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Once around a star in 10 minutes
Patrick A. Woudt and Brian Warner
Department of Astronomy, University of Cape Town
 Astronomers in southern Africa have exciting times ahead of them. One
year from now, toward the end of 2003, first light will be obtained at
the Southern African Large Telescope (SALT; see also www.salt.ac.za).
Following a commissioning period in which the performance of the
telescope and its instruments is to be crucially examined, astronomers
are scheduled to start using SALT towards the end of 2004, i.e., only two
years from now. Upon completion, SALT will be the largest single telescope
in the southern hemisphere, and will be used to make a more complete
inventory of the local Universe by studying the very faintest stellar systems
surrounding us, or to see what the Universe looked like ~5-8 billion
years ago by observing very distant galaxies.
A survey of faint interacting binary stars
At the University of Cape Town (UCT), scientists have been involved in a survey of
faint Cataclysmic Variable (CV) stars using the 1.9-m and 1.0-m telescopes of the South
African Astronomical Observatory (SAAO) in Sutherland and the UCT CCD
camera, a state-of-the-art electronic camera. Their survey is actively identifying
the most interesting specimens that will warrant more detailed spectroscopic
follow-up studies with SALT.
The stellar systems under investigation, CVs, are interacting binary stars,
consisting of a hot central star (a `white dwarf' which is a star with the mass of the
sun, but comparable in size to the earth) and a lower mass companion. Mass is
being transferred from the lower mass companion onto the central white dwarf,
either via an accretion disc, or directly along magnetic field lines if the
magnetic field of the central white dwarf is strong enough. The image above shows an
artist's impression of a Cataclysmic Variable (CV) with an accretion disc. Studying
CVs at high time resolution will throw important light on the physical processes
involved in mass accretion at the very centre of these systems.
ES Cet; A helium-transferring CV with an
orbital period of 10 minutes
The survey, dubbed the UCT CCD CV Survey, has, after two and a half
years of intensive observing in Sutherland, resulted in many new fascinating
observations. These scientists have more than doubled the number of known orbital
periods for southern Novae (a class of CVs) and in addition measured the
rotation period of the central white dwarf in four southern Novae.
However, the most interesting object discovered by the survey is a CV called
ES Cet, located in the constellation of Cetus
(the Whale). Their observations showed that ES Cet varies regularly in brightness
with a period of 620.26 seconds (or about 10 minutes); there is no other period in the system.
The most logical explanation would be that this is the orbital period of the
system. However, from a theoretical perspective, ordinary hydrogen-rich CVs (if
you can ever call them ordinary?) do not have orbital periods less than 50
minutes. Alternative explanations such as a rotation period of the central white dwarf,
or pulsations in the central white dwarf, can be rejected on the basis of an
absence of a second periodicity (the orbital period) and on the basis of its
spectrum.
The spectrum only shows Helium emission lines and this gives the strongest
support for the periodicity being orbital in nature. There is a small group of
CVs (only ~10 known to date) that have evolved through two phases of mass
exchange from normal composition binaries to a phase where both stars have lost
their hydrogen envelope and have their helium cores exposed. During their
orbital evolution, these systems pass through a minimum period near 4 minutes
(when the stars are very close to each other) and thereafter evolve to longer
periods. The change in the orbital period is due to the emission of
gravitational radiation. In order to understand why gravitational radiation is
emitted, it helps to
picture the system in ones mind:
ES Cet consists of two white dwarfs, one with 0.7 times the mass of the
sun and one with 0.067 times the mass of the sun. They are very compact and hot
stars, about the size of the earth, and their separation is only
100.000 km, ~0.25 times the separation between the earth and the moon! They
transfer matter at an incredible rate. With an orbital period of only 10
minutes, it means the orbital velocity is ~1000 km/s (just imagine the moon
going around the earth every 10 minutes, compared to its 27.3 days orbit). These
are the conditions when gravitational waves are emitted strongly.
In order to confirm their model of ES Cet, they have recently obtained
spectroscopic observations at the Hobby Eberly Telescope in Texas. This is
SALT's northern twin telescope, the telescope on which SALT's design is based.
If SALT were ready now, ES Cet would be a prime target on SALT.
There are currently only two other objects that resemble ES Cet with periods
of 5 and 10 minutes, respectively. Both are incredibly faint and need SALT-like
telescopes to understand them fully. At first sight they appear similar to ES
Cet, but there are differences. The 5-minute `binary' shows some hydrogen in its
spectrum, and that would make it impossible to be such a highly evolved system.
The other 10-minute `binary' shows no
helium in its spectrum and is (by chance) obscured behind a dense dust cloud.
The jury is still out on these two objects. It might
be that ES Cet is the only one of these three objects that emits gravitational
waves.
In the next six months the team will be involved in a large observing campaign
on ES Cet in which they will monitor the orbital period of ES Cet. If indeed
gravitational waves are emitted, one should be able to detect a
measurable change in the orbital period over the length of a year. In addition,
gravitational wave detectors in the US have become operational
this year and it could well be that ES Cet is the first object from which
gravitational waves are detected directly.
Picture copyright PASP
More Information and References
Norton, A.J., Haswell, C.A., Wynn, G.A., 2002, submitted to MNRAS, astro-ph/0206013
Warner, B., Woudt, P.A., 2002, PASP 114, 129
Wegner, G., McMahon, R.K., Boley, F.I., 1987, AJ 94, 1271
Woudt, P.A., Warner, B., 2001, MNRAS 328, 159
Woudt, P.A., Warner, B., 2002, MNRAS in press
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