Importance of fingering convection for accreting white dwarfs   in the framework of full evolutionary calculations: the case  of the hydrogen-rich white dwarfs GD 133 and G 29-38

Wachlin, F. C.; Vauclair, G.; Vauclair, S.; Althaus, Leandro Gabriel

doi:10.1051/0004-6361/201630094

Cited by 37 publications

(44 citation statements)

References 55 publications

Supporting

Mentioning

Contrasting

Order By: Relevance

“…Figure 4 also compares against MESA calculations assuming only diffusion governs the equilibrium state, where we use Equation 4 to inferṀ acc , and assume bulk earth abundances for the accreted material. For objects with T eff > 10, 000 K, the inferred accretion rate can increase by several orders of magnitude when accounting for thermohaline mixing, confirming the earlier work of Deal et al (2013) and Wachlin et al (2017).…”

Section: Accretion Rates and Compositionssupporting

confidence: 81%

“…Hence, we infer an accretion rate that is 40% larger than the value Xu et al (2014) report, but we find excellent agreement with their relative diffusion timescales and composition of the accreted material. Deal et al (2013) and Wachlin et al (2017) have noted that thermohaline mixing may significantly alter the accretion rates inferred from observed abundances in polluted WDs. Koester (2015) argued against the efficacy of this instability for a few cases.…”

Section: Gravitational Sedimentationmentioning

confidence: 99%

“…Much recent work in this field has relied on the assumption of equilibrium between accretion and diffusive sedimentation. The large mean molecular weight of accreted material compared to the hydrogen atmosphere can lead to additional mixing due to the thermohaline instability (Deal et al 2013;Wachlin et al 2017), though see Koester (2015) for a critique of its efficacy. We perform time-dependent stellar evolution calculations using MESA (Paxton et al 2011(Paxton et al , 2013(Paxton et al , 2015(Paxton et al , 2018.…”

Section: Introductionmentioning

confidence: 99%

See 2 more Smart Citations

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

Bauer

Bildsten

2018

ApJL

View full text Add to dashboard Cite

Many isolated, old white dwarfs (WDs) show surprising evidence of metals in their photospheres. Given that the timescale for gravitational sedimentation is astronomically short, this is taken as evidence for ongoing accretion, likely of tidally disrupted planetesimals. The rate of such accretion,Ṁ acc , is important to constrain, and most modeling of this process relies on assuming an equilibrium between diffusive sedimentation and metal accretion supplied to the WD's surface convective envelope. Building on earlier work of Deal and collaborators, we show that highṀ acc models with only diffusive sedimentation are unstable to thermohaline mixing and that models which account for the enhanced mixing from the active thermohaline instability require larger accretion rates, sometimes reachingṀ acc ≈ 10 13 g s −1 to explain observed Calcium abundances. We present results from a grid of MESA models that include both diffusion and thermohaline mixing. These results demonstrate that both mechanisms are essential for understanding metal pollution across the range of polluted WDs with hydrogen atmospheres. Another consequence of active thermohaline mixing is that the observed metal abundance ratios are identical to accreted material.

show abstract

Section: Accretion Rates and Compositionssupporting

confidence: 81%

Section: Gravitational Sedimentationmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

See 1 more Smart Citation

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

Bauer

Bildsten

2018

ApJL

View full text Add to dashboard Cite

show abstract

“…Corresponding author: Evan B. Bauer ebauer@physics.ucsb.edu While many have relied on elemental sedimentation timescales to make inferences about polluted WD accretion, recent work has revealed that thermohaline instability is active and substantially modifies the inferred accretion rateṀ acc (Deal et al 2013;Wachlin et al 2017). Our work in Bauer & Bildsten (2018, Paper I) greatly expanded the range of T eff explored for polluted WD models accounting for thermohaline mixing.…”

Section: Introductionmentioning

confidence: 90%

Polluted White Dwarfs: Mixing Regions and Diffusion Timescales

Bauer

Bildsten

2019

ApJ

View full text Add to dashboard Cite

Many isolated white dwarfs (WDs) show spectral evidence of atmospheric metal pollution. Since heavy element sedimentation timescales are short, this most likely indicates ongoing accretion. Accreted metals encounter a variety of mixing processes at the WD surface: convection, gravitational sedimentation, overshoot, and thermohaline instability. We present MESA WD models that explore each of these processes and their implications for inferred accretion rates. We provide diffusion timescales for many individual metals, and we quantify the regimes in which thermohaline mixing dominates over gravitational sedimentation in setting the effective settling rate of the heavy elements. We build upon and confirm earlier work finding that accretion rates as high as 10 13 g s −1 are needed to explain observed pollution in DA WDs for T eff > 15, 000 K, and we provide tabulated results from our models that enable accretion rate inferences from observations of polluted DA WDs. If these rates are representative of young WDs, we estimate that the total mass of planetesimal material accreted over a WD lifetime may be as high as 10 28 g, though this estimate is susceptible to potential selection biases and uncertainties about the nature of disk processes that supply accretion to the WD surface. We also find that polluted DB WDs experience much less thermohaline mixing than DA WDs, and we do not expect thermohaline instability to be active for polluted DB WDs with T eff < 18, 000 K.

show abstract

“…The presence of metals in the envelope can themselves lead to sharp gradients in density, triggering instabilities such as the thermohaline (fingering) instability. This can lead to an under-prediction of accretion rates by orders of magnitude (Deal et al 2013;Bauer & Bildsten 2018;Bauer & Bildsten 2019;Wachlin et al 2017), although its application to white dwarf convection zones has been questioned (Koester 2015). It is, however, clear from the above papers that the thermohaline instability is not important for the cool, helium-rich white dwarfs considered in this work.…”

Section: O R I Gmentioning

confidence: 99%

Bayesian constraints on the origin and geology of exoplanetary material using a population of externally polluted white dwarfs

Harrison

Bonsor

Kama

et al. 2021

Monthly Notices of the Royal Astronomical Society

View full text Add to dashboard Cite

White dwarfs that have accreted planetary bodies are a powerful probe of the bulk composition of exoplanetary material. In this paper, we present a Bayesian model to explain the abundances observed in the atmospheres of 202 DZ white dwarfs by considering the heating, geochemical differentiation, and collisional processes experienced by the planetary bodies accreted, as well as gravitational sinking. The majority (>60%) of systems are consistent with the accretion of primitive material. We attribute the small spread in refractory abundances observed to a similar spread in the initial planet-forming material, as seen in the compositions of nearby stars. A range in Na abundances in the pollutant material is attributed to a range in formation temperatures from below 1,000 K to higher than 1,400 K, suggesting that pollutant material arrives in white dwarf atmospheres from a variety of radial locations. We also find that Solar System-like differentiation is common place in exo-planetary systems. Extreme siderophile (Fe, Ni or Cr) abundances in 8 systems require the accretion of a core-rich fragment of a larger differentiated body to at least a 3σ significance, whilst one system shows evidence that it accreted a crust-rich fragment. In systems where the abundances suggest that accretion has finished (13/202), the total mass accreted can be calculated. The 13 systems are estimated to have accreted masses ranging from the mass of the Moon to half that of Vesta. Our analysis suggests that accretion continues for 11Myrs on average.

show abstract

Importance of fingering convection for accreting white dwarfs in the framework of full evolutionary calculations: the case of the hydrogen-rich white dwarfs GD 133 and G 29-38

Cited by 37 publications

References 55 publications

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

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

Polluted White Dwarfs: Mixing Regions and Diffusion Timescales

Bayesian constraints on the origin and geology of exoplanetary material using a population of externally polluted white dwarfs

Contact Info

Product

Resources

About