Where Are the Extrasolar Mercuries?

Abstract

Abstract We utilize observations of 16 white dwarf stars to calculate and analyze the oxidation states of the parent bodies accreting onto stars. Oxygen fugacity, which is a measure of overall oxidation state for rocks, is as important as pressure and temperature in determining the structure of a planet. We find that most of the extrasolar rocky bodies formed under oxidizing conditions, but that ∼1/4 of the polluted white dwarfs have compositions that are consistent with more reduced parent bodies. The difficulty in constraining the oxidation states of relatively reduced bodies is discussed and a model for the time-dependent evolution of the apparent oxygen fugacity for a hypothetical reduced body engulfed by a WD is investigated. The differences in diffusive fluxes of various elements through the WD envelope yield spurious inferred bulk elemental compositions and oxidation states of the accreting parent bodies under certain conditions. The worst case for biasing against detection of reduced bodies occurs for high effective temperatures. For moderate and low effective temperatures, the evidence for relatively reduced parent bodies is preserved under most circumstances for at least several characteristic lifetimes of the debris disk.