Increases to Inferred Rates of Planetesimal Accretion Due to Thermohaline Mixing in Metal Accreting White Dwarfs

TL;DR

This paper demonstrates that thermohaline mixing significantly increases inferred accretion rates in metal-polluted white dwarfs, challenging previous models that only considered diffusive sedimentation.

Contribution

It introduces models incorporating thermohaline mixing, showing that higher accretion rates are necessary to match observed metal abundances in white dwarfs.

Findings

Thermohaline mixing destabilizes high accretion rate models.

Active thermohaline mixing requires larger accretion rates, up to 10^{13} g/s.

Metal abundance ratios reflect the composition of accreted material.

Abstract

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, $\dot M_{\rm acc}$, 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 high $\dot M_{\rm acc}$ 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 reaching $\dot M_{\rm acc} \approx 10^{13} \, \rm g \, s^{-1}$ 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.