The Infrared Activity of Comet P/Halley 1986 III at Heliocentric Distances from 0.6 to 5.92 AU

Abstract

We present an analysis of the combined infrared data for comet 1P/Halley 1986 III acquired by Gehrz and Ney, Hanner and coworkers, Tokunaga and coworkers, Green and coworkers, Ryan and Campins, and Bregman and coworkers. This database, the largest single body of infrared photometric data for any comet, spans a wavelength range from 0.7 to 23??m and describes the activity of P/Halley at heliocentric distances from 0.6 to 5.92 AU. Coverage of the thermal infrared spectrum was obtained for heliocentric distances from 0.6 to 2.8 AU. The quantitative corrections and calibration procedures required to intercompare the individual data sets are described. Long-term trends in the heliocentric dependence of P/Halley's grain color temperature, silicate emission optical strength, average grain albedo, apparent luminosity, and infrared monochromatic fluxes are discussed. The normalized thermal infrared flux varied as r-2.2 preperihelion and r-2.1 postperihelion in accord with the slope of the water production rate, indicating a general correlation between dust and gas production. Large-amplitude short-term (hours) fluctuations in all the observable quantities were seen in small apertures; these changes correlated with gas production rates and the presence of jets. Changes in JHK colors and the incomplete correlation between superheat and silicate feature strength imply variability in the grain composition, as well as in the size distribution. Based on a comparison of infrared data, comet Hale-Bopp (C/1995 O1) ejected a much higher relative abundance of submicron grains than Halley, and this pattern persisted to larger heliocentric distances.