Magnesium abundances in cool metal-polluted white dwarfs

TL;DR

This study reanalyzed metal-polluted white dwarfs using advanced atmosphere models, revealing higher magnesium abundances and providing insights into the composition of accreted exoplanetesimals, thus addressing previous Mg depletion issues.

Contribution

It introduces improved atmosphere models for white dwarf analysis, leading to more accurate elemental abundance measurements and resolving prior Mg depletion discrepancies.

Findings

Higher Mg abundances than previous analyses.

Average log Mg/Ca ratio of 1.5, above chondritic values.

Supports longer Mg retention in white dwarf atmospheres.

Abstract

The accretion of rocky material is responsible for the presence of heavy elements in the atmospheres of a large fraction of white dwarf stars. Those objects represent a unique opportunity to infer the bulk composition of exoplanetesimals. This chemical characterization requires the use of detailed atmosphere models to determine the elemental abundances at the photospheres of white dwarfs. In this work, we use a state-of-the-art model atmosphere code to reanalyse the first large survey of metal-polluted white dwarfs for which abundances are found for multiple elements. We show that the improved constitutive physics of our models lead to systematically higher Mg abundances than previous analyses. We find an average log Mg/Ca number abundance ratio of 1.5. This value is significantly above the reference abundance for chondrites, which is expected as current diffusion models predict that for the cool helium-atmosphere white dwarfs of our sample Mg should remain in the atmosphere longer than Ca. This helps resolve a recently identified Mg depletion problem, where the planetesimals accreted by white dwarfs were reported to be Mg-deficient compared to the expected composition of their planetary systems.