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

RAS Web site:
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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))