2002A&A...390..363I

We have observed 9 rotational lines of methanol in comet Hale-Bopp (1995 O1) using the 45-m radio telescope at Nobeyama Radio Observatory in the frequency range 36-104GHz. The observed transitions have upper state rotational energy levels of E_u=7-159K. Assuming the Haser model for density distribution, we estimated the excitation temperature and the production rate to be T_ex=81±8K and Q(CH₃OH)=(1.2±0.1)x10²⁹ molecule/s, respectively. We also observed the HCN J=1-0 line at 88GHz to derive the HCN production rate. We compared the production rate of CH₃OH with that of HCN and that of other molecules whose production rates have been determined previously. The abundance ratio Q(CH₃OH)/Q(X) is estimated to be in the range 3-121, where X denotes HCN, HNC, H₂CO, CS and CH₃CN. When we compare these abundance ratios with those in interstellar clouds: the bipolar flow L1157 B1/B2, the star-forming region SgrB2(M) and the dark cloud TMC-1, they are close to the values in L1157 B1/B2 or in SgrB2(M). We found that the abundance ratio Q(CH₃OH)/Q(H₂O) was similar to solid methanol ratio relative to water ice in interstellar medium. These results support the hypothesis that cometary nuclei are formed from dust grains outside of the primordial solar nebula so that the grain composition of the nebula remains in cometary nuclei.