Measuring T and log g in Sirius-type binaries

Since the white dwarfs in Sirius-type binaries are unobservable optically, we have previously been unable to accurately establish their temperatures, surface gravities and masses by the usual method of fitting model atmospheres to the optical H Balmer absorption lines. The advent of far-UV spectroscopy from 900Å to 1200Å with the FUSE satellite, covering the entire hydrogen Lyman series, allows us a first opportunity to determine precisely these fundamental parameters for the white dwarfs in these binaries. We have programmes in FUSE Cycles 1 & 2 to pursue this goal.

While the Balmer line technique is well-established, only a few Lyman series spectra have been available to make equivalent measurements. An important precursor to using the Lyman lines, particularly for those binaries where the white dwarf Balmer lines cannot be observed, is to establish that this technique gives reproducible results which are consistent with the Balmer line measurements of isolated stars. We have carried out a survey of all the available Lyman line data from past missions such the Hopkins UV Telescope (HUT) and Orfeus, together with early release data from FUSE (Barstow et al. 2001, MNRAS, in press)We have developed a systematic technique for measuring T and log g along the lines of that established for dealing with Balmer line data, but taking into account the fact that the wings of the Lyman lines often overlap significantly. There seems to be good overall agreement between the Balmer and Lyman line measurements, although significant differences are found for a few stars, possibly arising from systematic effects in the observation and data reduction process. Provided these can be adequately controlled, the Lyman line technique does give valid results. Initial use of the Lyman series lines to study the white dwarfs in our sample of Sirius-like systems has been spectacular. Burleigh et al. (2001, MNRAS, in press) have shown that the white dwarf companion to the bright A1 giant b  Crateris possesses an unusually low mass, and has almost certainly evolved through binary interaction. This system could be a long sought remnant of Algol-type evolution.