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

Understand

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.