Mapping the ingredients of an exploded star
26 November 2001Probably 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 Agencys Technology and Research Centre, 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.
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.
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".
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 : +22.214.171.12456
Dr. Fred Jansen
XMM-Newton Project Scientist
ESA Science Programmes Communications