Element Abundances in the Unshocked Ejecta of Cassiopeia A

We analyze and model the infrared spectrum of the Cassiopeia A supernova remnant with the aim of determining the masses of various elements in the unshocked ejecta. In this way, we complement the survey of the X-ray-emitting ejecta to provide a complete census of the elemental composition of the Cas A ejecta. We calculate photoionization–recombination equilibria to determine the ionization balance of various elements in the ejecta as a function of density using the X-ray and UV emission from the forward and reverse shocks as the ionizing radiation. With the assumption that all emission lines are principally excited at the ejecta density that maximizes their emission, we can convert observed line intensities into element masses. We find that the majority of the ∼3 M _⊙ ejecta have already been through the reverse shock and are seen today in X-rays. A minority, ∼0.47 ± 0.05 M _⊙, with uncertainties quoted here coming from the data fitting procedure only, are still expanding inside the reverse shock and emitting in the infrared. This component is comprised mainly of O, Si, and S, with no Fe readily detectable. Incorporating uncertainties estimated to come from our modeling, we quote $0.47{\pm }_{0.24}^{0.47}$ M _⊙. We speculate that up to a further 0.07 M _⊙ of Fe may be present in diffuse gas in the inner ejecta, depending on the Fe charge state.