Stars in the fast lane - going round and round

Two white dwarfs orbit each other in the smallest known binary system in a mere 5 minutes

March 09, 2010

That is real fast: Two suns orbit each other in a mere 5.4 minutes. This makes HM Cancri the binary star system with by far the shortest known orbital period - and at the same time the smallest binary known. Its size is equivalent to no more than a quarter of the distance from the Earth to the Moon, about 100,000 kilometres. This has been shown by an international team of astronomers from the Max Planck Institute for Extraterrestrial Physics and other institutions (Astrophysical Journal Letters, March 10th, 2010).

The binary system HM Cancri consists of two white dwarfs. These are the burnt-out, dense cinders of stars such as our Sun. A typical white dwarf exists in a highly condensed form of helium, carbon and oxygen and has a bit more than half of the mass of the Sun, with a size, however, that is comparable to that of our Earth. The two white dwarfs in HM Cancri are so close together - no more than eight times the diameter of the Earth - that mass is flowing from one star to the other.

Already in 1999, HM Cancri was detected as an X-ray source by the MPE-lead ROSAT satellite. Light variations in the X-ray and optical bands soon showed the 5.4 minute period, but for a long time it remained unclear whether this period actually indicated the orbital period of the system. "The modulation was so short that we were reluctant to accept the possibility of an orbital period without solid proof," says Vadim Burwitz, who reported the X-ray periodicity of HM Cancri in the ROSAT data in 1999.

The team of astronomers, led by Dr. Gijs Roelofs of the Harvard-Smithsonian Center of Astrophysics, have now used the world´s largest telescope, the 10-m Keck-I telescope on Hawaii, to prove that the 5.4 minute period is indeed the binary period of the system. As the two stars revolve around each other, the orbital motion introduces a Doppler effect, which causes the spectral lines to periodically shift from blue to red and back. The scientists used this effect to detect the velocity variations in the spectral lines of the light emitted by HM Cancri.

The observations of HM Cancri were an ultimate challenge due to the extremely short period that needed to be resolved and the faintness of the binary system. At a distance of close to 16,000 light years from Earth, the binary shines a million times less bright than the faintest stars visible to the naked eye.

"HM Cancri is very faint, but in order to unriddle its nature, we had to obtain hundreds of spectra within a short time. This posed an extreme challenge to the currently existing observatories and only the largest telescope in the world, and favourable weather conditions, allowed us to succeed," says Arne Rau of the Max Planck Institute for Extraterrestrial Physics in Garching, Germany, who led the observations on Hawaii.

"The binary HM Cancri is a real challenge for our understanding of stellar and binary evolution," adds Dr. Gijs Nelemans of the Radboud University. "We know the system must have come from two normal stars that somehow spiralled together in two earlier episodes of mass transfer." The physics of this process however, is very poorly known.

Moreover, HM Cancri offers a nearly unique possibility to test Einstein´s General Theory of Relativity, since the system should be one of the most copious emitters of gravitational waves due to its very fast movement through space-time. "We hope to detect these distortions of space-time directly with the future LISA satellite and HM Cancri will be a cornerstone system for this mission," says Nelemans.

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