We study the outer-shock structure of the oxygen-rich supernova remnant G292.0+1.8 , using a deep observation with the Chandra X-ray Observatory .
We measure radial variations of the electron temperature and emission measure that we identify as the outer shock propagating into a medium with a radially decreasing density profile .
The inferred ambient density structure is consistent with models for the circumstellar wind of a massive progenitor star rather than for a uniform interstellar medium .
The estimated wind density ( n _ { \mathrm { H } } = 0.1 \sim 0.3 \mathrm { cm } ^ { -3 } ) at the current outer radius ( \sim 7.7 pc ) of the remnant is consistent with a slow wind from a red supergiant ( RSG ) star .
The total mass of the wind is estimated to be \sim 15 - 40 \mathrm { M } _ { \odot } ( depending on the estimated density range ) , assuming that the wind extended down to near the surface of the progenitor .
The overall kinematics of G292.0+1.8 are consistent with the remnant expanding through the RSG wind .