The discovery of oxygen-rich chromospheres in two massive DQ white dwarfs

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.