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RAS PN06/26 (NAM19): Exploding 'Star Within A Star'

Last Updated on Sunday, 01 December 2013 20:40
Published on Friday, 07 April 2006 00:00
RS_Oph.jpgOn 12 February 2006, amateur astronomers reported that a faint star in the constellation of Ophiuchus had suddenly become clearly visible in the night sky without the aid of a telescope. Records show that this so-called recurrent nova, RS Ophiuchi (RS Oph), has previously reached this level of brightness five times in the last 108 years, most recently in 1985. The latest explosion has been observed in unprecedented detail by an armada of space- and ground-based telescopes.

Speaking today (Friday) at the RAS National Astronomy  Meeting at Leicester, Professor Mike Bode of Liverpool John Moores University  and Dr Tim O'Brien of Jodrell Bank Observatory will present the latest results which are shedding new light on what happens when stars explode.

RS Oph is just over 5,000 light years away from Earth. It consists of a white dwarf star (the super-dense core of a star, about the size of the Earth, that has reached the end of its main hydrogen-burning phase of evolution and shed its outer layers) in close orbit with a much larger red giant star.

The two stars are so close together that hydrogen-rich gas from the outer layers of the red giant is continuously pulled onto the dwarf by its high gravity. After around 20 years, enough gas has been accreted that a runaway thermonuclear explosion occurs on the white dwarf's surface. In less than a day, its energy output increases to over 100,000 times that of the Sun, and the accreted gas (several times the mass of the Earth) is ejected into space at
speeds of several thousand km per second.

Five explosions such as  this per century can only be explained if the white dwarf is near the maximum  mass it could have without collapsing to become an even denser neutron star.

What is also very unusual in RS Oph is that the red giant is  losing enormous amounts of gas in a wind that envelops the whole system. As a  result, the explosion on the white dwarf occurs "inside" its companion's  extended atmosphere and the ejected gas then slams into it at very high  speed.

Within hours of notification of the latest outburst of RS  Oph being relayed to the international astronomical community, telescopes both  on the ground and in space swung into action. Among these is NASA's Swift  satellite which, as its name suggests, can be used to react rapidly to things  that change in the sky. Included in its armoury of instruments is an X-ray  Telescope (XRT), designed and built by the University of Leicester.

"We realised from the few X-ray measurements taken late in the  1985 outburst that this was an important part of the spectrum in which to  observe RS Oph as soon as possible," said Professor Mike Bode of Liverpool John  Moores University, who led the observing campaign for the 1985 outburst and now heads the Swift follow-up team on the current explosion.

"The  expectation was that shocks would be set up both in the ejected material and in  the red giant's wind, with temperatures initially of up to around 100 million  degrees Celsius - nearly 10 times that in the core of the Sun. We have not  been disappointed! "

The first observations by Swift, only three  days after the outburst began, revealed a very bright X-ray source. Over the  initial few weeks, it became even brighter and then began to fade, with the  spectrum suggesting that the gas was cooling down, although still at a  temperature of tens of millions of degrees. This was exactly what was expected  as the shock pushed into the red giant's wind and slowed down. Then something  remarkable and unexpected happened to the X-ray emission.

"About a  month after the outburst, the X-ray brightness of RS Oph increased very  dramatically," explained Dr. Julian Osborne of the University of Leicester.  "This was presumably because the hot white dwarf, which is still burning nuclear  fuel, then became visible through the red giant's wind.

"This new  X-ray flux was extremely variable, and we were able to see pulsations which  repeat every 35 seconds or so. Although it is very early days, and data are  still being taken, one possibility for the variability is that this is due to  instability in the nuclear burning rate on the white  dwarf."

Observatories Swing Into Action

Meanwhile,  observatories working at other wavelengths changed their programmes to observe  the event. Dr. Tim O'Brien of Jodrell Bank Observatory, who did his PhD thesis  work on the 1985 explosion, and Dr. Stewart Eyres of the University of Central  Lancashire, lead the team that is securing the most detailed radio observations  to date of such an event.

"In 1985, we were not able to begin  observing RS Oph until nearly three weeks after the outburst, and then with  facilities that were far less capable than those available to us today," said  Dr. O'Brien.

"Both the radio and X-ray observations from the last  outburst gave us tantalising glimpses of what was happening as the outburst  evolved. In addition, this time, we have developed very much more advanced  computer models. The combination of the two now will undoubtedly lead to a  greater understanding of the circumstances and consequences of the explosion.

"In 2006, our first observations with the UK's MERLIN system were  made only four days after the outburst and showed the radio emission to be much brighter than expected," added Dr. Eyres. "Since then it has brightened, faded, then brightened again. With radio telescopes in Europe, North America and Asia now monitoring the event very closely, this is our best chance yet of understanding what is truly going on."

Optical observations are  also being obtained by many observatories around the globe, including the robotic Liverpool Telescope on La Palma. Observations are also being conducted  at the longer wavelengths of the infrared part of the spectrum.

"For the first time we are able to see the effects of the  explosion and its aftermath at infrared wavelengths from space, with NASA's  Spitzer Space Telescope," said Professor Nye Evans of Keele University, who heads the infrared follow-up team.

"Meanwhile, the observations we  have already obtained from the ground, from the United Kingdom Infrared  Telescope on the summit of Mauna Kea in Hawai'i, already far surpass the data we had during the 1985 eruption.

