We have used the Owens Valley Radio Observatory Millimeter Array to obtain a map of the J = ( 1 { \rightarrow } 0 ) CO emission from the circumstellar shell around HD 179821 , a highly evolved G-type star which will probably explode as a supernova in the next 10 ^ { 5 } yr .
Very approximately , the gas presents as a circular ring with an inner diameter of 3 { \farcs } 95 , an outer diameter of { \sim } 12 { \arcsec } and with azimuthal variations in the CO brightness by about a factor of 2 .
Until about 1600 years ago , the star was a red hypergiant losing about 3 { \times } 10 ^ { -4 } M _ { \odot } yr ^ { -1 } at an average outflow speed of 32 km s ^ { -1 } .
We propose that when HD 179821 explodes as a supernova , it may resemble Kepler ’ s supernova remnant and thus some of the anisotropies in supernova remnants may be intrinsic .
If the factors which cause the anisotropic mass loss in HD 179821 persist to the moment when the star explodes as a supernova , the newly-born pulsar may receive a momentum “ kick ” leading to a space motion near { \sim } 700 km s ^ { -1 } .
Independent of the angular asymmetries , the radially detached shell around HD 179821 may be representative of environments which produce dust echoes from gamma-ray bursts .