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 of Hwang & Laming (2012) 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 $\sim 3 $M_sun ejecta have already been
through the reverse shock and are seen today in X-rays. A minority, $\sim
0.47\pm 0.05$ M_sun, 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 $0.47 \pm {0.47\atop 0.24}$ M_sun. We speculate
that up to a further 0.07 M_sun of Fe may be present in diffuse gas in the
inner ejecta, depending on the Fe charge state.
