Far-Ultraviolet Space Telescope Imaging Spectrograph Spectra of the White Dwarf REJ 1032+532. II. Stellar Spectrum

Abstract

We discuss the Hubble Space Telescope Space Telescope Imaging Spectrograph UV echelle spectrum of the hot DA white dwarf REJ 1032+532. The interstellar data from this spectrum are presented by Holberg and coworkers. In this paper we discuss a number of strong photospheric features due to C, N, and Si that are present in the REJ 1032+532 spectrum. While the inferred heavy element content of REJ 1032+532 roughly matches the predictions of radiative levitation for carbon and silicon, the observed nitrogen abundance greatly exceeds predictions by a factor of 50. The observed shapes of the N V lines provide the first evidence, at UV wavelengths, of heavy element stratification in a hot DA white dwarf. Homogeneous models are unable to reproduce the shape of the REJ 1032+532 N V lines, nor can they account for the relatively low degree of EUV opacity in the star. We present a simple stratified nitrogen model that resolves these problems. The high degree of stratification in REJ 1032+532 is the signature of ongoing mass loss in this star. The radial velocity of REJ 1032+532 obtained with Space Telescope Imaging Spectrograph differs by 44 km s^-1 from that obtained from the Balmer H I lines with the Multiple Mirror Telescope. This suggests that REJ 1032+532 is likely a member of a binary system containing either a late M star or another white dwarf.

Based on observations with the NASA/ESA Hubble Space Telescope obtained at the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Inc., under NASA contract NAS 5-26555.

Observations reported here have been obtained in part with the Multiple Mirror Telescope, a joint facility of the University of Arizona and Smithsonian Institution.