HST reveals the aftermath of "star wars" (Forwarded)

Royal Astronomical Society
London, U.K.

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Robert Massey
Tel: +44 (0)20 7734 4582
AND
Anita Heward
Tel: +44 (0)1483 420 904

NATIONAL ASTRONOMY MEETING PRESS ROOM (16 - 20 APRIL ONLY):
Tel: +44 (0)1772 892 613
+44 (0)1772 892 475
+44 (0)1772 892 477

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RAS National Astronomy Meeting web site:
http://nam2007.uclan.ac.uk

CONTACTS:

Prof Michael F. Bode
Astrophysics Research Institute, Liverpool John Moores University
Tel: +44 (0)151 231 2920 (direct), 2919(secretary)

Dr Daniel Harman
Astrophysics Research Institute, Liverpool John Moores University
United Kingdom
Tel: +44 (0)151 231 2906 (direct), 2919 (secretary)

Dr Stewart Eyres
Centre for Astrophysics
University of Central Lancashire
Tel: +44 (0)1772 893 742

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

Prof Sumner Starrfield
School of Earth and Space Exploration
Arizona State University
Tel: +1 480 965 7569

From 17 to 19 April, Dr. Bode can be contacted via the NAM press office
(see above).

PRESS INFORMATION NOTE: RAS PN 07/ 16 (NAM 12)

EMBARGOED FOR 00:01 BST, TUESDAY 17 APRIL 2007

HUBBLE SPACE TELESCOPE REVEALS THE AFTERMATH OF "STAR WARS"

An Anglo-American team of astronomers have used the Advanced Camera for
Surveys on the Hubble Space Telescope (HST) to obtain the first direct
optical images of the aftermath of a recent titanic explosion that took
place in a star system 5,000 light years from Earth.

In a talk on Tuesday 17 April at the Royal Astronomical Society National
Astronomy Meeting in Preston, Professor Michael Bode of Liverpool John
Moores University will describe how these unique observations shed new
light on the circumstances of such events.

Professor Bode will be speaking on behalf of the team which also comprises
Dan Harman and Matt Darnley (Liverpool JMU, UK), Tim O'Brien (Jodrell Bank
Observatory, University of Manchester, UK), Howard Bond (Space Telescope
Science Institute, USA), Sumner Starrfield (Arizona State University,
USA), Nye Evans (University of Keele, UK), Stewart Eyres (University of
Central Lancashire, UK) and Michael Shara (American Museum of Natural
History, USA).

During the night of 12 February 2006, Japanese amateur astronomers
reported that a star in the constellation of Ophiuchus (known as RS Oph
for short) had suddenly brightened and become visible even with the
unaided eye in the night sky. Although this was the latest in a series of
such outbursts of this star that have been spotted over the last hundred
years or so, it was the first one since 1985 and gave scientists an
opportunity to study it with new, more powerful, telescopes on the ground
and in space.

RS Oph consists of a white dwarf, a super-dense dead star about the size
of the Earth which was once the core of a star like the Sun and whose
outer layers have been lost into space, in close orbit with a much larger,
so-called red giant star. The two stars are so close together that the
strong gravitational field of the white dwarf continuously pulls
hydrogen-rich gas from the outer layers of the red giant. After around 20
years, so much gas builds up 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 a quantity of gas
equivalent to the mass of the Earth is ejected into space at speeds of
several thousand kilometres per second (several million miles per hour).

Explosions such as this on short timescales of decades can only be
explained if the white dwarf is near the maximum mass it could have
without having collapsed to become an even denser object -- a neutron star
-- during a supernova explosion.

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 effectively "inside" its
companion's atmosphere and the ejected gas then slams into it at very high
speed.

Professor Bode explains: "Immediately after the explosion, an observing
campaign was set in train that involved most of the major space
observatories, and many on the ground. We expected to see emission from
the blast waves set up as the ejecta from the white dwarf impacted the red
giant wind and we were not disappointed! For example, X-ray observations
revealed temperatures in the shocked gas of over 100,000,000 degrees
Celsius (around ten times that in the core of the Sun)."

