Nathan J. Dickinson

A comprehensive near- and far-ultraviolet spectroscopic study of the hot DA white dwarf G191-B2B

We present a detailed spectroscopic analysis of the hot DA white dwarf G191-B2B, using the best signal-to-noise ratio, high-resolution near- and far-UV spectrum obtained to date. This is constructed from co-added Hubble Space Telescope (HST) Space Telescope Imaging Spectrometer (STIS) E140H, E230H and FUSE observations, covering the spectral ranges of 1150-3145A and 910-1185 A, respectively. With the aid of recently published atomic data, we have been able to identify previously undetected absorption features down to equivalent widths of only a few mA. In total, 976 absorption features have been detected to 3σ confidence or greater, with 947 of these lines now possessing an identification, the majority of which are attributed to Fe and Ni transitions. In our survey, we have also potentially identified an additional source of circumstellar material originating from Si III. While we confirm the presence of Ge detected by Vennes et al., we do not detect any other species. Furthermore, we have calculated updated abundances for C, N, O, Si, P, S, Fe and Ni, while also calculating, for the first time, a non-local thermodynamic equilibrium abundance for Al, deriving Al III/H = 1.60+0.07-0.08× 10-7. Our analysis constitutes what is the most complete spectroscopic survey of any white dwarf. All observed absorption features in the FUSE spectrum have now been identified, and relatively few remain elusive in the STIS spectrum.

A photospheric metal line profile analysis of hot DA white dwarfs with circumstellar material

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.

The Stratification of Metals in Hot White Dwarf Atmospheres

The atmospheres of many hot white dwarfs contain significant quantities of metals. Various studies have reached different conclusions on the distribution of nitrogen in the atmosphere of REJ1032+532, a 44,350 K DA. Here, we examine this object again to gain a better understanding of its nitrogen distribution, and attempt to model the C IV features seen in the DA PG0948+534 (110,0000 K).