Far UV spectroscopy and imaging in Sirius-type binaries

Sirius-like binary systems consist of a white dwarf plus a main sequence, or slightly evolved star, of spectral type B-K. In most cases the white dwarf is undetectable optically because its flux is completely swamped by the bright primary. Over 20 new systems were identified in the 1990s through the detection of the white dwarf's EUV and soft X-ray flux. Sirius-like binaries are of astrophysical importance since they can be used to investigate the relationship between the mass of a main sequence star and its white dwarf progeny. Potentially they can also be used to place observational constraints on the theoretical white dwarf mass-radius relation.

Burleigh & Barstow (2000, A&A, vol 359, p977) identified a hot white dwarf companion to the B9.5V star 16 Dra (HD150100), using spectra obtained with EUVE. White dwarf companions to B stars are of significant importance since they must have evolved from massive progenitors, perhaps close to the theoretical 8M SOLAR MASSES maximum mass for all white dwarf progenitors (and, therefore, the minimum mass for producing a Type II supernova through core collapse).

Barstow et al. (2001, MNRAS, VOL 322, P891) attempted to resolve a number of Sirius-like binaries in the ultra-violet using the Wide Field Planetary Camera 2 on the Hubble Space Telescope . Out of 17 targets observed, 8 were successfully resolved. Although most of the implied orbital periods are hundreds of years, in at least three cases, 56 Persei (the figure shown below), z  Cygni and RE J1925-566, it should be possible to detect orbital motion within a few years, yielding dynamical masses for the white dwarf components.