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J. L. Provencal, H. L. Shipman, J. MacDonald (University of Delaware)
The composition and structure of white dwarf stars contain a wealth of information about the evolution of contemporary main sequence stars. We use this information to build theoretical models placing limits on star formation history, age, and chemical evolution of the Galactic disk. One of the largest uncertainties in our models is the interior composition of a white dwarf, assumed to be a mixture of carbon and oxygen. This uncertainty introduces a 20% error in the ages of white dwarfs.
DQs, helium rich white dwarfs displaying carbon spectroscopic features, offer an opportunity to directly observe the actual carbon/oxygen ratio in a white dwarf core. Theoretical work argues that oxygen should be detectable in the more massive DQ stars. We present STIS observations of the massive white dwarf G227-5 in which we detect the expected oxygen, but in emission indicating a stellar wind/chromospheric activity not expected in such a cool white dwarf. We discuss the implications of this phenomenon.