# Trace hydrogen in helium atmosphere white dwarfs as a possible signature of water accretion

## TL;DR

This study suggests that hydrogen detected in helium atmosphere white dwarfs likely originates from accreted water-rich planetesimals, indicating water is common in planetary systems with rocky bodies.

## Contribution

It provides evidence linking hydrogen in helium white dwarfs to water accretion, highlighting a correlation between hydrogen presence and metal pollution in these stars.

## Key findings

- Hydrogen is nearly twice as common in metal-polluted white dwarfs.
- GD 16 and GD 17 contain large amounts of atmospheric hydrogen.
- Water-bearing planetesimals are a plausible source of hydrogen in these stars.

## Abstract

A handful of white dwarfs with helium-dominated atmospheres contain exceptionally large masses of hydrogen in their convection zones, with the metal-polluted white dwarf GD 16 being one of the earliest recognised examples. We report the discovery of a similar star: the white dwarf coincidentally named GD 17. We obtained medium-resolution spectroscopy of both GD 16 and GD 17 and calculated abundances and accretion rates of photospheric H, Mg, Ca, Ti, Fe and Ni. The metal abundance ratios indicate that the two stars recently accreted debris which is Mg-poor compared to the composition of bulk Earth. However, unlike the metal pollutants, H never diffuses out of the atmosphere of white dwarfs and we propose that the exceptionally high atmospheric H content of GD 16 and GD 17 ($2.2\times 10^{24}$g and $2.9\times 10^{24}$g respectively) could result from previous accretion of water bearing planetesimals. Comparing the detection of trace H and metal pollution among 729 helium atmosphere white dwarfs, we find that the presence of H is nearly twice as common in metal-polluted white dwarfs compared to their metal-free counterparts. This highly significant correlation indicates that, over the cooling age of the white dwarfs, at least some fraction of the H detected in many He atmospheres (including GD 16 and GD 17) is accreted alongside metal pollutants, where the most plausible source is water. In this scenario, water must be common in systems with rocky planetesimals.