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Infrared spectroscopy of comets with ISO: what we learned on the composition of cometary dust

J. Crovisiera, * T.Y. Brookeb, K. Leech0, D. Bockelee-Morvana, E. Lelloucha, M.S. Hannerb, B. Altieric, H.U. Kellerd, T. Lime, T. Encrenaza, A. Salamac, M. Griffinf, T. de Graauw8, E. van Dishoeck'1 and R.F. Knacke1

aObservatoire de Paris-Meudon, F-92195 Meudon, France bJet Propulsion Laboratory, Pasadena, CA, USA

cIS0 Data Centre, Astrophysics Division of ESA, Villafranca, Spain dMPI fur Aeronomie, Katlenburg-Lindau, Germany eRutherford Appleton Laboratory, Didcot, Oxford, UK

fQueen Mary and Westfield College, Chilton, UK

gSRON, Groningen, The Netherlands hLeiden Observatory, The Netherlands

'Penn State University, Erie, PA, USA

The Infrared Space Observatory (ISO) offered us the opportunity to observe the spectrum of celestial bodies from 2.4 to 196 fim. This spectral range is of particular interest for the study of cometary dust. We report here on the ISO observations of comets C/1995 01 (Hale-Bopp) and 103P/Hartley 2, and discuss the nature of cometary dust.

1. COMET C/1995 Ol (HALE-BOPP)

The apparition of the long-period, exceptional comet C/1995 01 (Hale-Bopp) coincided with the ISO operation. The spectrum of this comet was observed as part of a target-of-opportunity programme by an international team. Observations took place pre-perihelion on April 1996 (rh = 4.6 AU) and September-October 1996 (rh ss 2.9 AU), and post-perihelion on December 1997 (rh = 3.9 AU) and April 1998 (rh = 4.9 AU) [1,24], Unfortunately, visibility constraints of the satellite precluded observations at smaller heliocentric distances.

High-resolution spectra with high signal-to-noise ratios in the thermal infrared region were obtained on September-October 1996 with the SWS instrument (Figure 1). These

'This work is based on observations with ISO, an ESA project with instruments funded by ESA Member States (especially the PI countries: France, Germany, the Netherlands and the United Kingdom) and with participation of ISAS and NASA.

C/1995 01 (Hale-Bopp) ISO/SWS 26 Sept. 1996

C/1995 01 (Hale-Bopp) ISO/SWS 26 Sept. 1996

Wavelength [/Am]

Figure 1. C/1995 01 (Hale-Bopp) on 26 September 1996 at rh = 2.9 AU. The 6 to 45 pm region of the SWS spectrum is represented here degraded to a resolution 5/6X = 500 (full dark line). The instrument apertures range from 14"x20" (short wavelengths) to 20"x33" (long wavelengths). The intensity scale has been normalized to a 14"x20" aperture assuming a l/p brightness distribution law and slight scale adjustments have been applied to make the spectrum continuous. As explained in [5], the observed spectrum has been decomposed into several emission components: blackbody 1 and 2, forsterite, ortho-pyroxene and amorphous silicates.

spectra show broad emission features superimposed over continuum thermal emission: the strongest appear at «¿10, ~19.5, 23.5 and 33.5 pm, and weaker ones are present at 11.9, 16.25, 20.8, 21.6, 26.1 and 28.1 pm. The wavelengths of all these peaks correspond to crystalline, Mg-rich olivine (forsterite). A careful study of the spectra shows also signs of the presence of crystalline pyroxenes and amorphous silicates [1,5,2-4]. The contributions of these various components have been quantitatively evaluated [5] (see also [6-9]). There is no indication of change in the dust composition with heliocentric distance, or before and after perihelion.

Crystalline water ice is detected in spectra observed at 2.8 AU pre-perihelion, from emission features at 45 and 65 pm and possibly an absorption feature at 3.1 pm [10]. This suggests that grains of water ices are still present at this distance from the Sun, and that they could contribute significantly to the release of water in the coma. No sign of PAHs could be found in the spectrum of comet Hale-Bopp (especially in the 6-9 /zm region, where they are expected to have strong bands not blended with silicate features, and where they are prominent in circumstellar dust discs).

2. COMET 103P/HARTLEY 2

The short-period comet 103P/Hartley 2 was observed in December 1997 to January 1998, close to its perihelion at 1 AU, as part of a guaranteed time programme [5,11] (see also [12]). Whereas Hale-Bopp was a long-period comet presumably coming from the Oort cloud, 103P/Hartley 2 is a Jupiter-family comet that supposedly originated from the Edgeworth-Kuiper belt. A low-resolution 5.5-17 /zm spectrum obtained with the CVF of the ISOCAM instrument reveals a silicate feature much less intense relative to continuum than in comet Hale-Bopp. The presence of crystalline silicates is also tentatively indicated from the shape of its spectrum in the 8-13 fim region.

Cometary dust has thus striking similarities (composition, crystallinity) with Solar System interplanetary dust and circumstellar dust observed in both evolved and new stars [13,14]. The differences between comet Hale-Bopp and comet P/Hartley 2 may be ascribed to their belonging to the 'dust-rich' and 'dust-poor' comet classes, respectively: the dust-rich comet Hale-Bopp has very small, super-heated dust grains, whose silicate band emission is enhanced.

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