2005ApJ...631..935K

We present the results of a systematic study using ASCA of the ionization state for six ``mixed-morphology'' supernova remnants (MM SNRs): IC 443, W49B, W28, W44, 3C 391, and Kes 27. MM SNRs show centrally filled, thermal X-ray emission, which contrasts with shell-like radio morphology, a set of characteristics at odds with the standard model of SNR evolution (e.g., the Sedov model). We have therefore studied the evolution of the MM SNRs 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 MM SNRs. 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 MM SNRs have experienced evolution similar to that of shell-like SNRs. In addition to the results above, we have discovered an ``overionized'' ionization state in W49B, similar 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 MM SNRs are near collisional ionization equilibrium, since the conduction timescale is roughly similar to the ionization timescale. Hence, we conclude that MM SNRs are those that have evolved over ∼10<sup>4</sup> yr. We call this phase the ``conduction phase.''