Observations of submillimetre lines of CH 3OH, HCN, and H 2CO in comet P/Swift-Tuttle with the James Clerk Maxwell Telescope

Abstract

We present submillimetre moleculaaar line observations of comet P/Swift Tuttle (1992t = 1992 XXVIII) which were undertaken on 7 and 8 december 1992 with the James Cleark Maxwell Telescope (JCMT). Three molecules were detected: (1) HCN through its J (4-3) rational line at 354–505 GHz; (2) H 2Co through its 5 15 − 4 14 ortho line at 351–769 GHz and (3) CH 3OH through several of its J (7-6) transitions at 338 Ghz and one line at 341 GHz. The lines are bluesifted with respect to the comet rest velocity,which is indicative of preferential outgassing towards the Sun.The relative intensities of the CH 3OH rotational lines show that the population distribution can be described by a rotational temperature of 45 ± 7 K,although deviations from thetrmal equilibrium are present. The comparison with an out-of-equilibrium models suggests that the collisional region where thermalization occurs is larger than is expected in an isotropic coma where neutral-neutral collisions prevail.The model is able to reproduced most of the observed methanol line intensites From these observations we deriveproduction rates of Q( HCN) = 1.3 × 10 27 s −1, Q( H 2 CO) = 5 × 10 27 s −1 and Q( CH 3 OH) = 2 × 10 28 s −1 , assuming direct releease from the nucleus and istropic sublimation. This correponds, to molecular abundances with respect to water of 0.3, 1 and 4%; for HCN, H 2CO and CH 3OH, respectively.