| Mapping
the ingredients of an exploded star
Probably the most detailed analysis of the composition and dynamics
of the supernova remnant Cassiopeia-A has been presented at the
symposium 'New Visions of the X-ray Universe in the XMM-Newton and
Chandra era' which is taking place this week at the European Space
Agency's Technology and Research Center, ESTEC, Noordwijk in the
Netherlands.
Cassiopeia A (Cas-A) is a young shell-shaped supernova remnant some
15 light years in diameter situated some 10 thousand light years
away. It is the remains of a massive star which, having exhausted
all its hydrogen fuel, exploded 320 years ago. The core of such
a collapsing star can give rise to a neutron star or black hole.
Its external parts are blown apart projecting stellar material,
glowing in X-rays, into the surrounding interstellar medium.
The stellar material contains many heavy elements which have been
forged from lighter elements in the progenitor star, and during
the explosion process. All the chemical elements in our human bodies
have their origins in such stellar explosions and the resulting
primordial broth.
Unprecedented accuracy
With its fine angular resolution, NASA's Chandra had already provided
a view of the fine knotted structure of Cas-A. It highlighted clumps
and filaments of ejected matter. Today with its combination of sensitivity
and superior X-ray collecting power, ESAs XMM-Newton has provided
element abundance maps of Cas-A of unprecedented accuracy.
Using the observatory's EPIC-MOS cameras, Cas-A was targeted in
late 2000 during XMM-Newton's performance verification and calibration
phase. The total observation time was practically 24 hours, collecting
sufficient X-ray photons to allow a full spectral analysis of each
individual pixel in a 15 x 15 grid covering the angular size of
the remnant's X-ray image.
The matrix of nine abundance maps shows the distribution and abundance
in Cassiopeia A of neon, magnesium, silicon, sulphur, argon, calcium,
the iron-L and iron-K ionization states and nickel, with a color
scale based on the solar values for these elements.
Large variations
The maps clearly highlight large abundance variations of the X-ray
emitting material across the face of the remnant. One notes however
that the distributions of silicon, sulphur, argon and calcium -all
oxygen burning products- are similar and distinct from the carbon
burning products, neon and magnesium.
Iron abundance is seen to vary over the remnant with little correlation
to other elements. The oxygen abundance, contrary to other elements,
is much higher than predicted by theory; perhaps because it has
a very different spatial distribution, concentrated to the North
and because the EPIC cameras have a poorer spectral resolution at
lower energies.
Fast moving iron
The study not only measured abundances but also elemental velocities
by determining Doppler shifts- the displacement of spectral lines
when elements are moving. Iron, produced at the heart of the stellar
explosion and present now in the furthest reaches of the remnant,
was apparently expelled considerably faster to overtake other elements
that were formed nearer to the stars surface. The dynamics of the
Cas-A X-ray emitting material indicates strong asymmetrical effects
in the supernovae explosion.
"Only XMM-Newton could give us all the data we required to
produce such reliable results," says lead author Dr. Dick Willingale
from the University of Leicester, UK. "These new maps of Cas-A
provide fresh insights into the elemental composition, the distribution
and the plasma dynamics of the remnant. It also gives us clues as
to its progenitor probably a star between 12 and 30 times the
mass of our Sun".
----
The paper "X-ray spectral imaging and Doppler mapping of Cassiopeia
A" by R.Willingale (University of Leicester), J.A.M.Bleeker,
K.J. van der Heyden and J.Kaastra (SRON National Institute for Space
Research, Utrecht Netherlands) and J.Vink (Columbia University,
New York) is to appear in Astronomy and Astrophysics.
For more information:
Dr. Dick Willingale
Department of Physics and Astronomy
University of Leicester
Leicester LE1 7RH, UK
Tel : +44.116.252.3556
Email: rw@star.le.ac.uk
Dr. Fred Jansen
XMM-Newton Project Scientist
Tel: +31.71.565.4426
Email: fjansen@astro.estec.esa.nl
Monica Talevi
ESA Science Programs Communications
Tel: +31.71.565.3223
Email: mtalevi@estec.esa.nl
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