Outbursting star creates double-lobed nebula

BY KEITH COOPER

ASTRONOMY NOW

Posted: 22 April, 2009

An artist's impression of the RS Oph system. Image: David Hardy (www.astroart.org)/STFC.

A cataclysmic variable that outburst three years ago now has debris expanding into a 'peanut-shaped' nebula, according to new Hubble Space Telescope images that have been released today at the National Astronomy Meeting, part of the European Week of Astronomy and Space Science at the University of Hertfordshire.

The star, RS Ophiuchi (RS Oph for short), is actually two stars: a swollen red giant orbited closely by the remnant of a dead star, a white dwarf. The white dwarf is so close that it is actually inside the peripheral layers of the red giant's atmosphere, and so it manages to gather up quite a large amount of the red giant's gas on its surface. Over time the build-up of material on the surface of the white dwarf eventually reaches catastrophic proportions and a thermonuclear reaction takes place resulting in a huge explosion on its surface. This happens every 20 years or so, and since 2006 new observations and understanding of the nova - termed a cataclysmic variable because it brightens dramatically in the sky during its outbursts from an increase in energy output 100,000 times greater than the Sun - have become a mainstay of the National Astronomy Meeting.

During each outburst, enough gas and dust is ejected from the white dwarf to make the equivalent of several Earth-mass planets, and this debris races away at up to 3,000 kilometres per second.

Hubble Space Telescope image of the double-lobed nebula in RS Ophiuchi. Image: NASA/HST and Valerio Ribeiro.

The Hubble images, taken 155 and 449 days after the outburst, were backed up by ground-based spectroscopy, and show how the outburst debris is evolving as it expands into a double-lobed, peanut-shaped nebula. The nebula is elongated perpendicular to the plane of the RS Oph system, where there is less gas to stop its expansion.

There will be more outbursts to occur in the RS Oph system during the coming decades, but eventually there may be one outburst too many, and a chain reaction will possibly rip the white dwarf apart. "There are some astronomers who believe systems like this will ultimately explode as supernovae," says Valerio Ribeiro from Liverpool John Moores University, who presented the images. "Our continuing work will help us find out if that is possible."

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