Broadband infrared photometry of comet Hale-Bopp with ISOPHOT

Abstract

Comet Hale-Bopp was observed ve times with ISOPHOT, the photometer on board ESA's Infrared Space Observatory (ISO) between 4.6 and 2.8 AU. Each time, broadband photometry was performed using 4 dierent detectors, 5 apertures and 10 lters covering the range between 3.6 and 170 m. Background ob- servations were performed with identical instrument settings at the same positions on the sky several days after the comet observations. The observation strategy and the data reduction steps are described in some detail, including the techniques to correct for variable detector responsivity. The resulting inband power values of the Hale-Bopp observations and their uncertainties are given. The mean uncertainty is 25%. The nal fluxes were computed, taking into account the zodiacal background, possible oset of the comet's position from the center of the aperture, the brightness distribution within the coma, and the spectral energy distribution of the comet's emission. Strong thermal emission from a broad size distribution of dust particles was detected in all of the data sets, even at r =4 :6{4:9 AU pre-perihelion and 3.9 AU post-perihelion; the total thermal energy varied as r 3 . The 7.3{12.8 m color temperature was1.5 times the blackbody temperature, higher than that observed in any other comet. Silicate features at 10 and 25 m were prominent in all 5 data sets, the largest heliocentric distances that silicate emission has been detected in a comet. The presence of crystalline water ice grains is suggested from the 60 m excess emission at 4.6{4.9 AU, consistent with the observed QOH if the icy grains were slightly warmer than an equilibrium blackbody. The average albedo of the dust is higher than that of comet P/Halley, but lower than other albedo measurements for Hale-Bopp nearer perihelion. There is no evidence for a component of cold, bright icy grains enhancing the scattered light at 4.6 AU. Simple models for a mixture of silicate and absorbing grains were t to the ISO spectra and photometry at 2.8 AU. The observed flux at >100 mr equires a size distribution in which most of the mass is concentrated in large particles. Dust production rates of order 1:5 10 5 kg s 1 at 2.8 AU and 3 10 4 kg s 1 at 4.6 AU have been found. They correspond to dust to gas mass ratios of 6 to 10.