Once around a star in 10 minutes

Patrick A. Woudt and Brian Warner Department of Astronomy, University of Cape Town

Picture copyright PASP 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