Increases to Inferred Rates of Planetesimal Accretion due to Thermohaline Mixing in Metal-accreting White Dwarfs

Bauer, Evan B.; Bildsten, Lars

doi:10.3847/2041-8213/aac492

Cited by 51 publications

(72 citation statements)

References 46 publications

Supporting

Mentioning

Contrasting

Order By: Relevance

“…Thermohaline instabilities result from an inverted mean molecular weight gradient in the atmosphere of white dwarfs. This instability results in the depletion of all elements equally (Bauer & Bildsten 2018). However, thermohaline instabilities only dominate over gravitational settling if they occur on a shorter timescale, and Bauer & Bildsten (2018) conclude that thermohaline instability only dominates in white dwarfs with thin, hot, H dominated atmospheres.…”

Section: Mg Depletionmentioning

confidence: 91%

“…This instability results in the depletion of all elements equally (Bauer & Bildsten 2018). However, thermohaline instabilities only dominate over gravitational settling if they occur on a shorter timescale, and Bauer & Bildsten (2018) conclude that thermohaline instability only dominates in white dwarfs with thin, hot, H dominated atmospheres. The sample we have used consists of cool white dwarfs with He dominated atmospheres which are expected to be dominated by gravitational settling.…”

Section: Mg Depletionmentioning

confidence: 91%

See 1 more Smart Citation

Modelling the distributions of white dwarf atmospheric pollution: a low Mg abundance for accreted planetesimals?

Turner

Wyatt

2019

Monthly Notices of the Royal Astronomical Society

View full text Add to dashboard Cite

The accretion of planetesimals onto white dwarf atmospheres allows determination of the composition of this polluting material. This composition is usually inferred from observed pollution levels by assuming it originated from a single body. This paper instead uses a stochastic model wherein polluting planetesimals are chosen randomly from a mass distribution, finding that the single body assumption is invalid in > 20% of cases. Planetesimal compositions are modelled assuming parent bodies that differentiated into core, mantle and crust components. Atmospheric levels of Ca, Mg and Fe in the model are compared to a sample of 230 DZ white dwarfs for which such pollution is measured. A good fit is obtained when each planetesimal has its core, mantle and crust fractions chosen independently from logit-normal distributions which lead to average mass fractions of f Cru = 0.15, f Man = 0.49 and f Cor = 0.36. However, achieving this fit requires a factor 4 depletion of Mg relative to stellar material. This depletion is unlikely to originate in planetesimal formation processes, but might occur from heating while the star is on the giant branch. Alternatively the accreted material has stellar abundance, and either the inferred low Mg abundance was caused by an incorrect assumption that Mg sinks slower than Ca and Fe, or there are unmodelled biases in the observed sample. Finally, the model makes predictions for the timescale on which the observed pollutant composition varies, which should be the longer of the sinking and disc timescales, implying variability on decadal timescales for DA white dwarfs.

show abstract

Section: Mg Depletionmentioning

confidence: 91%

Section: Mg Depletionmentioning

confidence: 91%

Modelling the distributions of white dwarf atmospheric pollution: a low Mg abundance for accreted planetesimals?

Turner

Wyatt

2019

Monthly Notices of the Royal Astronomical Society

View full text Add to dashboard Cite

show abstract

“…In stars and WDs, thermohaline mixing has the effect of introducing mixing into radiative regions where mixing would otherwise be negligible. This mixing is important in scenarios like planetesimal accretion (Bauer & Bildsten 2018) and carbon-enhanced metal-poor stars (Stancliffe & Glebbeek 2008), where heavier elements accrete on top of a lightweight atmosphere. In our scenario, radiative levitation tends to push heavy elements upwards, while gravitational settling and thermohaline mixing counteract levitation.…”

Section: Thermohaline Mixingmentioning

confidence: 99%

The Long-term Evolution and Appearance of Type Iax Postgenitor Stars

Fuller

Schwab

Foley

2019

ApJ

View full text Add to dashboard Cite

Type Iax supernovae may arise from failed explosions of white dwarfs 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 white dwarf (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 T eff ≈ 50 − 100 × 10 3 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 white dwarfs 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.

show abstract

“…In White Dwarfs undergoing accretion of material from a surrounding debris disk, a fingering region is thought to be located just beneath the surface (c.f. Deal et al 2013;Bauer & Bildsten 2018, for instance), and could participate in draining the excess metallicity of accreted material inward in addition to the effect of gravitational settling. Assuming a steady-state balance between the rate of debris accretion and the inward subsurface flux of heavy elements, one can in theory predict their expected surface abundances.…”

Section: Implications For Stellar Interiors and Future Workmentioning

confidence: 99%

The Interaction between Shear and Fingering (Thermohaline) Convection

Garaud

Kumar

Sridhar

2019

ApJ

View full text Add to dashboard Cite

Fingering convection is a turbulent mixing process that can occur in stellar radiative regions whenever the mean molecular weight increases with radius. In some cases, it can have a significant observable impact on stellar structure and evolution. The efficiency of mixing by fingering convection as a standalone process has been studied by Brown et al. (2013), but other processes such as rotation, magnetic fields and shear can affect it. In this paper, we present a first study of the effect of shear on fingering (thermohaline) convection in astrophysics. Using Direct Numerical Simulations we find that a moderate amount of shear (that is not intrinsically shear-unstable) always decreases the mixing efficiency of fingering convection, as a result of the tilt it imparts to the fingering structures. We propose a simple analytical extension of the Brown et al. (2013) model in the presence of shear that satisfactorily explains the numerically-derived turbulent compositional mixing coefficient for moderate shearing rates, and can trivially be implemented in stellar evolution codes. We also measure from the numerical simulations a turbulent viscosity, and find that the latter is strongly tied to the turbulent compositional mixing coefficient. Observational implications and caveats of the model are discussed.

show abstract

Increases to Inferred Rates of Planetesimal Accretion due to Thermohaline Mixing in Metal-accreting White Dwarfs

Cited by 51 publications

References 46 publications

Modelling the distributions of white dwarf atmospheric pollution: a low Mg abundance for accreted planetesimals?

Modelling the distributions of white dwarf atmospheric pollution: a low Mg abundance for accreted planetesimals?

The Long-term Evolution and Appearance of Type Iax Postgenitor Stars

The Interaction between Shear and Fingering (Thermohaline) Convection

Contact Info

Product

Resources

About