The IR spectrum of Hale-Bopp

Comet C/1995 O1 (Hale-Bopp) is a long period comet discovered in mid-1995 when it was 7 AU from the Sun. At that time it was already very active and 200 times brighter than comet Halley at the same distance, making it an exceptional comet. It was therefore selected as an ISO Target-of-Opportunity. It was first observed on 27 April 1996 with the PHOT-S spectrometer of ISOPHOT when it was at 4.6 AU from the Sun. The resolution in the 2.5-12 m region was about 85, that in the 6-12 m range was about 95. Integration time was of 4096 seconds, and a sky background observation was made at the same sky position some days later.

The data shows that the continuum in the 2.5-5 m region (see figure 1) is too low to be detected. An obvious feature in this spectrum is the CO₂ v₃ band. The observed intensity corresponds to a CO production rate of about 1.3 10²⁸ molecules s ^-1. Various other emission bands are expected in this 2.5-5 m region, such as the CO 4.67 m line, the v₃ (2.66 m) and v₂ (6.27 m) bands of water, the 3.2-3.6 m emission band from methanol and other unidentified species. These are not detected, but their upper limits agree with what is expected from this comet at such a distance from the sun. The ISO observations show that CO , CO and water are major volatiles that drive cometary activity at large heliocentric distances. This contrasts with the activity at small heliocentric distances, driven by water with much less CO and CO.

The 6-12 m spectrum (figure 2) shows thermal emission at a colour temperature of 162 K (6-8 m range) and a strong silicate band around 10 m, with a narrow feature at 11.2 m indicative of crystalline silicates. This feature has traditionally been interpreted as due to the presence of crystalline olivine grains. The absence of emission at 6.2, 7.7 and 8.6 m rules out PAHs as candidates for the 11.2 m emission. Olivine bearing submicrometric grains exhibit their main IR bands between 11.2 and 11.3 m , the exact location depending on composition, and Olivine also shows structure near 10.5 m similar to the comet spectrum. It is the leading candidate for these features.

Data reduction has started on further spectroscopic observations of comet Hale-Bopp made in September with PHOT-S, SWS and LWS. The comet was closer to the Sun and therefore more active.

Figure 1: The 2.5-5 m spectrum of Comet Hale-Bopp as observed by PHOT-S.

Figure 2: The 6-12 m spectrum of Comet Hale-Bopp as observed by PHOT-S.