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Stellar merger may have sparked brilliant outburst
Posted: 15 April

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Eight years of detailed monitoring of a distant star that underwent an explosive outburst, causing it to became a million times more luminous than the Sun, is suggesting that it likely suffered a violent merger with another star. V838 Monocerotis, whose outburst in 2002 became famous thanks to the spectacular Hubble Space Telescope images, was the subject of a poster presentation at the Royal Astronomical Society’s National Astronomy Meeting this week at the University of Glasgow.

The evolution of the light echo illuminating the gas cloud around V838 Mon following its outburst in 2002. Image: NASA/ESA/H E Bond (STScI).

V838 Mon is less than 25 million year old star – very young for a star – and has a mass eight times that of our Sun, but infrared spectroscopy conducted by a team led by Mark Rushton of the University of Central Lancashire shows that the star is currently one of the coolest ever seen, with a temperature matching that of a small, cool brown dwarf. Rushton attributes this to the outer layers of the star having become bloated, and cooling as they expanded from what had been a perfectly healthy, hot, massive star. This is the opposite of what happens during a conventional nova, when pulses cause the star to contract, heat up and brighten. Instead, the leading theory is that V838 Mon merged with another star, the shock waves from which caused the outer layers of the star to puff up, and Rushton’s spectroscopic analysis is corroborating this theory.

“The merging process is fairly rapid, but it then takes decades for the star to settle down,” Mark Rushton tells Astronomy Now. The outburst, which lit up the cavernous cloud of gas around it with a ‘light echo’, had three peaks in brightness, each about a month apart. The first peak has been interpreted as the two stars grazing one another, and the second, brightest, peak as the actual coming together, followed by a third peak indicating that the resulting turbulence is beginning to settle down. Spectroscopic observations made in 2004 show that some of the outer envelope of the star is beginning to slowly but surely contract to its original level. Over the past eight years spent monitoring the star, Rushton and his colleagues Tom Geballe of the Gemini Observatory in Hawaii, Aneurin Evans of Keele University, and Stewart Eyres of the University of Central Lancashire, have seen this contraction begin to alter the star’s spectrum, going from strong absorption lines of water vapour, carbon monoxide and metal oxides, to the likes of sulphur dioxide that has only ever been seen in one other star, a Mira variable called UX Cygni.

If V838 Mon is beginning to contract back to its original state, it should begin to heat up again. X-ray observations made by NASA’s orbiting Chandra Observatory suggest that it is indeed heating up as the star’s photosphere contracts, causing it to rotate faster, just as an ice skater spins faster when she pulls in her arms. Another outlying companion star has also been detected in the system, but it is currently uncertain whether it gravitational influence could have spurred on the merger.

“These kinds of mergers are expected to occur every few decades in the Galaxy,” says Rushton, and they are particularly common in dense clusters of stars; for instance blue stragglers in globular clusters, which are hotter and bluer than they should be for their age, are thought to derive from stellar mergers. It seems there will still be plenty of opportunities for catching stars in the act of merging in the future.