U. WARWICK (UK)—The stars in HM Cancri, the smallest known binary, revolve around each other in just 5.4 minutes—the shortest known orbital period for any binary.
The system, which consists of two white dwarfs, is no larger than 8 times the diameter of the Earth—or the equivalent of no more than a quarter of the distance from the Earth to the Moon. The two stars contain a highly condensed form of helium, carbon, and oxygen.
The findings by an international team of astronomers are published in Astrophysical Journal Letters.
“This is an intriguing system in a number of ways,” says Tom Marsh, physics professor at the University of Warwick. “It has an extremely short period; mass flows from one star and crashes down onto the equator of the other in a region comparable in size to the English Midlands where it liberates more than the Sun’s entire power in X-rays.
“It could also be a strong emitter of gravitational waves, which may one day be detected from this type of star system.”
Marsh says the two white dwarfs in HM Cancri are so close together that mass is flowing from one star to the other.
The binary was first noticed as an X-ray source in 1999 showing a 5.4 minutes periodicity but it remained unclear whether this period also indicated the actual orbital period of the system. The period was so short that astronomers were reluctant to accept the possibility without solid proof.
The research team used the world’s largest telescope, the Keck telescope on Hawaii, to detect the velocity variations in the spectral lines in the light of HM Cancri.
Velocity variations are induced by the Doppler effect, caused by the orbital motion of the two stars revolving around each other. The Doppler effect causes the lines to periodically shift from blue to red and back.
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 at a brightness no more than one millionth of the faintest stars visible to the naked eye.
“A few years ago we proposed that HM Cancri was indeed an interacting binary consisting of two white dwarfs and that the 5.4 minute period was the orbital period,” says Warwick researcher and team member Danny Steeghs. “It is very gratifying to see this model confirmed by our observations, especially since earlier attempts had been thwarted by bad weather.”
“This type of observation is really at the limit of what is currently possible,” explains Paul Groot, a professor at the Radboud University Nijmegen in the Netherlands. “Not only does one need the biggest telescopes in the world, but they also have to be equipped with the best instruments available.”
“The binary HM Cancri is a real challenge for our understanding of stellar and binary evolution,” adds 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, but the physics of this process is very poorly known.
“The system is also a big opportunity for general relativity. It must be one of the most copious emitters of gravitational waves.
“These distortions of space-time we hope to detect directly with the future LISA satellite, and HM Cancri will be a cornerstone system for this mission.”
Additional researchers from Radboud University and the Harvard-Smithsonian Center of Astrophysics also contributed to the study.
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