Runaway star deepens mystery

via Technology Review.

Jack Hills at Los Alamos National Laboratories came up with a nice hypothesis in 1988. If two stars locked together in a binary system encounter the supermassive black hole in the centre of the galaxy and hit it just right, one will be captured and the other accelerated away at such great speeds they exceed the escape velocity of the galaxy and so will be flung out. These stellar analogues to Hawking radiation (Stephen Hawking’s idea about particles created via pair production, electromagnetically joined, but with one absorbed by a black hole and the other flung out) would travel at 1000 km/s and are known as Hypervelocity stars.

Using the Doppler shift of light with increasing velocities, astronomers took Hill’s advice, searched for and found several stars at these massive velocities, his hypothesis became a theory and seemed to be vindicated. Now, however, a deeper look at one of the stars has ruled it out of being one of Hill’s runaways.

The Doppler shift provides the line of sight velocity – the speed at which a thing is moving towards or away from you (there is a transverse term at really high speeds due to relativistic time dilation, but not at 1000km/s). Careful observations of how a star’s position changes on the night sky can then give velocities in the other two dimensions, allowing the path the star has taken to be traced backward.

HD 271791 was the first hypervelocity star. It has eleven times the mass of the Sun and unfortunately, it has never been anywhere near the central black hole. Indeed, the closest it is likely to have been during its lifetime is three thousand light years away on the edge of the galactic disc. Now astronomers are wondering what sent this one flying.

Maybe it was accelerated through a similar mechanism, but involving several stars many times more massive than the Sun. This is the suggestion of Vasilii Gvaramadze at Moscow State University. Alternative explanations include tidal interactions between the Milky Way and a satellite galaxy – though there’s no indication that any occurred in the vicinity of this star – and a supernova explosion of a companion star blasted away this one. The latter explanation is favoured by other astronomers, but Gvaramadze feels some interaction with other stars is required.

Observations of the composition of the runaway should help determine matters – if its been slammed by the shell of a supernova, the star should carry indications of effectively staining by materials ejected in the explosion.

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