2. Spectroscopic observations
P/Gehrels 3 has been observed at Mauna Kea Observatory, Hawaii, on June 20th, 1996 using the 3.6 m Canadian-French-Hawaiian telescope equipped with the MOS (MultiObject Spectrograph) and the CCD STIS2 (2048x2048 pixels). The grism used is the V150 with a dispersion of 433 Å/mm. The spectral range covered is about 400 nm 980 nm with a spectral dispersion of 7 Å/pixel. A spectral resolution of about 30 Å has been obtained with a slit aperture of 1.7 arcsec. In this wavelength range both emission and absorption features can be seen. This range includes some of the most diagnostic features needed for the spectral classification: the 0.9 µm absorption feature, which is critical for the interpretation of the surface reflectance spectra and the C2 emission bands.
The observations of comet nuclei are very difficult: when the comet is active, the nucleus is obscured by the coma and dust emissions. Contamination can be removed only with careful modelling and the spectrum of the bare nucleus can be obtained. Luu (1993), using this technique, observed 5 comets at large distance, finding that the optical spectra are very different from each other, ranging from blue, as for Chiron, to red, as for Tempel 2. This variety of slopes was also found in the Trojan population (Jewitt & Luu 1990).
The observations have been performed when the comet had passed the perihelion, at 4.5 AU from the Sun and 3.5 AU from the Earth; the visual magnitude was of 21.8 and the airmass was 1.5. Several solar analog spectra have been secured during the night and the reflectivity of the comet, shown in Fig. 1, results from the division by the spectrum of 16 Cygnus B. The spectrum is normalized at 5500 Å. No sign of activity has been detected in the comet spectrum. The spike near 0.76 micron is due to a not perfect removal of the atmospheric telluric absorption.
The optical spectrum shows a featureless, red continuum with a reflectivity gradient S' equal to 13.30.1%/ Å in the wavelength range 550 and 800 nm. This value is statistically consistent with the mean slope of the optical spectra of comet nuclei, S' = 145%/ Å (Jewitt & Luu 1990; Fitzsimmons et al. 1994), with that of Trojans, S' = 104%/ Å (Jewitt & Luu 1990), D-type asteroids, S' = 13.51.0%/ Å (Lazzarin et al. 1995), and some Centaurs (Barucci et al. 1999). The value of the reflectivity gradient S' suggests a similarity of the P/Gehrels 3 surface composition with that of Trojans and dark asteroids. The similar red colors and low albedos may be evidence for common organic compounds (possibly mixed with a limited amount of ice) on the surface of the two populations (dark asteroids and comets), but we must remember the strong influence of the grains size on the spectra and also on the influence of dark material on the spectral reflectance. Considerable reduction of reflectivity of pure ice is achieved even with very small amounts of inclusions: the pure snow reflectivity which, in the visible, is approximately 0.9, can be reduced to less than 0.3 with a dust to ice mass ratio of only (Warren & Wiscombe 1980). Very small amount of grains can reduce the reflectance spectra on an ice surface (Clark 1982). The presence of carbonaceous material mixed with water ice, even in low percentages ( 20 percent), will mask the ice spectral features and reduce considerably its albedo (Clark & Lucey 1984). The presence of organic material on the surface of comet nucleus can mask the absorption features of silicates in the visible and in the near IR.
© European Southern Observatory (ESO) 2000
Online publication: February 25, 2000