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

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 mediumresolution 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 (2.2×10 24 g and 2.9×10 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.

From hydrogen to helium: the spectral evolution of white dwarfs as evidence for convective mixing

“…white dwarfs and their accretion of hydrogen have suggested that this is an extremely rare scenario (Gentile Fusillo et al 2017;Rolland et al 2018). As such our assumption that the relative number of helium-atmosphere white dwarfs should increase monotonically with cooling age is likely a robust one.…”

“…In principle the hydrogen abundances in DBA stars could be used to reconstruct their past evolution, but this is not accounting for the fact that accretion of planetary debris can significantly impact their hydrogen content. In fact, several DBA white dwarfs have orders of magnitude more hydrogen than would be possible by the convective dilution or convective mixing scenarios, and it is thought that the accretion of water-rich asteroids is the most likely explanation for the hydrogen abundance in these objects (Farihi et al 2011;Raddi et al 2015;Gentile Fusillo et al 2017). The study of spectral evolution is clearly complex and involves many competing models that need to be constrained with well defined samples of the local white dwarf population.…”

Throwing Icebergs at White Dwarfs

“…However, there have been detailed abundance measurements for only about 15 polluted WDs so far (see Jura & Young 2014;Jura et al 2015;Wilson et al 2015;Farihi et al 2016;Melis & Dufour 2017;Xu et al 2017;Gentile Fusillo et al 2017, and references therein), two of which show signs of planetary pollution in terms of composition and amount (see Section 1 for planetary pollution) (Jura et al 2009;Zuckerman et al 2011), and one with signs of volatile material pollution in the form of nitrogen (Xu et al 2017). While these are so far very small number statistics, these current results could imply that the planetary and volatile pollution rates are relatively high, on the order of ∼ 10 %, roughly consistent with our findings that ∼ 7.5% of all WDs could be polluted by Kuiper belt analog objects, while about 1% could be polluted by Neptune-like planets.…”

Compositions of Planetary Debris around Dusty White Dwarfs

“…In a volume-limited sample of DBs, the fraction of water is less than 1% of the total accreted mass ) even though there are some exceptions where the WDs have accreted a significant amount of water (e.g. Farihi et al 2013;Raddi et al 2015;Gentile Fusillo et al 2017). At least one WD has accreted a Kuiper-Belt-Object analog with 30% water and 10% carbon by mass (Xu et al 2017).…”