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We present Keck/HIRES data with model atmosphere analysis of thehelium-dominated polluted white dwarf GD 40, in which we measure atmosphericabundances relative to helium of 9 elements: H, O, Mg, Si, Ca, Ti, Cr, Mn, andFe. Apart from hydrogen whose association with the other contaminants isuncertain, this material most likely accreted from GD 40's circumstellar dustdisk whose existence is demonstrated by excess infrared emission. The data arebest explained by accretion of rocky planetary material, in which heavyelements are largely contained within oxides, derived from a tidally disruptedminor planet at least the mass of Juno, and probably as massive as Vesta. Therelatively low hydrogen abundance sets an upper limit of 10% water by mass inthe inferred parent body, and the relatively high abundances of refractoryelements, Ca and Ti, may indicate high-temperature processing. While theoverall constitution of the parent body is similar to the bulk Earth being over85% by mass composed of oxygen, magnesium, silicon and iron, we findn(Si)/n(Mg) = 0.30 +/- 0.11, significantly smaller than the ratio near unityfor the bulk Earth, chondrites, the Sun, and nearby stars. This result suggeststhat differentiation occurred within the parent body.

Chemical Abundances in the Externally Polluted White Dwarf GD 40: Evidence of a Rocky Extrasolar Minor Planet