Observations of HCN, HNC, and NH 3 in Comet Hale‐Bopp

Abstract

We have observed the J=1-0 rotational lines of HCN and HNC, and the (J, K)=(1, 1), (2, 2), and (3, 3) inversion lines of NH_3 toward comet Hale-Bopp during 1997 April 19-21. We have made a cross-scan toward the coma, and have found the source sizes of HCN and HNC to be 38^''+/-7^'' and 50^''+/-18^'', respectively. The [HNC]/[HCN] abundance ratios obtained are 0.13+/-0.03 and 0.36+/-0.32 at the center of the coma and in the outer part, respectively. Although the source size of HNC might be slightly larger than that of HCN, the distributions of these two species almost agree with each other within the error limits. Since it has been suggested that HCN and HNC are a parent molecule and a product of coma chemistry, respectively, the present results indicate that the production of HNC is occurring in the inner coma region. We have calculated the density distributions of HCN and HNC in the coma of comet Hale-Bopp by assuming a simple chemical model. The calculated HNC/HCN ratio is almost constant in the coma, which is consistent with our observations. In addition, the average excitation temperature is found from the observations of the NH_3 lines to be 54+/-45 K with the 74" beam, corresponding to a coma size of 8.6x10^4 km.