Distribution of emission measure versus electron temperature (kT_e) and ionization age (n_et) at the epoch of Cas A derived from model W15-2-cw-IIb-HD+dec compared with the results of Chandra X-ray observations. The large white cross marks the ranges of kT_e and n_e t derived from an X-ray survey of ejecta in Cas A (Hwang & Laming 2012); the small yellow crosses mark the values inferred from the analysis of regions dominated by thermal emission of shocked ambient gas (Lee et al. 2014).