HST/STIS spectroscopy of the exposed white dwarf in the short-period dwarf nova EK TrA

Abstract

We present high resolution Hubble Space Telescope ultraviolet spectroscopy of the dwarf nova EK TrA obtained in deep quiescence. The Space Telescope Imaging Spectrograph data reveal the broad Lyalpha absorption profile typical of a moderately cool white dwarf, overlayed by numerous broad emission lines of He, C, N, and Si and by a number of narrow absorption lines, mainly of \C I and \Si II. Assuming a white dwarf mass in the range 0.3-1.4 M_sun we derive T_eff=17 500-23 400 K for the primary in EK TrA; T_eff =18 800 K for a canonical mass of 0.6 M_sun. From the narrow photospheric absorption lines, we measure the white dwarf rotational velocity, vsin i=200+/-100 km s^-1. Even though the strong contamination of the photospheric white dwarf absorption spectrum by the emission lines prevents a detailed quantitative analysis of the chemical abundances of the atmosphere, the available data suggest slightly sub-solar abundances. The high time resolution of the STIS data allows us to associate the observed ultraviolet flickering with the emission lines, possibly originating in a hot optically thin corona above the cold accretion disk. Based on observations made 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.