The gaseous coma of Hale-Bopp was observed at the IRAM Plateau de Bure Interferometer in October and November 1996 and from March 10 to March 20 1997. The spatial resolution was 1.5'' at 1mm and 3.5'' at 3 mm, corresponding to about 1500 and 3500 km on the comet in March 1997. Emission lines of HCN, HNC, CO, H CO, CH OH, CS and H S were successfully detected in cross-correlation mode. High quality maps were obtained for these species. SO was not detected in interferometry. In addition SO (Wink et al. Circ. 6591, 1997) and HCOOH ( Wink et al. Circ. 6599, 1997) were successfully detected in auto-correlation mode, providing the first identification of these species in cometary atmospheres.
These interferometric observations have several goals. First, the spatial distribution of CO (fig. 1, H CO and HNC should put strong constraints on their origin. In situ observations aboard the Giotto spacecraft have shown that a large fraction of CO and H CO is released in the coma rather than from the sublimation of the nucleus. This distributed source could be organic grains or complex species, as polymers. The observations at Plateau de Bure should be able to distinguish the fraction of CO and H CO coming from the nucleus and constrain the size of the distributed sources. The abundance of the very unstable species HNC in cometary nuclei is another important issue, since it should be an important diagnostic on the conditions of formation of cometary material. The non-detection of SO in interferometric mode brought the proof that SO is mainly a daughter species, i.e. a photodissociation product (Wink et al. IAU Circ. 6591, 1997). Detailed modelling is needed to establish whether SO (Wink et al. IAU Circ. 6591, 1997) is its main parent. The observations will also provide a 3-D reconstruction of the morphology of the gaseous coma, the high spectral resolution giving the informations along the line of sight. Several lines of CH OH, CO and CS were observed to study the evolution of the excitation temperature in the coma.
The abundance of SO and HCOOH in the nucleus of comet Hale-Bopp will allow to better understand the links between cometary and interstellar material and the origin of comets.
Figure 1: Map of the J(2-1) transition of CO at 230 GHz obtained on March 11, 1997. The spatial resolution is 1.5''.
See also the Comet page on IRAM's Web page
Editor's Note: The continuum emission from the nucleus has also been detected with the Plateau de Bure Interferometer by Altenhof, Wink et al. (IAU telegram 6587), allowing an estimate of the size of the nucleus (diameter km).