Molecular Observations of C/1995O1 (Hale-Bopp) with the IRAM Telescopes near Perihelion: Interferometric Maps and New Parent Molecules

D. Bockelée-Morvan (Obs. Paris), J. Wink (IRAM, Grenoble), D. Despois (Obs. Bordeaux), N. Biver, P. Colom, J. Crovisier, E. Gérard, B. Germain, E. Lellouch (Obs. Paris), R. Moreno, G. Paubert (IRAM, Granada), J.K. Davies, W.R.F. Dent (JAC, Hilo), H. Rauer (DLR, Berlin)

Comet C/1995O1 (Hale-Bopp) was observed in October 1996 and March 1997 with the Institut de Radioastronomie Millimétrique (IRAM) interferometer at Plateau de Bure (France). Interferometric maps of the HCN J(1-0) and CO J(2-1) lines were obtained in October 1996. In March 1997, the radio lines of eight species (CO, HCN, HNC, CH tex2html_wrap_inline17 OH, H tex2html_wrap_inline19 CO, H tex2html_wrap_inline19 S, CS, SO) were mapped. The spatial resolution ranged from 1.5 to 3.5 arc sec and the map sizes were from 20 tex2html_wrap_inline23 20 arc sec to 1 tex2html_wrap_inline23 1 arc min. Our goal was three-fold: i) investigate the spatial distribution of parent molecules to relate gas and dust jets; ii) constrain the origin of CO, H tex2html_wrap_inline19 CO, CS and SO which are thought to be released at least partly from a distributed source rather than from direct sublimation from the nucleus; iii) study the evolution of the excitation conditions of CH tex2html_wrap_inline17 OH and CO as a function of radial distance, by mapping several of their rotational lines. A preliminary analysis of these observations will be presented. SO was clearly detected in single-dish mode whereas it was not in interferometry: this shows that it is mainly a photodissociation product.

In March 1997, the IRAM interferometer was also used with its five 15-m antennas in a single-dish mode to search for minor parent molecules. We obtained the first identifications of sulphur dioxide (SO tex2html_wrap_inline19 ) and formic acid (HCOOH) in a comet. Searches for minor volatiles with the IRAM 30-m telescope at Pico Veleta (Spain) in early April 1997 allowed the identifications of formamide (NH tex2html_wrap_inline19 CHO) and methyl formate (CH tex2html_wrap_inline17 OCHO). Abundances relative to CH tex2html_wrap_inline17 OH are estimated to 0.5 %, 3% and 3 % for NH tex2html_wrap_inline19 CHO, HCOOH and CH tex2html_wrap_inline17 OCHO, respectively. The abundances of SO and SO tex2html_wrap_inline19 relative to water are significantly larger than upper limits derived from UV observations of previous comets (Kim and A'Hearn, 1991, Icarus 90, 79). Stringent upper limits were also obtained for a number of species.