On the Lithology and Mineralogy of Polluted White Dwarf Materials

To better understand rocky exoplanet compositions we compare materials being accreted by “polluted” White Dwarfs (WDs; n = 22) to meteorites, terrestrial planets, and Sun-like stars from the Hypatia Catalog (FGKM class; n = 4,350). We use Mg, Si, Ca, and Fe as these elements comprise >95% of terrestrial planetary cations (Ni and Al comprising most of the remainder) and are sufficient to broadly identify planetary rock types; they are also simultaneously available for nearly two dozen WDs. In the event, we find some overlap between Hypatia and WD compositions, however 32% (7) of WDs range to Ca contents above levels observed for Hypatia stars, while another 18% (4) have both uniquely high Fe and low Mg. White Dwarfs also range beyond composition estimates for bulk terrestrial planets, but they do not exceed the compositional ranges for meteorites, indicating that most WDs could represent a—perhaps random—sampling of differentiated planetary material. For example, mesosiderites (mixtures of pyroxene and metallic Fe) appear to be good analogs for three WDs: HE0106-3253, PG1015+161, and WD1145+017, while one WD (PG0843+517) is strikingly like an iron meteorite (although a different report of PG0843 by Gänsicke et al. [2012] indicates an admixture of silicate material). However, most meteorite classes are imprecise analogs: (a) four WDs overlap nicely with urelites (primitive, C-rich achondrites) for Mg, Fe and Si, but only two (GD61 and WDJ1242+5226) have sufficiently low Ca; (b) another four have Mg and Si that matches chondritic meteorites, but again, only two (WD2207+121 and WD1551+175) are not overly enriched in Ca; (c) the four most Ca-enriched WDs have Ca contents similar to Howardites (achondrites, thought to derive from Vesta) but dissimilar with respect to other cations. There are several possibilities for such mismatches, among which are accretionary dynamics, and observational bias; the most intriguing possibility, though, is that WD materials represent fractionation paths not accessible to (or yet discovered in) our solar system.