial Mass Rotation Overshooting Wd Model Masssupporting

confidence: 90%

Residual Carbon in Oxygen–Neon White Dwarfs and Its Implications for Accretion-induced Collapse

Rocha

2019

ApJ

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“…If future work shows that these flames do quench before reaching the center, then these models would not evolve towards AIC. In Brooks et al (2017), we showed that such a 'hybrid' WD is in fact unstable to convection as it cools. However, in the accretion scenario considered in this paper, the relatively short timescale to grow to M Ch ( Myr) means that the WD would not be fully mixed at the time it nears M Ch .…”

Section: Structure At Aicmentioning

confidence: 99%

“…We start with a 1.1M WD that, due to convective boundary mixing, had a 0.4M cold C/O core beneath a hot O/Ne mantle. As in Brooks et al (2017), we then allowed for complete mixing over 10 Myr and then placed this WD in a 3 hr orbital period binary with a 1.5M helium star. When the WD reaches 1.25M via accretion and steady helium burning, a carbon flame in the hot C/O ashes ignites and propagates into the star, as shown in Figure 6.…”

Section: C/o Wd Accretors As Aic Progenitorsmentioning

confidence: 99%

Accretion-induced Collapse from Helium Star + White Dwarf Binaries

Brooks

Bildsten

et al. 2017

ApJ

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Accretion-induced collapse (AIC) occurs when an O/Ne white dwarf (WD) grows to nearly the Chandrasekhar mass (M Ch ), 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 1.5M He star in a 3 hour orbital period with 1.1−1.3M 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,Ṁ ≈ 3 × 10 −6 M /yr, 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, allowing M WD → M Ch , triggering 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.

“…1 We find that the convective regions grow and mix material on a timescale set by the selected cooling timescale in our simulations. Likewise, the mixing prescription assumed in the stellar evolution calculations of Brooks et al (2017) led to the mixing of the stellar model on its cooling time. where ∇ T and ∇ ad are the actual and adiabatic temperature gradients, g is the local gravitational acceleration, ρ and P are the density and total pressure, and χ T = ∂ ln P/∂ ln T and χ ρ = ∂ ln P/∂ ln ρ are the compressibilities.…”

Section: Cooling Hybrid Wdsmentioning

confidence: 99%

“…In Section 3 we outline the equations being solved in our numerical simulations and devise an idealized problem that captures the key features of the cooling hybrid WD. In Section 4 we compare the results of our simulations to models that adopt the approach taken by Brooks et al (2017). In Section 5 we summarize and conclude.…”

Section: Introductionmentioning

confidence: 99%

Mixing via Thermocompositional Convection in Hybrid C/O/Ne White Dwarfs

Garaud

2019

ApJ

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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.