Nuclear explosion on a dead star

Astronomers probe the aftermath of a giant outburst of RS Ophiuchi

The discovery, reported in the 20th July issue of Nature, was made by bringing together many of the world's radio telescopes into arrays capable of seeing the aftermath of the explosion in incredible detail.

During the night of 12th February this year Japanese astronomers reported that a star called RS Ophiuchi had suddenly brightened and become clearly visible in the night sky. Although this was the latest in a series of such outbursts that have been spotted over the last hundred years or so, it was the first since 1985 and therefore an opportunity to bring to bear new, more powerful, telescopes in an effort to understand the causes and consequences of these eruptions.

Dr Tim O'Brien of The University of Manchester's Jodrell Bank Observatory requested urgent observations with the VLBA (the Very Long Baseline Array of radio telescopes extending from Hawaii to the Caribbean). "Our first observations, made only two weeks after the explosion was reported, showed an expanding blast wave already comparable in size to Saturn's orbit around the Sun. However, we needed to use the world's most powerful radio telescopes because, from a distance of 5,000 light years, its apparent size on the sky was only 5 millionths of a degree - the size of a football seen from 2,700 km away."

The blast wave results from a huge nuclear explosion which takes place on the surface of one of a pair of stars, about 5,000 light years from Earth, which are closely circling one another. Gas captured from one star, a red giant, builds up on the surface of its white dwarf companion (a super-dense dead star about the size of the Earth which was once the core of a star like the Sun whose outer layers have been lost into space).

Eventually enough gas collects on the white dwarf for thermonuclear reactions to begin, similar to those which power the Sun but which runaway into a massive explosion. In less than a day, its energy output increases to over 100,000 times that of the Sun, and the gas (about the mass of the Earth) is thrown into space at speeds of several thousand km per second. This ejected matter then slams into the extended atmosphere of the bloated red giant and sets up blast waves that accelerate electrons to almost the speed of light. The electrons release radio waves as they move through a magnetic field that are then picked up by the telescope arrays.

Over the following months, the team continued to track the outburst using the European VLBI Network (EVN) which includes telescopes in South Africa and China, the MERLIN array of radio telescopes in the UK, and the Very Long Baseline Array (VLBA) and Very Large Array (VLA) in the USA, a truly global effort.

Dr Richard Porcas of the Max Planck Institute for Radio Astronomy in Bonn, who was also involved in the 1985 observing campaign of the last outbreak of RS Oph, coordinated the European VLBI Network observations. "A week after our first observations, we combined telescopes across Europe with two in China and another in South Africa and were surprised to find that the blast wave had become distorted. Over the next few months our observations have shown it turning from a ring into a cigar-like shape. It's going to need a lot more work to understand exactly what causes this but either the explosion shoots jets of matter in opposite directions or somehow the atmosphere of the red giant is shaping the ejected material."

Once the outburst is over, gas will again build up on the white dwarf until at some point, maybe another 20 years in the future, RS Oph should explode again. An important question which the astronomers hope to answer is whether in each explosion the white dwarf throws off all the matter it has collected from the red giant or whether it is hoarding some material and therefore gradually increasing in mass.

Dr Tim O'Brien, who also studied RS Oph's previous outburst in 1985 for his doctoral thesis, concludes "If the white dwarf is increasing in mass then it will eventually be ripped apart in a titanic supernova explosion and the cycle of outbursts will come to an end."

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The European VLBI Network (EVN) is a joint facility of European, Chinese, South African and other radio astronomy institutes funded by their national research councils, conducting unique, high resolution, radio astronomical observations of cosmic radio sources. It is the most sensitive VLBI array in the world, thanks to the collection of extremely large telescopes like the Effelsberg 100 metre telescope that contribute to the network.

MERLIN is the Multi-Element Radio Linked Interferometer Network, an array of seven radio telescopes distributed around Great Britain, with separations of up to 217 km. At a frequency of 5GHz, the resolution of MERLIN is better than 50 milliarcseconds, somewhat greater than that of the Hubble Space Telescope. It is operated by the University of Manchester as a National Facility of the UK Particle Physics and Astronomy Research Council.

The Very Long Baseline Array (VLBA) is a system of ten radio-telescope antennas, each with a dish 25 metres in diameter and weighing 220 tonnes. From Mauna Kea on the Big Island of Hawaii to St. Croix in the U.S. Virgin Islands, the VLBA spans more than 8000 km, providing astronomers with the sharpest vision of any telescope on Earth or in space. Dedicated in 1993, the VLBA has an ability to see fine detail equivalent to being able to stand in New York and read a newspaper in Los Angeles. The National Radio Astronomy Observatory is a facility of the National Science Foundation, operated under cooperative agreement by Associated Universities, Inc.


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