Cas A is a Galactic supernova remnant whose supernova explosion is observed to be of Type IIb from spectroscopy of its light echo ( 20 ) .
Having its SN type known , observational constraints on the mass loss history of Cas A ’ s progenitor can provide crucial information on the final fate of massive stars .
In this paper , we study X-ray characteristics of the shocked ambient gas in Cas A using the 1 Msec observation carried out with the Chandra X-ray Observatory ( 19 ) , and try to constrain the mass loss history of the progenitor star .
We identify thermal emission from the shocked ambient gas along the outer boundary of the remnant .
Comparison of measured radial variations of spectroscopic parameters of the shocked ambient gas to the self-similar solutions of Chevalier ( 3 ) show that Cas A is expanding into a circumstellar wind rather than into a uniform medium .
We estimate a wind density n _ { \mathrm { H } } \sim 0.9 \pm 0.3 \mathrm { cm } ^ { -3 } at the current outer radius of the remnant ( \sim 3 pc ) , which we interpret as a dense slow wind from a red supergiant ( RSG ) star .
Our results suggest that the progenitor star of Cas A had an initial mass around 16 \mathrm { M } _ { \odot } , and its mass before the explosion was about 5 \mathrm { M } _ { \odot } , with uncertainties of several tens of percent .
Furthermore , the results suggest that , among the mass lost from the progenitor star ( \sim 11 \mathrm { M } _ { \odot } ) , a significant amount ( more than 6 \mathrm { M } _ { \odot } ) could have been via its RSG wind .