  Observations of planetary material polluting the atmospheres of white dwarfs
are an important probe of the bulk composition of exoplanetary material.
Medium- and high-resolution optical and ultraviolet spectroscopy of seven white
dwarfs with known circumstellar dust and gas emission are presented. Detections
or meaningful upper limits for photospheric absorption lines are measured for:
C, O, Na, S, P, Mg, Al, Si, Ca, Ti, Cr, Fe, and Ni. For 16 white dwarfs with
known observable gaseous emission discs (and measured photospheric abundances),
there is no evidence that their accretion rates differ, on average, from those
without detectable gaseous emission. This suggests that, typically, accretion
is not enhanced by gas drag. At the effective temperature range of the white
dwarfs in this sample (16,000-25,000K) the abundance ratios of elements are
more consistent than absolute abundances when comparing abundances derived from
spectroscopic white dwarf parameters versus photometric white dwarf parameters.
Crucially, this highlights that the uncertainties on white dwarf parameters do
not prevent white dwarfs from being utilised to study planetary composition.
The abundances of oxygen and silicon for the three hydrogen-dominated white
dwarfs in the sample with both optical and ultraviolet spectra differ by 0.62
dex depending on if they are derived from the optical or ultraviolet spectra.
This optical/ultraviolet discrepancy may be related to differences in the
atmospheric depth of line formation; further investigations into the white
dwarf atmospheric modelling are needed to understand this discrepancy.
