  Characterizing the ejecta in young supernova remnants is a requisite step
towards a better understanding of stellar evolution. In Cassiopeia A the
density and total mass remaining in the unshocked ejecta are important
parameters for modeling its explosion and subsequent evolution. Low frequency
(<100 MHz) radio observations of sufficient angular resolution offer a unique
probe of unshocked ejecta revealed via free-free absorption against the
synchrotron emitting shell. We have used the Very Large Array plus Pie Town
Link extension to probe this cool, ionized absorber at 9 arcseconds and 18.5
arcseconds resolution at 74 MHz. Together with higher frequency data we
estimate an electron density of 4.2 electrons per cubic centimeters and a total
mass of 0.39 Solar masses with uncertainties of a factor of about 2. This is a
significant improvement over the 100 electrons per cubic centimeter upper limit
offered by infrared [S III] line ratios from the Spitzer Space Telescope. Our
estimates are sensitive to a number of factors including temperature and
geometry. However using reasonable values for each, our unshocked mass estimate
agrees with predictions from dynamical models. We also consider the presence,
or absence, of cold iron- and carbon-rich ejecta and how these affect our
calculations. Finally we reconcile the intrinsic absorption from unshocked
ejecta with the turnover in Cas A's integrated spectrum documented decades ago
at much lower frequencies. These and other recent observations below 100 MHz
confirm that spatially resolved thermal absorption, when extended to lower
frequencies and higher resolution, will offer a powerful new tool for low
frequency astrophysics.
