A comparative study of the atmospheric composition of the DA white dwarfs Feige 24 and G191-B2B

We analyze Hubble Space Telescope Imaging Spectrograph spectra of two white dwarfs, Feige 24 and G191-B2B, and we compare the measured abundance patte rns. Except for carbon, which appears overabundant in G191-B2B by 0.3-0.5 d ex relative to Feige 24, the average heavy-element abundance in Feige 24 is 0.17 dex larger than in the cooler, hence older, G191-B2B, indicating a sl ow but perceptible decline of metallicity with time. We also found that the C IV lambda 1550 doublet in G191-B2B is composed of a photospheric compone nt and a second component, either interstellar or circumstellar, separated by only Delta upsilon = 15 km s(-1). We find a simple correspondence betwee n the measured element abundance, its solar abundance value, and its atomic weight from which we deduce the likely presence of additional elements suc h as neon and magnesium in the photospheres of Feige 24 and G191-B2B.