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Hubble Captures Peanut-Shaped Stellar Explosion

April 22, 2009

Using the NASA / ESA Hubble Space Telescope (HST), an international team of astronomers have taken the first optical images of a dramatic stellar outburst and discovered a peanut-shaped bubble expanding rapidly into space. Team member Valerio Ribeiro, a graduate student from Liverpool John Moores University will present their results on Wednesday 22nd April at the European Week of Astronomy and Space Science conference at the University of Hertfordshire.

The scientists looked at a star in the constellation of Ophiuchus (known as RS Oph) which has undergone a series of outbursts over the last century. On 12th February 2006, Japanese amateur astronomers reported it had brightened once again and had even become visible to the unaided eye. This was the first eruption of RS Oph since 1985 and gave scientists the unprecedented opportunity to study it using new more powerful telescopes on the ground and in space, including the HST.

RS Oph consists of a white dwarf, a dead star about the size of Earth in orbit around a much larger star, a so called red giant. Due to its proximity the white dwarf pulls hydrogen rich gas from the outer layers of the red giant and roughly every 20 years the build up of gas on the white dwarf’s surface causes a cataclysmic thermonuclear explosion. The rise to maximum brightness takes place in less than a day and at its height the energy output of RS Oph increases to over 100,000 times that of the Sun. The eruption ejects a quantity of material equivalent to the mass of the Earth at speeds of several thousand kilometers per second.

The red giant is also continuously losing enormous amounts of gas in a wind that envelops the whole system. As a result, the explosion on the white dwarf occurs effectively inside its companion’s atmosphere and the ejected gas then slams into it at very high speed.

Using the HST, observations of RS Oph were made 155 and 449 days after the outburst. Combined with spectroscopy from ground-based telescopes, the first images revealed a double-lobed “peanut” structure with material expanding outwards at between 1000 and 3000 km per second.

The team attribute the shaping of the nebula to the pre-existing red giant wind. In a binary system like this, material gathers towards the plane of the stars’ orbits while at the poles it is less dense. When the outburst takes place, the ejected material hits the high density gas in the orbital plane and slows down rapidly, while at the poles it moves more quickly. The result is the peanut shape seen in the HST images and confirmed earlier observations made using radio telescopes on the ground.

Valerio Ribeiro now hopes to watch RS Oph over the years to come. He comments, “There are some astronomers who believe systems like this will ultimately explode as supernovae. Our continuing work will help us find out if that will happen.”

Image Caption: An artist’s impression of the binary star system RS Ophiuchi: hydrogen-rich gas transferred from a red giant onto the surface of a white dwarf has just exploded. (Credit: David A. Hardy http://www.astroart.org & Science and Technology Facilities Council)

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