Above left: Optical and ultraviolet spectra (in red) of the Sirius-type binary BD , discovered with ROSAT and IUE in The white dwarf is clearly visible in the far-UV but its companion star dominates in the optical. The blue line represents the white dwarf’s flux. Above right: Historic photograph of Sirius, the brightest star in the sky, with its tiny white dwarf companion. Sirius-type Binary Systems Matt Burleigh and Martin Barstow The majority of catalogued white dwarfs are isolated objects, identified on the basis of their blue colour or high proper motions. Very few white dwarfs have been found in binaries, simply because the brighter companion stars completely swamp their optical flux. It is intriguing to consider that if the brightest star in the night sky, Sirius, was much farther away, its tiny white dwarf companion might never have been identified. The ROSAT all-sky x-ray and extreme ultraviolet (EUV) surveys discovered many new hot white dwarfs. At these energies, white dwarfs are far brighter than most normal stars, and with ROSAT’s help we have been able to identify over 20 of these degenerate objects in binaries with bright, normal companions, just like the Sirius system. At optical wavelengths the white dwarfs are unresolvable from the ground, because they are so close to the companion star, but we have been able to study them in detail instead with ultra-violet satellites such as the International Ultraviolet Explorer (IUE), the Extreme Ultraviolet Explorer (EUVE), and the Hubble Space Telescope (HST). These new systems are important because the binary population of white dwarfs has never before been studied, and, more importantly, they offer us the chance to explore some of the fundamental relations in stellar astrophysics. The maximum mass for a white dwarf progenitor Theoretically, the maximum mass of a white dwarf-forming star is about 8 solar masses. Stars more massive than that explode as supernovae, creating neutron stars. However, this theoretical limit has never been backed up by observational evidence. One of our new Sirius-type binaries, HR2875 (y Pup), consists of a white dwarf and a massive B5V star. Since massive stars evolve faster than less massive ones, we know for sure that this white dwarf must have evolved from a progenitor more massive than a B5V star - about 6.5 times the mass of the Sun. For the first time observationally, we have been able to place a lower limit on the maximum mass for white dwarf progenitor stars. Testing the white dwarf mass-radius relation Chandrasekar’s Nobel prize-winning description of degenerate matter included the relationship between a white dwarf’s mass and its radius: the more massive a white dwarf is, the smaller it is. However, this relation has barely been tested in detail observationally, primarily because it is extremely difficult to measure the distance to a white dwarf (and therefore its radius). The Hipparcos satellite measured the distances to thousands of stars, but it could not see most isolated white dwarfs because they are so faint. However, for the white dwarfs in Sirius-type binaries, it could measure the distances of the bright, normal companions. By combining these distances with the white dwarf temperatures and gravities, which we will measure with a new NASA satellite called the Far Ultraviolet Spectroscopic Explorer (FUSE), we will be able to test the white dwarf mass-radius relation in detail, and check whether the theoreticians have got their calculations correct! UNI LOGO HERE Resolving the systems with HST and WFPC2 Last summer we began to image all of these new binary systems in the UV with HST and the WFCPC2 camera, to try and resolve the white dwarfs. So far, 13 systems have been observed, and we have been able to resolve the two stars in 6 cases. The closest separation is a mere 0.21 arcseconds. In that system (RE ) the physical distance between the two stars is less than the orbit of Neptune around the Sun! Now that we have been able to locate these white dwarfs, it will be possible (again, with HST) to obtain their optical spectra and to determine the binary orbits. This will allow us to test the mass-radius relation to an even greater accuracy. Above: EUVE discovery spectrum of the white dwarf companion to the B5V star HR2875 (y Pup). This white dwarf could only be identified in the EUV since the B star still dominates in the far-UV. This white dwarf must have descended from a parent star close to the maximum mass for white dwarf progenitors. Left: HST/WFPC2 UV images of three of these new Sirius-type binary systems. At the far left is RE In this system the two stars are separated by just 21AU, less than the distance between the Sun and Neptune (represented by the green circle in each image). The middle image is HR1358, and on the right is HD2133