THE INFRARED SPECTRUM OF COMET HELE–BOPP AS SEEN BY THE INFRARED SPACE OBSERVATORY


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Abstract

Spectra of comet C/1995 O1 (Hale–Bopp)) were obtained with the Infrared Space Observatory (ISO)) at medium resolution with the grating spectrometer in the photometer (PHT-S)) and/or at high resolution with the short wavelength spectrometer (SWS)) and long wavelength spectrometer (LWS)) in April 1996 (Crovisier et al., 1996)), September–October 1996 (Crovisier et al., 1997a, b)) and December 1997, at distances from the Sun of 4.6, 2.9 and 3.9 AU, respectively. For the first time, high-resolution spectra of a comet covering the entire 2.4 to 200 μm spectral range were obtained.The vibrational bands of H2O, CO2and CO are detected in emission with PHT-S. Relative production rates of 100:22:70 are derived for H2O : CO2 : CO at 3 AU pre-perihelion. H2O is observed at high spectral resolution in the ν3group of bands around 2.7 μm and the ν2group around 6 μm with SWS, and in several rotational lines in the 100–180 μm region with LWS. The high signal-to-noise ratio of the ν3band observed on September–October 1996 allows accurate determinations of the water rotational temperature (28 K)) and of its ortho-to-para ratio (2.45 ± 0.10, which significantly differs from the high temperature limit and corresponds to a spin temperature of 25 K)).Longward of 6 μm the spectrum is dominated by dust thermal continuum emission, upon which broad emission features are superimposed. The wavelengths of the emission peaks correspond to those of Mg-rich crystalline olivine (forsterite)). In the September–October 1996 spectra, emission features at 45 and 65 μm and possible absorption at 2.9–3.2 μm suggest that grains of water ice were present at 3 AU from the Sun.The observations made post-perihelion in late December 1997 led to the detections of H2O, CO2and CO at 3.9 AU from the Sun (Figures 1 and 2)). The production rates were ≈3.0 × 1028, 3.5 × 1028and ≈1.5 × 1029s-1, respectively. This corresponds to H2O : CO2 : CO = 100 : 110 : 500 and confirms that at such distances from the Sun, cometary activity is dominated by sublimation of CO and CO2rather than by H2O.

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