Sinking and spreading of metal pollution in magnetic white dwarfs
Abstract
Observations of magnetic polluted white dwarfs indicate that most of them have higher concentrations of metals near the poles compared to lower latitudes. Maintaining such abundance gradients requires gravitational sinking times t↓ below the convection zone that are shorter than the horizontal spreading time t↔ across the surface by convective eddies. We show analytically that t↓/t↔∝ T8/3, where T is the temperature at the base of the convection zone, which rises by more than an order of magnitude as convection penetrates deeper into the atmosphere. Correspondingly, t↓/t↔ jumps by several orders of magnitude, clearly delineating between hot white dwarfs with abundance variations and cold ones with homogenous surfaces. We incorporate magnetic fields self-consistently into the stellar structure and compute for the first time the sinking to spreading time-scale ratio as a function of B. Magnetic white dwarfs have shallower convection zones at a given Teff, and the inward penetration of convection shifts to lower Teff. Quantitatively, B∼ 105 G extends the range for enhanced abundance patches from Teff≳ 30 000 K (13 000 K) to Teff≳ 15 000 K (6000 K) for helium (hydrogen) dominated atmospheres, which is insufficient to explain the observed patchy white dwarfs, which are even colder. A possible solution is variability in the accretion rate on a time-scale of a decade, as recently detected in another white dwarf.
Related
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BibTeX
@misc{shiftan2026sinking,
title = {{Sinking and spreading of metal pollution in magnetic white dwarfs}},
author = {Elad Shiftan and Sivan Ginzburg},
howpublished = {arXiv},
year = {2026},
month = sep,
doi = {10.48550/arxiv.2609.00117},
eprint = {2609.00117},
archivePrefix = {arXiv},
url = {https://arxiv.org/abs/2609.00117},
}
RIS
TY - UNPB
TI - Sinking and spreading of metal pollution in magnetic white dwarfs
AU - Elad Shiftan
AU - Sivan Ginzburg
PY - 2026
DA - 2026/09/01
PB - arXiv
DO - 10.48550/arxiv.2609.00117
UR - https://arxiv.org/abs/2609.00117
AB - Observations of magnetic polluted white dwarfs indicate that most of them have higher concentrations of metals near the poles compared to lower latitudes. Maintaining such abundance gradients requires gravitational sinking times t_↓ below the convection zone that are shorter than the horizontal spreading time t_↔ across the surface by convective eddies. We show analytically that t_↓/t_↔∝ T^8/3, where T is the temperature at the base of the convection zone, which rises by more than an order of magnitude as convection penetrates deeper into the atmosphere. Correspondingly, t_↓/t_↔ jumps by several orders of magnitude, clearly delineating between hot white dwarfs with abundance variations and cold ones with homogenous surfaces. We incorporate magnetic fields self-consistently into the stellar structure and compute for the first time the sinking to spreading time-scale ratio as a function of B. Magnetic white dwarfs have shallower convection zones at a given T_eff, and the inward penetration of convection shifts to lower T_eff. Quantitatively, B∼ 10^5 G extends the range for enhanced abundance patches from T_eff≳ 30 000 K (13 000 K) to T_eff≳ 15 000 K (6000 K) for helium (hydrogen) dominated atmospheres, which is insufficient to explain the observed patchy white dwarfs, which are even colder. A possible solution is variability in the accretion rate on a time-scale of a decade, as recently detected in another white dwarf.
ER -