OBSERVATIONS OF SUBMILLIMETRE LINES OF CH3OH, HCN, AND H2CO IN COMET P SWIFT-TUTTLE WITH THE JAMES-CLERK-MAXWELL-TELESCOPE/

We present submillimetre molecular line observations Of comet P/Swift- Tuttle (1192t = 1992 XXVIII) which were undertaken on 7 and 8 December 1992 with the James Clerk Maxwell Telescope (JCMT). Three molecules w ere detected. (1) HCN through its J (4-3) rotational line at 354.505 G Hz; (2) H2CO through its 5(15)-4(14) ortho line-at 351.769 GHz and (3) CH3OH through several of its J (7-6) transitions at 338 GHz and one l ine at 341 GHz. The lines are blueshifted with. respect to the comet r est, velocity, which is indicative of preferential outgassing towards the Sun. The relative intensities of the CH3OH rotational lines show t hat the population distribution can,be described by a rotational tempe rature Of 45 +/- K, although deviations from thermal equilibrium are p resent. The comparison with an out-of-equilibrium model suggests that the collisional region where thermalization occurs is larger than is e xpected in an isotropic coma where neutral(neutral collisions prevail. The model is able to reproduce most of the observed methanol line int ensities. From these observations, we derive production rates of Q(HCN ) - 1.3 x 10(27) s(-1), Q(H2CO) = 5 x 10(27) s(-1) and Q(CH3OH) = 2 x 10(28) s(-1), assuming direct release from the nucleus and isotropic s ublimation This corresponds to molecular abundances with respect to wa ter of 0.3, 1 and 4%, for HCN, H2CO and CH3OH, respectively.