Strange Abundances in a White Dwarf: Evidence for Simultaneous Accretion of Rocky and Icy Bodies?

We present analysis of optical and ultraviolet spectra of the hot, bright, white dwarf G238-44 obtained with FUSE, Keck/HIRES, HST/COS and STIS. Ten elements heavier than helium (C, N, O, Mg, Al, Si, P, S, Ca, Fe) are detected, and we derive their photospheric abundances and diffusion fluxes. Since the atmospheric diffusion timescales of these elements are on the order of days, accretion-diffusion is currently ongoing in a steady-state. By comparing equivalent width measurements from 10 nights of HIRES data over a baseline of 24 years, we conclude that accretion onto the surface of G238-44 is essentially constant and continuous. Thus, the star must be surrounded by a substantial quantity of orbiting material even though no infrared excess emission has ever been associated with the white dwarf. The conventional interpretation for the presence of heavy elements in a white dwarf spectrum is accretion of material from a planetary body, most often something similar to a dry stony asteroid with near chondritic abundances. But for some white dwarfs the accreted parent body displays evidence of differentiation, for example into core and mantle, or appears to have been water-rich, or, on rare occasions, so volatile rich that it appears similar to a Kuiper Belt object. In any event, all published analyses suggest that white dwarf photospheric abundances can be understood as due to accretion of a single parent body (or a collection of bodies with very similar elemental abundances) that has a solar system analog. The situation at G238-44 appears to be quite different. We are unable to match the relative elemental abundances in G238-44 with those of any known solar system object. We therefore suggest that, if exo-planetary bodies with solar system analogs are relevant for explaining the element abundances in G238-44, then the star is now accreting at least two such bodies that have very different compositions. Our best fit to the dataset suggests that G238-44 is currently accreting a mix of a rocky planetesimal and an analog of an icy Kuiper Belt object with an approximate, respective, mass ratio of 3:1.