24 May 2007

Stars merge in new cosmic explosion

Cosmos Online
A new class of stellar explosion caused by the merger of two stars has been discovered by scientists in the U.S.
Stars merge in new cosmic explosion

A stellar outburst in this galaxy, Messier 85, may be a new type of cosmic explosion caused by the collision of two stars Credit: NOAO/AURA/NSF

SYDNEY: A new class of stellar explosion caused by the merger of two stars has been discovered by scientists in the U.S.

“The discovery of this enigmatic event is merely the proverbial tip of the iceberg for an emerging class of cosmic transients,” said Shrinivas Kulkarni of the California Institute of Technology (Caltech) in Pasadena, who led the research.

Flash of inspiration

The finding is based on observations of a flash seen in the Virgo cluster in a galaxy known as Messier 85. According to the researchers, the event was probably caused by the merger of two ordinary stars some 49 million years ago.

The flash was unlike any known category of cosmic explosion: it was brighter than stellar outbursts called novae, but dimmer than the massive explosions known as supernovae. “I was simply floored. In a short time we went from speculation to a real discovery. It was an exciting moment for me,” said Arne Rau, a postdoctoral fellow working with Kulkarni.

Novae occur when old stars suck enough energy from a companion star to trigger nuclear fusion on their surfaces, and supernovae occur when stars die in cataclysmic explosions. According to the researchers, this new object flared to about 10 times brighter than the brightest nova, but stayed at least 10 times dimmer than a supernova.

Star search

The explosion was first discovered in January 2006 during California’s Lick Observatory Supernova Search. Kulkarni and his Caltech colleagues, who had been speculating on possible new classes of cosmic explosions, mounted a major follow-up.

Using the Palomar Observatory in California, the Keck telescope in Hawaii and the Spitzer Space telescope, the team found that the object, named M85 OT 2006-1, maintained its brightness for about two months before gradually fading away.

Stars are often found in pairs, and Kulkarni believes the observations point to a merger between companions in a binary system. He estimates that the stars involved in the collision were about as massive as our Sun.

Dubbed a “luminous red nova” because of its brilliance and disctinctive red colour, the object adds to a growing list of temporarily bright objects in the sky that can be classified neither as novae or supernovae.

These other objects range in brightness and age, and include the stars M31 RV and V4332 Sagittarii, which both also turned red when they brightened. Another star called V838 Monocerotis temporarily illuminated a surrounding shell of dust during an outburst in late 2002.

Stuart Ryder of the Anglo-Australian Observatory in Sydney, Australia, described the new finding as “potentially very exciting”, but cautioned against creating a whole new class of cosmic explosions too hastily.

He points out that the brightness of cosmic outbursts depends on many factors including the orbit of the companion star in a binary, and the relative amounts of elements like hydrogen, helium, oxygen and iron in the stars.

“We’d want to identify a lot more objects like M85 OT 2006-1 before we accept this as the prototype of a new class,” he said.

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