On the ground, radio observations from telescopes spread around the globe
also allowed the team to probe the initial stages of the outburst.
Professor Bode comments, "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. Over the next few
months we were surprised to find our radio observations apparently showing
it turning from a ring into a cigar-like shape with two more extended
blobs ("jets") gradually emerging, one on either side."

In order to determine more precisely what was happening, optical
observations with the orbiting Hubble Space Telescope (HST) were made in
July 2006. Dr Dan Harman of Liverpool JMU took on the task of analysing
the resulting data. "The problem here was that, seen from a distance of
5,000 light years, we were looking for what would appear to be very tiny
and very faint features buried within the glare from the bright central
star -- a bit like trying to read the registration (licence) plate of an
approaching car with its headlights on at night. However, after carefully
removing the confusing effects of the star we were astounded by the
results".

Professor Bode continues, "Archival images taken before the latest
outburst show no extended structure, but our latest HST images clearly
show what appear to be two overlapping rings of total extent around 0.4
seconds of arc in size. At a distance of 5,000 light years, that equates
to 8 times the diameter of Pluto's orbit around our Sun and an inferred
speed of expansion from the time of the explosion of around 3,200
kilometres per second (over 7 million miles per hour). The overall size
and orientation are consistent with continued expansion of the largest
structures (so-called "jets") seen in the later radio images, but the
picture is, perhaps unsurprisingly, not the simple one that had been
assumed prior to the 2006 outburst."

What Mike Bode and the team think we may be seeing is emission from the
boundary of a rapidly expanding region shaped something like a peanut, but
inclined towards us at an angle of around 40 degrees. The central stars
orbit around each other in the plane of the "waist" region and the rings
we see are a natural consequence of us looking through this inclined
structure. They are now working with astronomers in Mexico who have high
resolution optical spectra taken from the ground at around the time of the
HST observations, and with these they expect to be able to tie down the
geometry more precisely. "Further scheduled HST observations should also
help in this regard", says Bode. As Professor Sumner Starrfield adds, "The
HST images clearly resolve the effects of high velocity material that has
been explosively ejected from the white dwarf and then impacting the
environment of the companion star: Star Wars in Action."

The big question is what causes this shaping in the first place? It is
thought unlikely that it originates in the explosion itself. More probable
is that the environment into which the material is ejected is denser in
some directions (most likely the plane of the binary star orbit) than
others. This will have important wider implications for our understanding
of the explosion and how jet-like structures are formed in many other
astronomical objects.

NOTES FOR EDITORS

The 2007 RAS National Astronomy Meeting is hosted by the University of
Central Lancashire. It is sponsored by the Royal Astronomical Society and
the UK Science and Technology Facilities Council.

This year the NAM is being held together with the UK Solar Physics (UKSP)
and Magnetosphere, Ionosphere and Solar-Terrestrial (MIST) spring
meetings. 2007 is International Heliophysical Year.

IMAGES:

High resolution images, and an animation of the model, can be obtained
over the Internet via
http://www.astro.livjm.ac.uk/press/

Captions:
Artist's impression of RS Ophiuchi
In this 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)

Hubble Space Telescope image of RS Ophiuchi
This image of the expanding remnant of RS Oph was taken 155 days after
outburst as seen in an emission line of doubly ionised Oxygen. The "rings"
lie almost EW and are of total extent 0.4 arc seconds. This is a false
colour image and the white central region is where the very bright central
star is situated.
(Credit: NASA, ESA and M.F. Bode, D. Harman (Liverpool JMU))

A Simple Initial Model
The HST image may be explained in terms of emission from an expanding,
"peanut-shaped" shell. Here we see the shell side on (left), inclined at
an angle consistent with the central binary orbit (middle), and as the
middle image, except at the spatial resolution of the HST (right). Note
that the very bright central star is not shown in the model.
(Credit: D. Harman (Liverpool JMU))