Do White Dwarfs Sample Water-rich Planetary Material?

Polluted white dwarfs offer a unique way to directly probe the compositions of exoplanetary bodies. We examine the water content of accreted material using the oxygen abundances of 51 highly polluted white dwarfs. Within this sample, we present new abundances for three H-dominated atmosphere white dwarfs that showed promise for accreting water-rich material. Throughout, we explore the impact of the observed phase and lifetime of accretion disks on the inferred elemental abundances of the parent bodies that pollute each white dwarf. Our results indicate that white dwarfs sample a range of dry to water-rich material, with median uncertainties in water mass fractions of ≈15%. Among the He-dominated white dwarfs, 35/39 water abundances are consistent with corresponding H abundances. While for any individual white dwarf it may be ambiguous as to whether or not water is present in the accreted parent body, when considered as a population, the prevalence of water-rich bodies is statistically robust. The population as a whole has a median water mass fraction of ≈25%, and enforcing chondritic parent body compositions, we find that 31/51 WDs are likely to have nonzero water concentrations. This conclusion is different from a similar previous analysis of white dwarf pollution and we discuss reasons why this might be the case. Pollution in H-dominated white dwarfs continues to be more water-poor than in their He-dominated cousins, although the sample size of H-dominated white dwarfs remains small and the two samples still suffer a disjunction in the range of host star temperatures being probed.