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

Abstract

We present a detailed spectroscopic analysis of the hot DA white dwarf G191-B2B, using the best signal to noise, high resolution near and far UV spectrum obtained to date. This is constructed from co-added HST STIS E140H, E230H, and FUSE observations, covering the spectral ranges of 1150-3145Å and 910-1185Å 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 mÅ. 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. (2005), 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, an NLTE 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.

Hot DA white dwarf model atmosphere calculations: including improved Ni PI cross-sections

“…Finally, in Model 4, we use the Ku11 transitions and AUTOSTRUCTURE PICS. In all four cases, we Preval et al (2013), where the values with the lowest statistical uncertainty were used.…”

“…These abundances were taken from Preval et al (2013), measured in their analysis of the hot DA WD G191-B2B. In the case where there was more than one ionization stage considered, we used the abundances with the smallest uncertainty.…”

“…Using a similar method to Preval et al (2013), we measured the abundances for G191-B2B using all four of the models described above. The observational data for G191-B2B consists of three high S/N spectra constructed by co-adding multiple data sets.…”

“…The first spectrum uses data from the Far Ultraviolet Spectroscopic Explorer spanning 910-1185 Å, and the other two use data from the Space Telescope Imaging Spectrometer aboard the Hubble Space Telescope spanning 1160-1680 Å and 1625-3145 Å, respectively. A full list of the data sets used, and the coaddition procedure, is given in detail in Preval et al (2013). The model grids for each metal was constructed by using SYNSPEC.…”

“…Studies of WD metal abundances, such as those of Barstow et al (1998); Vennes & Lanz (2001); Preval et al (2013), used model atmospheres incorporating the atomic data of Kurucz (1992, hereafter Ku92) in conjunction with photoionization cross-section (PICS) data from the opacity project (OP) for Fe, and approximate hydrogenic PICS for Ni. A more comprehensive data set has since been calculated by Kurucz (2011, hereafter Ku11), containing a factor ∼10 more transitions and energy levels for Fe and Ni IV-VI than its predecessor.…”