Transcript Siriusposter

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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.
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Above left: Optical and ultraviolet spectra (in red) of the Sirius-type binary BD+27 1888,
discovered with ROSAT and IUE in 1994. 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
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 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 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!
Resolving the systems with HST and WFPC2
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.
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 (RE1925+566) the physical distance between the two stars is less
than the orbit of Neptune around the Sun!
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.
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.
Left: HST/WFPC2 UV images of three of these
new Sirius-type binary systems. At the far left is
RE1925+566. 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