"The shocked red giant wind and the  material ejected in the explosion give rise to emission not only at X-ray,  optical and radio wavelengths, but also in the infrared, via coronal lines  (so-called because they are prominent in the Sun's very hot corona). These will  be crucial in determining the abundances of the elements in the material ejected  in the explosion and in confirming the temperature of the hot gas."

26 February 2006 was a highlight of the observational  campaign. In what must surely be a unique event, four space satellites, plus  radio observatories around the globe, observed RS Oph on the same day.

"This star could not have exploded at a better time for  international ground and space based studies of an event which has been changing  every time we look at it," said Professor Sumner Starrfield of Arizona State  University, who heads the U.S. side of the collaboration. "We are all very  excited and exchanging many emails every day trying to understand what is  happening on that day and then predict the behaviour on the  next."

What is apparent is that RS Oph is behaving like a "Type II" supernova remnant. Type II supernovae represent the catastrophic death of a star at least 8 times the mass of the Sun. They also eject very high velocity material which interacts with their surroundings. However, the full evolution of  a supernova remnant takes tens of thousands of years. In RS Oph, this
evolution  is literally occurring before our eyes, around 100,000 times faster.

"In the 2006 outburst of RS Oph, we have a unique opportunity of understanding much more fully such things as runaway thermonuclear explosions and the end-points of the evolution of stars," said Professor Bode.

"With the observational tools now at our disposal, our efforts 21  years ago look rather primitive by  comparison."

CONTACTS

Prof. Michael F.  Bode
Astrophysics Research Institute
Liverpool John Moores  University
Twelve Quays House, Egerton  Wharf                          |
Birkenhead CH41 1LD
Tel: +44 (0)151-231-2920 (direct) -2919  (secretary)
Mob: +44 (0)7968-422360
E-mail:  This email address is being protected from spambots. You need JavaScript enabled to view it.

From 5 to 7 April, Prof. Bode can be  contacted via the NAM press office. Tel: +44 (0)116-229-7474     or     229-7475      or     252-3312       or   252-3531

Dr. Tim  O'Brien
Jodrell Bank Observatory
University of Manchester
Tel: +44  (0)1477-571321
Mob: +44 (0)7963-624162
E-mail: This email address is being protected from spambots. You need JavaScript enabled to view it.

On  7 April, Dr O'Brien can be contacted via the NAM press office (see above).

Dr. Julian Osborne
Department of Physics and  Astronomy
University of Leicester
Tel: +44 (0)116-252-3598
E-mail:  This email address is being protected from spambots. You need JavaScript enabled to view it.

Dr. Stewart Eyres
Centre for  Astrophysics
University of Central Lancashire
Tel: +44  (0)1772-893742
Mob: +44 (0)7792-777750  
E-mail -  This email address is being protected from spambots. You need JavaScript enabled to view it.

Professor Nye Evans
Astrophysics  Group
University of Keele
Tel: +44 (0)1782-583342
E-mail:  This email address is being protected from spambots. You need JavaScript enabled to view it.

Professor Sumner Starrfield
Department of Physics  and Astronomy
Arizona State University
Tel: +1 480-965-7569
E-mail:  This email address is being protected from spambots. You need JavaScript enabled to view it.

NOTES FOR EDITORS

SWIFT.  The Swift satellite was designed to study gamma ray bursts. It includes a large  field-of-view burst detector, and sensitive X-ray and UV/optical telescopes. The  NASA Goddard Space Flight Center manages the Swift project, and the satellite is  controlled from Penn State University, using a ground station in Kenya. Swift is  a NASA mission with the
participation of the UK Particle Physics and Astronomy  Research Council (PPARC) and the Italian Space Agency.

MERLIN is an  array of seven radio telescopes distributed across the United Kingdom. It is  operated by the University of Manchester as a National Facility of the UK  Particle Physics and Astronomy Research Council (PPARC).

UKIRT. The  world's largest telescope dedicated solely to infrared astronomy, the United  Kingdom Infrared Telescope (UKIRT) is sited in Hawai'i near the summit of Mauna  Kea at an altitude of 4194m above sea level. It is owned by the UK Particle  Physics and Astronomy Research Council (PPARC) and operated by the staff of the  Joint Astronomy Centre, Hilo.

SPITZER. The Spitzer Space Telescope is the  fourth and final element in NASA's family of Great Observatories. The  Observatory carries an 85-centimeter telescope and three cooled science  instruments capable of performing imaging and spectroscopy in the infrared  micron range, most of which is inaccessible from the ground.

The 2006 RAS  National Astronomy Meeting is hosted by the University of Leicester. It is  sponsored by the Royal Astronomical Society, the UK Particle Physics and  Astronomy Research Council (PPARC), the University of Leicester and the National  Space Centre, Leicester.

This is a joint release with  PPARC.

IMAGES:

Images and a movie showing a  conceptualisation of the explosion of RS Ophiuchi will be posted  at: http://www.swift.ac.uk/RSOph.shtml
(Movie courtesy Dr Andrew  Beardmore, University of Leicester)  

FURTHER  INFORMATION

Swift  mission:
http://www.swift.ac.uk/

MERLIN:
http://www.merlin.ac.uk/

UKIRT:
http://www.jach.hawaii.edu/UKIRT/

Liverpool  Telescope:
http://telescope.livjm.ac.uk/