31st Annual Meeting of the DPS, October 1999
Session 17. Comet Comae Posters
Poster Group I, Monday-Wednesday, October 11, 1999, , Kursaal Center

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[17.03] The composition of comet C/1999H1 (Lee) from radio observations

D. Bockel\'ee-Morvan (Obs. Paris), N. Biver (Univ. Hawaii), P. Colom, J. Crovisier, E. G\'erard, F. Henry (Obs. Paris), J.K. Davies, H. Matthews (JAC), D.C. Lis, T.G. Phillips (Caltech), F. Rantakyro (SEST), H. Weaver (J. Hopkins Univ.)

We report on coordinated radio spectroscopic observations of comet C/1999H1 (Lee) undertaken from May to September 1999. Some early results were given in Biver et al. (1999, IAU Cir. 7203).

The 18-cm lines of the OH radical were monitored at the Nançay radio telescope, providing measurements of the water production rate Q(\rm H2O) from mid-may to mid-august. For the May-June period, the following average Q(\rm H2O) were derived: 5.5 1028 s-1 (May 19--23); 8. 1028 s-1 (May 24--June 3); 1.2 1029 s-1 (June 4-13); 1.5 1029 s-1 (June 14-25). Observations of parent molecules were made with the Swedish-ESO telescope (SEST), the Caltech Submillimeter Observatory (CSO), the James Clerk Maxwell telescope (JCMT), and the 30-m telescope of the Institut de Radioastromie millimétrique (IRAM). The HCN, HNC, CH3OH, H2CO, and CS species were detected through one or several of their rotational transitions. Abundances relative to water are estimated to 0.1% (HCN), 0.012% (HNC), 4% (CH3OH), 1.3% (H2CO), and 0.08% (CS). Observations of H2S and CO yield upper limits of 0.9 % and 5%, respectively.

Molecular abundances are within the range of values measured in other comets. When compared to comets C/1995O1 (Hale-Bopp) and C/1996B2 (Hyakutake), comet Lee is enriched in CH3OH by a factor of ~ 2 and depleted in CO by a factor of ~ 4. The most surprising result is the HNC/HCN mixing ratio of 12%. This value is close to that measured in comet Hale-Bopp near perihelion and twice higher than that inferred in comet Hyakutake. Observations of HNC in comet Hale-Bopp and chemical modelling have suggested that HNC could be the product of coma chemistry. This new measurement in a moderatly active comet should provide further constraints on the origin of HNC in comets.

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