Abstract

Characterizing the ejecta in young SNRs 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" and 18".5 resolution at 74 MHz. Together with higher frequency data we estimate an electron density of 4.2 cm^-3 and a total mass of 0.39 Mʘ with uncertainties of a factor of ~2. This is a significant improvement over the 100 cm^-3 upper limit offered by infrared [S III] line ratios from the Spitzer Space Telescope.