,
D. Bockelée-Morvan
,
P. Colom
,
J. Crovisier
,
D. Despois
,
L. Jorda
,
R. Moreno
,
G. Paubert
,
H. Rauer
Observatoire de Paris-Meudon
Observatoire de Bordeaux
IRAM (Granada) 
= 10.5, which is about 250 times
brighter than comet Halley at the same heliocentric
distance. Comet Hale-Bopp is a long-period comet, with a
period of about 3000 years. It will reach perihelion on
April 1, 1997 at 0.91 AU from the Sun and, although
predictions of cometary evolution are highly unreliable, it
might be at that time a very bright object, with possibly a
negative magnitude. We could then have an unprecedented
opportunity to investigate a very bright comet with modern
instrumentation.
Comet Hale-Bopp is already an interesting object worth of detailed studies. Its coma shows rapidly evolving dust jets. The reason for this activity cannot be sublimation of water --- which is the driver of cometary activity at small heliocentric distances --- because it is inefficient at this distance.
Thanks to the IRAM staff and to observers who accepted
last-minute changes to their observing schedule,
observations of comet Hale-Bopp could take place as early as
mid-August. We searched for volatile species in the coma.
The CO 
line was already marginally detected on Aug.
16 and 23. It was then confidently detected on Sept. 20 and
21 (Rauer et al. 1995, IAU Circ. No 6236) and
observed at several subsequent periods. The same line was
also independently detected at the JCMT (Matthews
et al. 1995, IAU Circ. No 6234).
This CO spectrum resembles the one observed in another
exceptional object, the short-period comet
P/Schwassmann-Wachmann 1, which harbours gaseous
activity although located beyond Jupiter on a nearly
circular orbit (Crovisier et al. 1995, Icarus
115, 213; also IRAM Newsletter No. 19). The line
is blue shifted by 0.4 km s
, which indicates
outgassing from the day-side of the nucleus. This
anisotropic outgassing can be related to the dust jets seen
in the visible images of the comet. Assumming a CO
rotational temperature of 10 K --- as was retrieved from
observations of the 
and 
lines in
P/Schwassmann-Wachmann 1 --- the CO production rate
ranges between 1.3 and 
molec. s
(on the order of one ton per second), with indications of
time variations, for our observations between Aug. 16 and
Oct 22, 1995.
Although important, this CO production may not be sufficient
to explain the huge dust production of this comet observed
in the visible. We also searched for other volatiles: HCN,
H
CO, CH
OH, which have sublimation temperatures
between those of CO and H
O. The search was negative,
with upper limits corresponding to a few percent the
production rate of CO.
An open question is whether the activity of comet Hale-Bopp is due to the sublimation of carbon monoxide ice, or triggered by the amorphous-to-crystalline water ice transition, an exothermic process which induces the release of trapped volatiles. This would have different implications on the physical state of cometary ices and on their formation scenarios.
We plan to monitor the CO production of this comet and to watch for the onset of sublimation of other species as the comet approaches the Sun. Very sensitive spectral observations could be undertaken at IRAM in one year if this comet becomes indeed very bright. They would allow us to investigate the composition of the cometary ices and to obtain clues to their outgassing processes.
Figure 6: The CO 
line observed in comet
Hale-Bopp, average of September 20-21, 1995.