GD424 -- a helium-atmosphere white dwarf with a large amount of trace hydrogen in the process of digesting a rocky planetesimal

TL;DR

This study analyzes GD424, a helium-atmosphere white dwarf with trace hydrogen, revealing it accreted planetary debris possibly rich in water, and provides detailed atmospheric and compositional parameters.

Contribution

It presents a hybrid analysis method combining spectroscopy, photometry, and astrometry to determine white dwarf parameters and trace hydrogen origin.

Findings

Identified 11 photospheric metals in GD424.

Estimated the white dwarf's mass, radius, and cooling age.

Suggested the hydrogen originated from water-rich planetary debris.

Abstract

The photospheric metal pollution of white dwarfs is now well-established as the signature of the accretion of planetary debris. However, the origin of the trace hydrogen detected in many white dwarfs with helium atmospheres is still debated. Here, we report the analysis of GD424: a metal-polluted, helium-atmosphere white dwarf with a large amount of trace hydrogen. We determined the atmospheric parameters using a hybrid analysis that combines the sensitivity of spectroscopy to the atmospheric composition, $\log(\mathrm{H/He})$, with that of photometry and astrometry to the effective temperature, $T_{\mathrm{eff}}$, and surface gravity, $\log g$. The resulting white dwarf mass, radius, and cooling age are $M_{\mathrm{WD}}=0.77\pm0.01\,\mathrm{M}_{\odot}$, $R_{\mathrm{WD}}=0.0109\pm0.0001\,\mathrm{R}_{\odot}$, and $\tau_\mathrm{cool}=215\pm10$ Myr, respectively. We identified and measured the abundances of 11 photospheric metals and argue that the accretion event is most likely either in the increasing or steady state, and that the disrupted planetesimal resembles either CI chondrites or the bulk Earth in terms of its composition. We suggest that the observed $1.33\times 10^{22}$ g of trace hydrogen in GD424 were at least partly acquired through accretion of water-rich planetary debris in an earlier accretion episode.