Some hot DA white dwarfs have circumstellar high ion absorption features in
their spectra, in addition to those originating in the photosphere. In many
cases, the line profiles of these absorbing components are unresolved. Given
the importance of the atmospheric composition of white dwarfs to studies of
stellar evolution, extra-solar planetary systems and the interstellar medium,
we examine the effect of including circumstellar line profiles in the abundance
estimates of photospheric metals in six DA stars. The photospheric C and Si
abundances are reduced in five cases where the circumstellar contamination is
strong, though the relative weakness of the circumstellar Si IV absorption
introduces minimal contamination, resulting in a small change in abundance. The
inability of previous, approximate models to reproduce the photospheric line
profiles here demonstrates the need for a technique that accounts for the
physical line profiles of both the circumstellar and photospheric lines when
modelling these blended absorption features.Comment: 7 pages, 5 figues, 3 tables, accepted for publication in MNRA
