Accretion Rates and Thermohaline Convection in Polluted White Dwarfs
- 1California Institute of Technology
- 2McGill University
Abstract
The atmospheres of polluted white dwarfs contain information about the composition of disrupted planetary material. However, interpreting the observed abundances requires a quantitative description of the mixing processes that transport accreted material through their envelopes. In particular, the accretion of metal-rich debris drives thermohaline convection in the star’s radiative zone. Previous studies have modeled this form of turbulence as an enhanced diffusive process, often inferring mass accretion rates M ̇ acc that exceed X-ray constraints. Here we develop a time-dependent model that treats thermohaline mixing as a propagating front whose evolution is coupled self-consistently to the surface abundance of heavy elements. We perform hydrodynamical simulations which demonstrate that thermohaline mixing drives the composition gradient toward values slightly above marginal thermohaline stability. Using this condition, we derive analytic solutions for the mixed layer depth h and surface abundance X surf , finding that both scale as t 1/2 at early times, transitioning to h ∝ t 4/27 and X surf ∝ t 10/27 at later times. Without gravitational settling, the thermohaline front never reaches a steady state, while including settling leads to an equilibrium surface abundance X surf , eq ∝ M ̇ acc 30 / 43 . Applied to G 29–38, our model matches the observed heavy-element abundance for M ̇ acc ∼ 1 × 1 0 10 –2 × 10 ¹⁰ g s ⁻¹ , comparable to previous findings using stellar evolution codes. These rates exceed the X-ray estimate by about an order of magnitude, which may point to a partially suppressed X-ray luminosity from the accretion boundary layer. We also present a practical prescription for inferring accretion rates directly from the observed surface abundance and stellar parameters for other polluted white dwarfs.
Versions
Related
- Transport of Magnetic Fields by Thermohaline Convection in Crystallizing White Dwarfs
- EPRV asteroseismology of the K dwarfs GJ 570 A and GJ 783 A with ESPRESSO. Empirical mode-amplitude scaling in tension with 3D convection simulations
- EPRV Asteroseismology of the K dwarfs GJ 570 A and GJ 783 A with ESPRESSO. Empirical mode-amplitude scaling in tension with 3D convection simulations
- Planetary Accretion Is Less Frequent in Wide Binaries: Evidence from Metal-Enriched White Dwarfs in DESI DR1
- Planetary Accretion Is Less Frequent in Wide Binaries: Evidence from Metal-enriched White Dwarfs in DESI DR1
BibTeX
@article{fuentes2026accretion,
title = {{Accretion Rates and Thermohaline Convection in Polluted White Dwarfs}},
author = {J. R. Fuentes and Matias Castro-Tapia and Jim Fuller and Marcus N. King},
journal = {The Astrophysical Journal Letters},
year = {2026},
month = nov,
doi = {10.3847/2041-8213/aeb6b1},
url = {https://iopscience.iop.org/article/10.3847/2041-8213/aeb6b1},
note = {CC BY 4.0},
}
RIS
TY - JOUR
TI - Accretion Rates and Thermohaline Convection in Polluted White Dwarfs
AU - J. R. Fuentes
AU - Matias Castro-Tapia
AU - Jim Fuller
AU - Marcus N. King
PY - 2026
DA - 2026/11/02
JO - The Astrophysical Journal Letters
DO - 10.3847/2041-8213/aeb6b1
UR - https://iopscience.iop.org/article/10.3847/2041-8213/aeb6b1
AB - The atmospheres of polluted white dwarfs contain information about the composition of disrupted planetary material. However, interpreting the observed abundances requires a quantitative description of the mixing processes that transport accreted material through their envelopes. In particular, the accretion of metal-rich debris drives thermohaline convection in the star’s radiative zone. Previous studies have modeled this form of turbulence as an enhanced diffusive process, often inferring mass accretion rates M ̇ acc that exceed X-ray constraints. Here we develop a time-dependent model that treats thermohaline mixing as a propagating front whose evolution is coupled self-consistently to the surface abundance of heavy elements. We perform hydrodynamical simulations which demonstrate that thermohaline mixing drives the composition gradient toward values slightly above marginal thermohaline stability. Using this condition, we derive analytic solutions for the mixed layer depth h and surface abundance X surf , finding that both scale as t 1/2 at early times, transitioning to h ∝ t 4/27 and X surf ∝ t 10/27 at later times. Without gravitational settling, the thermohaline front never reaches a steady state, while including settling leads to an equilibrium surface abundance X surf , eq ∝ M ̇ acc 30 / 43 . Applied to G 29–38, our model matches the observed heavy-element abundance for M ̇ acc ∼ 1 × 1 0 10 –2 × 10 ¹⁰ g s ⁻¹ , comparable to previous findings using stellar evolution codes. These rates exceed the X-ray estimate by about an order of magnitude, which may point to a partially suppressed X-ray luminosity from the accretion boundary layer. We also present a practical prescription for inferring accretion rates directly from the observed surface abundance and stellar parameters for other polluted white dwarfs.
SN - 2041-8213
SN - 2041-8205
ER -