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Session 28 - Stellar Evolution: Beyond the Main Sequence.
Oral session, Monday, June 10
Humanities 3650,
\vskip -0.2truecm Composite supernova remnants (SNRs) appear center-filled in X-rays and have a shell-like radio morphology. I have used ROSAT and ASCA data to study a sample of \sim20 composite SNR, six of these in detail, to understand the origin of centrally enhanced X-ray emission. Seven of the sample remnants (W44, W28, 3C400.2 Kes 27, MSH 11-61A, 3C391, and CTB 1) were found to be clearly mixed morphology (M-type) composites, and their central surface brightness is a factor of 2-5 times that at the edge. Despite their X-ray morphological similarity to plerions, the dominant X-ray spectral component is thermal (e.g. ASCA spectra show line emission). Another six remnants (e.g. W63, and HB21) are possible M-type composites. The remnants IC443, Kes 79 and HB 3 are similar to these composites; however, other physical processes are required to explain their properties. MSH 11-54 is definitely not a M-type composite, because the emission is enhanced by the ejecta from the progenitor.
For M-type composites, the temperature of the X-ray emitting plasma is largely uniform across the remnant, and the pressure and density either do not vary across the remnant, or slightly increase radially inwards, contrary to the classical Sedov solution. I have examined several hypotheses which purport to explain the origin of the X-ray emission: ``fossil'' radiation from the hot interior, a reverse shock, a stellar wind, large scale ISM structures, a reflected shock, and evaporating clouds. The hypothesis invoking evaporation of clouds in the SNR interior (McKee 1981; White amp; Long 1991) appears to be most consistent with the X-ray data, e.g. in the temperature, density, and pressure profiles. The clouds within a multi-phase interstellar medium are the source of the X-ray centrally peaked morphology. A simulation of the enhanced X-ray emission due to evaporation around a cloud is presented for the cases of both classical and saturated conduction based on Cowie amp; McKee (1977). For the classical conduction, the emission is enhanced by less than a factor of 3 for T<10^7 K. However, when the conduction becomes saturated, which is often the case around evaporating clumps, the X-ray emission can be enhanced by a factor of 3-20 and the enhancement appears smooth around the clouds.
