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

The Density and Mass of Unshocked Ejecta in Cassiopeia A through Low Frequency Radio Absorption