Exploding 'star within a star' (Forwarded)

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EMBARGOED FOR 00:01 BST, FRIDAY, 7 APRIL 2006 (23:01 GMT, THURSDAY 6
APRIL)

Ref.: PN 06/26 (NAM 19)

EXPLODING 'STAR WITHIN A STAR'

On 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)

From 5 to 7 April, Prof. Bode can be contacted via the NAM press
office (see above).

Dr. Tim O'Brien
Jodrell Bank Observatory
University of Manchester
Tel: +44 (0)1477-571321

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

Dr. Stewart Eyres
Centre for Astrophysics
University of Central Lancashire
Tel: +44 (0)1772-893742

Professor Nye Evans
Astrophysics Group
University of Keele
Tel: +44 (0)1782-583342

Professor Sumner Starrfield
Department of Physics and Astronomy
Arizona State University
Tel: +1 480-965-7569

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

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/