Astron. Astrophys. 353, 1101-1114 (2000)

1. Introduction

Comets contain a record of the chemical composition of the primitive Solar Nebula at the place where they formed, 4.6 Gyr ago. Because cometary nuclei accreted in the coldest regions of the Solar Nebula, an important dilemma is the extent to which they inherited the molecular composition of the interstellar grains in the natal presolar dense cloud (Greenberg 1982) versus interstellar grains in the natal presolar dense cloud (Greenberg 1982) versus the role of subsequent chemistry in the Solar Nebula in establishing their composition (Lewis & Prinn 1980; Fegley 1993; Aikawa et al. 1999). By comparing the volatile composition of comets to that of interstellar matter in star-forming regions, we can obtain insights into the origin of cometary material and clues to the evolution of matter from interstellar clouds to planetary systems.

Improvements in millimetre and infrared spectroscopic techniques made possible remote detection of volatiles outgassed from cometary nuclei. Recently, the close approach to Earth of comet C/1996 B2 (Hyakutake) in March 1996 led to major progress in our knowledge of cometary composition with the first identifications of the HNC (Irvine et al. 1996; Lis et al. 1997), CH₃CN (Dutrey et al. 1996), HNCO (Lis et al. 1997), and OCS (Woodney et al. 1997) molecules at millimetric wavelengths, as well as CH₄, C₂H₆ (Mumma et al. 1996) and C₂H₂ (Brooke et al. 1996) in the infrared. These species were found to be present as traces in the coma, with abundances from 1% to only 0.01 %, with respect to water, the main constituent of cometary ices (Bockelée-Morvan 1997). The bright comet C/1995 O1 (Hale-Bopp) offered a new opportunity to search for cometary molecules. This comet was exceptionally active with a total visual magnitude of 10.5, when discovered in July 1995 at a distance = 7 AU from the Sun. It was shown by radio spectroscopy that this remarkable activity at large was due to the sublimation of CO (Jewitt et al. 1996; Biver et al. 1996). The water production rate began to surpass the CO production rate at = 3 - 4 AU (Biver et al. 1997; Biver et al. 1999a, hereafter referred to as Paper I). Around perihelion on April 1, 1997 at = 0.91 AU, reached 10³¹ mol. s^-1 (Colom et al. 1999; Combi et al. 1999; Dello Russo et al. 1998a). This enormous production rate (roughly 20 times that of P/Halley at the same heliocentric distance) provided a new opportunity to identify cometary compounds.

We present here the results of a successful search for new cometary species using millimetre and submillimetre spectroscopy. These observations were undertaken from February to April 1997, at the time comet Hale-Bopp was most active, using several radio telescopes, as described in Sect. 2. They resulted in the first identifications of cyanoacetylene (HC₃N), formamide (NH₂CHO), formic acid (HCOOH), sulphur monoxide (SO), sulphur dioxide (SO₂) and methyl formate (HCOOCH₃), and confirmed the presence of OCS and HNCO, marginally seen in comet C/1996 B2 (Hyakutake). In parallel, molecular lines of eight other species (HCN, HNC, CH₃CN, CO, CH₃OH, H₂CO, H₂S, CS) were observed, as part of a long-term monitoring program at radio wavelengths which began in August 1995 (Biver et al. 1997; Paper I). In Sects. 3 and 4, we present and discuss the derived molecular production rates and relative abundances of the newly detected species. In Sect. 5, we discuss striking similarities with molecular abundances measured in several interstellar sources.

© European Southern Observatory (ESO) 2000

Online publication: January 18, 2000
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