The Discovery of Oxygen-rich Chromospheres in Two Massive DQ White Dwarfs

Abstract

We present HST observations of the massive DQ white dwarfs G227-5 and G35-26 in which we find the first clear evidence of chromospheric activity in cool field white dwarfs. The chromospheric spectra, obtained over a very narrow wavelength range, show lines of the usually expected species O I, N I, C II, and possibly Si III and Mg II. The O I/C II line strengths are considerably higher than usually found in main-sequence chromospheres. However, the hypothesis that these high line strengths represent a genuine oxygen overabundance in these stars is less likely than the interpretation that the oxygen abundance is enhanced by one or more of the well-known effects that can alter chromospheric abundances.