Fully Sampled Maps of Ices and Silicates in Front of Cepheus A East with Spitzer

Abstract

We report the first fully sampled maps of the distribution of interstellar CO2 ices, H2O ices and total hydrogen nuclei, as inferred from the 9.7 micron silicate feature, toward the star-forming region Cepheus A East with the IRS instrument onboard the Spitzer Space Telescope. We find that the column density distributions for these solid state features all peak at, and are distributed around, the location of HW2, the protostar believed to power one of the outflows observed in this star-forming region. A correlation between the column density distributions of CO2 and water ice with that of total hydrogen indicates that the solid state features we mapped mostly arise from the same molecular clumps along the probed sight lines. We therefore derive average CO2 ice and water ice abundances with respect to the total hydrogen column density of X(CO2)_ice~1.9x10^-5 and X(H2O)_ice~7.5x10^-5. Within errors, the abundances for both ices are relatively constant over the mapped region exhibiting both ice absorptions. The fraction of CO2 ice with respect to H2O ice is also relatively constant at a value of 22% over that mapped region. A clear triple-peaked structure is seen in the CO2 ice profiles. Fits to those profiles using current laboratory ice analogs suggest the presence of both a low-temperature polar ice mixture and a high-temperature methanol-rich ice mixture along the probed sightlines. Our results further indicate that thermal processing of these ices occurred throughout the sampled region.