Binary star has a five-minute orbit

An international team of astronomers has discovered a weird pair of stars which orbit around each other in just five and a half minutes.

HM Cancri is thus the binary star with by far the shortest known orbital period. It’s also the smallest known binary, at about a quarter of the distance from the Earth to the Moon.

The binary system consists of two white dwarfs, which are so close  together that mass is flowing from one star to the other.

HM Cancri was first noticed as an X-ray source in 1999 showing a 5.4 minutes periodicity. But this was so short that for a long time astronomers were reluctant to accept that it really was the orbital period of the system.

But the team has now proved this using the world’s largest telescope, the Keck telescope on Hawaii. This was done by detecting the velocity variations in the spectral lines in the light of HM Cancri.

Professor Tom Marsh from the University of Warwick said; “This is an intriguing system in a number of ways: 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.”

The team believes that the system must have come from two normal stars that somehow spiralled  together in two earlier episodes of mass transfer, but says 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,” says Dr Gijs Nelemans of the Radboud University.

“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.”

A longer article will appear in Astrophysical Journal Letters.