ESA Science & Technology - Mapping the ingredients of an exploded star

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 remnants 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 ionisation states and nickel, with a colour 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 abundancies 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".

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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: rwstar.le.ac.uk

Dr. Fred Jansen
XMM-Newton Project Scientist
Tel: +31.71.565.4426
Email: fjansenastro.estec.esa.nl

Monica Talevi
ESA Science Programmes Communications
Tel: +31.71.565.3223
Email: mtaleviestec.esa.nl