RADIO LINE OBSERVATIONS OF COMET 109P SWIFT-TUTTLE AT IRAM/

Observations are presented of comet 109P/Swift-Tuttle (1992t and 1992 XXVIII in the old designations) obtained at the IRAM 30m millemetre ra dio telescope both before (Nov. 1992) and after perihelion (Jan. 1993) , when r(h) was similar to 1 AU and Delta between 1 and 2 AU. The mole cules HCN, H2S, H2CO and CH3OH were detected, with good signal-to-nois e ratios (up to 30). The line profiles are strongly asymmetric with a cusp at negative velocities; this leads to an important shift (-0.45 k m s(-1)) of the mean gas velocity with respect to the nucleus. This pr ofile is most probably linked to the jets seen at visual wavelengths. From methanol rotation diagrams, average rotational temperatures of 70 K for November 21 and 45 K for January are estimated. An isotropic di stribution of the moleclules is assumed, and Q(H2O)similar to 4.0 and 3.5 x 10(29) molec.s(-1) for November 21 and January, respectively. Re lative production rates Q/Q(H2O) of 0.1, 0.4, 0.5 and 4-7% on November 21, 1992 for HCN, H2S, H2CO and CH3OH, respectively, and 0.05, 0.2 an d 2% on January 6-7, 1993, for HCN, H2CO and CH3OH, respectively, are derived. The effect of coma anisotropy on the derivation of these rate s is briefly discussed. The decrease of non-water parent molecules fro m January, to be confirmed, raises questions about the nucleus homogen eity or sublimation process. Copyright (C) 1996 Published by Elsevier Science Ltd