The tight correlation between the variation in abundance of Si, S, Ar, Ca over an absolute abundance range of two orders of magnitude is strong evidence for the nucleosynthesis of these ejecta elements by explosive O-burning and incomplete explosive Si-burning due to the shock heating of these layers in the core collapse supernova. Full mixing of the burning products is implied by the excellent fit to the plasma model. However the Fe emission, both in the Fe-K and the Fe-L lines, does not show this correlation in any sense. A significant fraction of the Fe-K emission is seen at larger radii than Si-K and S-K as convincingly demonstrated in our Doppler derived 3-D reprojection, Fig. 11. Moreover the Fe-K emission is patchy, reminiscent of large clumps of ejecta material, rather than shock heated swept up circumstellar material. In fact the bulk of the Fe-K emission arises in two limited regions possibly indicating that the core collapse threw off material in two opposing clumps which we clearly see in Fig. 11. If we interprete these Fe-rich ejecta as the nucleosynthesis product of complete explosive burning of the Si-layer, spatial inversion of the O- and Si-burning products has occurred and large scale bulk mixing of the explosion products is an inevitable consequence. A similar conclusion was obtained by Hughes et al. (2000) for ejecta material at the east side of the remnant based on the morphological features of the high spatial resolution Chandra data.
The largely bi-polar distribution of the Fe-K emission and, to some degree,
the Si-K and S-K emission may indicate that the original explosion was
aspherical, possibly with axial symmetry.
The recent jet-induced models of Khokhlov et al. (1999)
and Höflich et al. (2001) produce a butterfly-shaped density profile
for the heavier elements a few hundred seconds after the explosion and this
might evolve into a distribution similar to our present results.
Therefore the progenitor mass estimate of 12 
derived from
the spherically symmetric models of Woosley & Weaver (1995) may be
inappropriate and the mass could be significantly larger.
The Fe-K emission requires a relatively high temperature in the range 2-6 keV. This temperature cannot be generated by the reverse shock wave, but only by the primary blast wave. Heating is certainly provided by the primary shock but preheating of the ambient medium by clumps that move ahead of the primary shock could contribute, see Hamilton (1985).
In the plane of the sky image Fig. 11
the Fe-K emission to the East is at a radius of
140 arcsec, near the primary shock, with an implied shock velocity of
in the range 3500-4500 kms-1 (see previous section).
It is coincident with a cluster of three FMFs,
4, 5, 6 listed by Fesen et al. (1988). They all have a proper
motion of
arcsecyr-1 and a mean radius from the
expansion centre in 1976 of 
arcsec. Assuming
a distance of 3.4 kpc and age in 1976 of 296 years
this corresponds to a transverse velocity of
kms-1and a deceleration parameter of
.
The same Fe-K emission is also coincident with the radio knots
89, 90, 92 and 93 listed by Anderson & Rudnick (1995).
These have a mean proper motion
of
arcsecyr-1 and a mean radius from the
expansion centre in 1987 of 
arcsec.
This corresponds to a transverse velocity of
kms-1and a deceleration parameter of
.
These radio knots also correspond to the bow shock feature D identified
using morphology and polarimetry by Braun et al. (1987). They
estimate the Mach number of this feature as 5.5, the highest in their
list of 11 such features.
The Fe-K emission at large radii is highly reminiscent of SNR shrapnel discovered by Aschenbach et al. (1995) around the Vela SNR. These are almost certainly bullets of material which were ejected from the progenitor during the collapse and subsequent explosion. They would initially be expected to have a radial velocity less than the blast wave but as the remnant develops, and the shock wave is slowed by interaction with the surrounding medium, the bullets would overtake the blast wave and appear outside the visible shock front as is the case in Vela. It was suggested by Aschenbach et al. (1995) that the X-ray emission from the Vela bullets arises from shock-heating of the ambient medium by supersonic motion. If this is the case the X-rays will be seen from Mach cones which trail the bullets extending back towards the centre of the remnant.
The generation of radio emission associated with the deceleration of
ejecta bullets has been discussed at length by several authors,
Bell (1977), Braun et al. (1987), Anderson & Rudnick (1995).
The optical emission arises from shocks penetrating dense
ejecta clumps. When these internal shocks have crossed the clump
deceleration sets in accompanied by a strong turn-on of radio
synchrotron emission. Electrons are accelerated in the bow-shock and
the magnetic field is amplified in shearing layers between the
dense ejecta and the external medium. The amplified magnetic
field in the wake of ejecta bullets
is predominately radial in agreement with radio polarization
measurements, Anderson et al. (1995).
The supersonic flow associated with this scenario has been
simulated by Coleman & Bicknell (1985).
The same situation could also give rise to X-ray emission. The bulk of
the electrons are heated to 
3 keV by the bow shock. As the
shocked material drifts back into the wake the plasma slowly comes
into ionization equilibrium and X-ray line emission is produced.
Our analysis of the abundances clearly indicates that
the matter responsible for the line emission is ejecta and this must
have been ablated from the bullets rather than swept up by the shock.
The velocities of both the radio and
X-ray emission in the East are about half that of the optical.
This is consistent with the peak of the radio and X-ray emission
falling in the wake of the bullet trailing behind the peak
of the optical emission.
What is the heating mechanism responsible for the cool component? Our present analysis clearly indicates this component is dominated by ejecta material. It is conventional to assume that the primary source of ejecta heating which produces the bright ring of X-ray emission in Cas A is the reverse shock (McKee 1974; Gull 1975). However the primary shock seen in X-rays and radio at a radius of 150 arcsec is not very bright and it is not clear that the reverse shock has been or is presently very strong. The Chandra image shows much fragmentation consistent with dense bullets and it is likely that significant heating arises, again, from the interaction of these bullets with the material pre-heated by the primary shock.
Acknowledgements
The results presented are based on observations obtained with XMM-Newton, an ESA science mission with instruments and contributions directly funded by ESA Member States and the USA. JV acknowledges support in the form of the NASA Chandra Postdoctoral Fellowship grant No. PF0-10011, awarded by the Chandra X-ray Center.
Copyright ESO 2002