dominates. However, cases that did not fit either morphology were found and referred to as either thermal composite, because of their centrally concentrated and yet thermal X-ray emission along with edge brightened radio emission, or mixed morphology supernova remnants (MMSNR).

@Rho+Petre_1998 first argued that the mixed morphology remnants formed a truly distinct class of SNRs. In more recent years more remnants in this class have been found and evidence has been presented that these remnants are, as a rule, undergoing interactions with dense clouds. In particular @Yusef-Zadeh_etal_2003 showed a strong correlation of OH maser emission with mixed morphology, indicating that shocks in molecular clouds are present around MMSNRs. In addition, shock-cloud interaction indicators such as broad velocity wings [@Frail+Mitchell_1998] or an enhanced $^{12}\mathrm{CO}(J=2-1)/^{12}\mathrm{CO}(J=1-0)$ ratio [@Seta_etal_1998] are observed in many MMSNRs [see review by @Slane_etal_2015].

At present, there are $\sim 40$ known MMSNRs, but the nature of the central X-ray emission is poorly understood. X-ray spectra show evidence for the presence of ejecta in some such MMSNRs, but the inferred mass of the X-ray emitting material appears to be much larger than any reasonable ejecta contributions. The observed temperature profiles show little variation with radius, in contrast to the steep profile 