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 patterns. Except for carbon, which appears overabundant in G191-B2B by 0.3-0.5 dex 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 slow but perceptible decline of metallicity with time. We also found that the C IV λ1550 doublet in G191-B2B is composed of a photospheric component and a second component, either interstellar or circumstellar, separated by only Δv = 15 km s^-1. We find a simple correspondence between the measured element abundance, its solar abundance value, and its atomic weight from which we deduce the likely presence of additional elements such as neon and magnesium in the photospheres of Feige 24 and G191-B2B.