Broadband infrared photometry of comet Hale-Bopp with ISOPHOT

Comet Hale-Bopp was observed five times with ISOPHOT, the photometer on boa rd ESA's Infrared Space Observatory (ISO) between 4.6 and 2.8 AU. Each time , broadband photometry was performed using 4 different detectors, 5 apertur es and 10 filters covering the range between 3.6 and 170 mum. Background ob servations were performed with identical instrument settings at the same po sitions on the sky several days after the comet observations. The observati on strategy and the data reduction steps are described in some detail, incl uding the techniques to correct for variable detector responsivity. The res ulting inband power values of the Hale-Bopp observations and their uncertai nties are given. The mean uncertainty is 25%. The final fluxes were compute d, taking into account the zodiacal background, possible offset of the come t's position from the center of the aperture, the brightness distribution w ithin the coma, and the spectral energy distribution of the comet's emissio n. 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-perihelio n and 3.9 AU post-perihelion; the total thermal energy varied as r(-3). The 7.3-12.8 mum color temperature was similar to1.5 times the blackbody tempe rature, higher than that observed in any other comet. Silicate features at 10 and 25 mum were prominent in all 5 data sets, the largest heliocentric d istances that silicate emission has been detected in a comet. The presence of crystalline water ice grains is suggested from the 60 mum excess emissio n at 4.6-4.9 AU, consistent with the observed Q(OH) if the icy grains were slightly warmer than an equilibrium blackbody. The average albedo of the du st is higher than that of comet P/Halley, but lower than other albedo measu rements for Hale-Bopp nearer perihelion. There is no evidence for a compone nt of cold, bright icy grains enhancing the scattered light at 4.6 AU. Simp le models for a mixture of silicate and absorbing grains were fit to the IS O spectra and photometry at 2.8 AU. The observed flux at lambda >100 mum re quires a size distribution in which most of the mass is concentrated in lar ge particles. Dust production rates of order 1.5 x 10(5) kg s(-1) at 2.8 AU and 3 x 10(4) kg s(-1) at 4.6 AU have been found. They correspond to dust to gas mass ratios of 6 to 10.