1994A&A...290..198W

Spatially and kinematically resolved images of emission in the ground state J=1->0 F=2->1 line of HC₃N at λ=3.3cm are presented which illustrate the structure of the molecular shell surrounding the evolved carbon star IRC+10 216. This ground state line, which traces cool material, should provide a good probe of outlying material. The emission appears ringlike, peaking at a radial distance of 12", but with substantial contributions from the entire envelope, extending to a radial distance of about 30" from the star. The J=1->0 F=2->1 line emission in the vibrationally excited 2ν₇² state has not been detected. A model is developed which reproduces both the images shown here, and also other images from lines in the millimeter wavelength region. The results show that the ringlike appearance of HC₃N emission occurs because of the shell form of HC₃N abundance distribution, with a peak at radius 15" falling off more slowly exterior to the peak than interior to it. A second model produces a better fit to the observations, but requires an interior region of unrealistically high HC₃N abundance and a low mass loss rate.