What Next for the Likely Presupernova HD 179821?

Abstract

We have used the Owens Valley Radio Observatory Millimeter Array to obtain a map of the J = 1→0 CO emission from the circumstellar shell around HD 179821, a highly evolved G-type star that will probably explode as a supernova in the next 105 yr. Very approximately, the gas presents as a circular ring with an inner diameter of 395, an outer diameter of ~12'', and azimuthal variations in the CO brightness of a factor of about 2. Until about 1600 years ago, the star was a red hypergiant, losing about 3 × 10-4 M☉ 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 that cause the anisotropic mass loss in HD 179821 persist until the moment when the star explodes as a supernova, the newly born pulsar may receive a momentum kick, leading to a space motion of close to 700 km s-1. Independent of the angular asymmetries, the radially detached shell around HD 179821 may be representative of environments that produce dust echoes from gamma-ray bursts.