Space

An Odd Parasite Star

And its dwarf host

By Stefan Anitei, Science Editor

13th of September 2007, 06:44 GMT

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An artist's depiction shows the planet-mass object (SWIFT J1756.9-2508)in the foreground with a pulsar (upper right), which is tidally distorting the companion into a teardrop-shaped object, and ripping gas from it

Not only worms can be parasites. But stars, too. A pulsar (dead spinning star) has been discovered swallowing material from its companion star, dwindling it until the latter becomes an object smaller than many common planets.

"This object is merely the skeleton of a star. The pulsar has eaten away the star's outer envelope, and all that remains is its helium-rich core." said co-author Craig Markwardt of NASA's Goddard Space Flight Center in

Maryland.

Pulsars are the central part of burnt out "neutron" stars that spin at rhythms of hundreds of times per second.

The system was found in June when NASA's Swift and Rossi X-ray Timing Explorer (RXTE) satellites detected an emission of X-rays and gamma rays in the direction of the Milky Way galactic center, inside the Sagittarius constellation. The distance between the "host" and the "parasite" is of just around 230,000 mi (368,000 km), a little less than that between the Earth and moon.

The host has a minimum mass of just 7 Jupiters, but it cannot be considered a planet due to the way it emerged.
"It's essentially a white dwarf that has been whittled down to a planetary mass," said co-author Christopher Deloye of Northwestern University.

Billions of years ago, the system was made of an extremely massive star and a smaller star, 1 to 3 times as big as our sun. The huge star evolved rapidly and died in a supernova explosion, being reduced to the neutron star. Meanwhile, the smaller star evolved to a red giant whose outer envelope embedded the neutron star.

This made the two stars attract each other while simultaneously the red giant's envelope was lost into space. After billions of years, this is what's left of the system.

"It's been taking a beating, but that's part of nature," said co-author Hans Krimm, also of NASA Goddard.

The two objects are now so close that the neutron star's powerful gravity sucks gas from its dwindling companion forming a spinning disk on its outer zone. From time to time, the disk expels big amounts of gas onto the neutron star, causing an outburst like the one observed in June.

This is the eighth pulsar system with a period of about one millisecond found to be accreting mass from its companion, but only another one has such a small "host".

TAGS:

star | pulsar | parasite

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