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Hubble
Space Telescope Reveals The Aftermath Of "Star Wars"
04/17/07
For
clarity, this shows how the model looks, firstly by tilting
it, then by reducing the resolution until it is seen at the
same pixel scale as the HST, and then finally by comparing
it with the observations.
Credit:
D. Harman (Liverpool JMU)
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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 kilometers 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.
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 & PPARC
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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 analyzing 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
(license) 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 kilometers 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.
Source:
RAS
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