The spectrum of comet Hale-Bopp as seen by ISO

Jacques Crovisier ¹, K. Leech ², D. Bockelée-Morvan ¹, E. Lellouch ¹, T.Y. Brooke ³, M.S. Hanner ³, B. Altieri ², H.U. Keller ⁴, & T. Lim ^2,5

¹ Observatoire de Paris, Meudon, France

² ISO Science Operations Centre, Astrophysics Division of ESA, Villafranca, Spain

³ Jet Propulsion Laboratory, Pasadena, USA

⁴ MPI für Aeronomie, Katlenburg-Lindau, Germany

⁵ Queen Mary and Westfield College, London, UK

Comet C/1995 O1 (Hale-Bopp) was observed with the Infrared Space Observatory within the target-of-opportunity comet programme. Spectra were obtained pre-perihelion on April 1996 and September-October 1996, and post-perihelion on December 1997 and April 1998, spanning heliocentric distances between 2.8 and 4.9 AU. At the largest heliocentric distances, only PHT-S spectra were obtained. At the smallest r's, a high-resolution spectrum covering the full 2.4 to 195 m spectral range was obtained for the first time in a comet, using SWS and LWS. Spectra of dedicated regions (H₂O, CO and CO₂ bands) were also observed with SWS. The vibrational bands of water, carbon dioxide and carbon monoxide are detected in emission, as expected from molecular fluorescence models. Relative production rates for H₂O:CO₂:CO vary, as a function of heliocentric distance, as could be expected from the different volatilities of these species. H₂O was observed at high spectral resolution in the group of bands around 2.7 m, the group around 6 m with SWS, and in several rotational lines with LWS. The ro-vibrational lines of the band were observed with a high S/N in September-October 1996. This allows an accurate determination of the water rotational temperature (28 K) and of its ortho-to-para ratio ( , which significantly differs from the high temperature limit and corresponds to a spin temperature of 25 K). Longward of 6 m, the spectrum is dominated by dust thermal continuum emission, upon which broad emission features are superimposed whose wavelengths correspond to those of Mg-rich crystalline olivine (forsterite). In the September-October spectra, emission features at 45 and 65 m and possible absorption at 2.9-3.2 m suggest that grains of water ice were present at 3 AU from the Sun.