Cas A: The Bright X-ray Knots and Oxygen Emission

Chandra High-Energy Transmission Grating (HETG) X-ray spectra are extracted from 17 bright, narrow regions of Cas A and provide unique measurements of their kinematic and plasma states. From the dominant emission lines, e.g. He-like Si, we derive accurate Doppler shifts in the range -2500 to +4000 km/s; these agree well with transverse-velocity measurements and allow the features to be located in 3D. Plasma diagnostics of these regions indicate temperatures largely around 1 keV with some above 3 keV. Using as well the non-dispersed zeroth-order data, we determine NEI model parameters for the regions which lead to density estimates. Values of n_e ~ 100 /cm^3 are likely the maximum of a range of densities in this X-ray emitting material. The common "oxygen-rich" assumption is coarsely tested by comparing the integrated O VIII line flux and continuum levels. It appears that most of the continuum is due to another source, e.g., from He.