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, HCO, CHOH, which have sublimation temperatures between those of CO and HO. 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.