Ionization States and Plasma Structures of Mixed-morphology SNRs   Observed with ASCA
  We present the results of a systematic study using ASCA of the ionization state for six ``mixed-morphology'' supernova emnants (MMSNRs): IC 443, W49B, W28, W44, 3C391, and Kes 27. MMSNRs show centrally filled thermal X-ray emission, which contrasts to shell-like radio morphology, a set of haracteristics at odds with the standard model of SNR evolution (e.g., the Sedov model). We have therefore studied the evolution of the MMSNRs from the ionization conditions inferred from the X-ray spectra, independent of X-ray morphology. We find highly ionized plasmas approaching ionization equilibrium in all the mmsnrs. The degree of ionization is systematically higher than the plasma usually seen in shell-like SNRs. Radial temperature gradients are also observed in five remnants, with cooler plasma toward the limb. In IC 443 and W49B, we find a plasma structure consistent with shell-like SNRs, suggesting that at least some MMSNRs have experienced similar evolution to shell-like SNRs. In addition to the results above, we have discovered an ``overionized'' ionization state in W49B, in addition to that previously found in IC 443. Thermal conduction can cause the hot interior plasma to become overionized by reducing the temperature and density gradients, leading to an interior density increase and temperature decrease. Therefore, we suggest that the ``center-filled'' X-ray morphology develops as the result of thermal conduction, and should arise in all SNRs. This is consistent with the results that MMSNRs are near collisional ionization equilibrium since the conduction timescale is roughly similar to the ionization timescale. Hence, we conclude that MMSNRs are those that have evolved over$\sim10^4$ yr. We call this phase as the ``conduction phase.'' 
Masahiro T. Kawasaki (MHI/ISAS), Masanobu Ozaki (ISAS), Fumiaki Nagase   (ISAS), Hajime Inoue (ISAS), and Robert Petre (GSFC)