The composition of ices in comet C/1995 O1 (Hale-Bopp) from radio spectroscopy

Further results and upper limits on undetected species

¹ Observatoire de Paris, 92195 Meudon, France
² Observatoire de Bordeaux, BP 89, 33270 Floirac, France
³ California Institute of Technology, MS 320-47, Pasadena, CA 91125, USA

Corresponding author: J. Crovisier, jacques.crovisier@obspm.fr

Received: 14 November 2003
Accepted: 26 January 2004

Abstract

From radio spectroscopic observations of comets, more than 22 molecules, radicals and ions, plus several isotopologues, were detected, the majority of them being recently revealed in comets C/1996 B2 (Hyakutake) and C/1995 O1 (Hale-Bopp). Among them, 6 molecules were detected for the first time (Bockelée-Morvan et al. [CITE]) in the course of a spectral survey conducted at radio wavelengths in comet Hale-Bopp with the CSO, the IRAM 30-m telescope and Plateau de Bure interferometer. In addition, many species were searched for unsuccessfully, some of them with stringent upper limits. We present here a review of these observations and further analysis of their results. This include: (i) confirmed detection of acetaldehyde (CH₃CHO); (ii) limits on small molecules such as ketene (H₂CCO) or methanimine (CH₂NH); (iii) limits on the abundance ratios in homologous series such as HC₅N/HC₃N, ethanol/methanol, acetic acid/formic acid; (iv) searches for precursors of key cometary species such as atomic Na and HNC; (v) constraints on more exotic species ranging from water dimer (H₂O)₂ to glycine; (vi) detection of the HS isotopic species and independent observations of HDO and DCN; (vii) limits on several other deuterated species; (viii) limits on several radicals and ions and a tentative detection of the C₂H radical; (ix) the presence of unidentified lines. Typical abundance upper limits of 2–5 relative to water are achieved for many species. Better upper limits are obtained for some linear molecules with high dipole moments. But more complex molecules such as dimethyl ether or glycine are poorly constrained. These results should give important clues to the chemical composition of cometary ices, to the formation mechanisms of cometary material, and to the chemical processes which occur in the inner coma.