Interferometric imaging of the sulfur - bearing molecules H 2 S , SO and CS in comet C / 1995 O 1 ( Hale - Bopp ) ⋆

Context. We present observations of rotational lines of H 2 S, SO and CS performed in comet C/1995 O1 (Hale-Bopp) in March 1997 with the Plateau de Bure interferometer of Institut de Radioastronomie Millimétrique (IRAM). Aims. The observations provide informations on the spatial and velocity distributions of these molecules. They can be used to constrain their photodissociation rate and their origin in the coma, i.e. nucleus or parent source. Methods. We use a radiative transfer code which allows us to compute synthetic line profiles and interferometric maps, to be compared to the observations. Results. Both single-dish spectra and interferometric spectral maps show evidence for a day/night asymmetry in the outgassing. From the analysis of the spectral maps, including the astrometry, we show that SO and CS present in addition a jet-like structure that may be the gaseous counterpart of the dust high-latitude jet observed in optical images. A CS rotating jet is also observed. Using the astrometry provided by continuum radio maps obtained in parallel at IRAM, we conclude that there is no need to invoke the existence of nongravitational forces acting on this comet, and provide an updated orbit. The radial extension of H 2 S is found to be consistent with direct release of H 2 S from the nucleus. SO displays an extended radial distribution. Assuming that SO 2 is the parent of SO, the photodissociation rate of SO is measured to be 1.5 × 10 −4 s −1 at 1 AU from the Sun. This is lower than most laboratory-based estimates and may suggest that SO is not solely produced by SO 2 photolysis. From the observations of J(2–1) and J(5–4) CS lines, we deduce a CS photodissociation rate of 1 − 5 × 10 −5 s −1. The photodissociation rate of CS 2 , the likely parent of CS, cannot be constrained due to insufficient angular resolution, but our data are consistent with published values. Conclusions. These observations illustrate the cometary science that will be performed with the future ALMA interferometer.