  We present Keck/HIRES data with model atmosphere analysis of the
helium-dominated polluted white dwarf GD 40, in which we measure atmospheric
abundances relative to helium of 9 elements: H, O, Mg, Si, Ca, Ti, Cr, Mn, and
Fe. Apart from hydrogen whose association with the other contaminants is
uncertain, this material most likely accreted from GD 40's circumstellar dust
disk whose existence is demonstrated by excess infrared emission. The data are
best explained by accretion of rocky planetary material, in which heavy
elements are largely contained within oxides, derived from a tidally disrupted
minor planet at least the mass of Juno, and probably as massive as Vesta. The
relatively low hydrogen abundance sets an upper limit of 10% water by mass in
the inferred parent body, and the relatively high abundances of refractory
elements, Ca and Ti, may indicate high-temperature processing. While the
overall constitution of the parent body is similar to the bulk Earth being over
85% by mass composed of oxygen, magnesium, silicon and iron, we find
n(Si)/n(Mg) = 0.30 +/- 0.11, significantly smaller than the ratio near unity
for the bulk Earth, chondrites, the Sun, and nearby stars. This result suggests
that differentiation occurred within the parent body.
