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 style designations) obtained at the IRAM 30 m millimetre radio telescope both before (Nov. 1992) and after perihelion (Jan. 1993), when rh was ∼ 1 AU and Δ between 1 and 2 AU. The molecules HCN, H2S, H2CO and CH2OH were detected, with good signal-to-noise ratios (up to 30). The line profiles are strongly asymmetric with a cusp at negative velocities; this leads to an important shift (−0.45 km s−1) of the mean gas velocity with respect to the nucleus. This profile 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 distribution of the molecules is assumed, and Q(H2O) ∼ 4.0 and 3.5 × 1029 molec. s−1 for November 21 and January, respectively. Relative production rates QQ(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 and 2% on January 6–7, 1993, for HCN, H2CO and CH3OH, respectively, are derived. The effect of coma anisotropy on the derivation of these rates is briefly discussed. The decrease of non-water parent molecules from November to January, to be confirmed, raises questions about the nucleus homogeneity or sublimation process.