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).
