The nucleus of comet C/1983 H1 IRAS-Araki-Alcock

We present a synthetic analysis of all available infrared (2-20 μm) and radio (1.3-6.1 cm) observations of comet C/1983 H1 IRAS-Araki-Alcock performed during its close approach to Earth in May 1983. We implement a model based on a spherical nucleus with a macroscopic mosaic of small and numerous active and inactive regions, and take into account the strong phase effect in the calculations of the thermal flux (often neglected in past interpretations). The orientation of the spin axis is assumed to be that determined by Sekanina [1988. Astron. J. 95, 1876-1894]. Additional constraints coming from visible photometry, measurements of the water production rate and the temporal variations of the cometary activity are introduced. We derive an equivalent nucleus radius of 3.4±0.5 km, consistent with a geometric albedo of 0.04 ±0.01 and a phase coefficient β=0.04mag deg−1 in the visible, and an active fraction of 2.9 ±1.9%. Although the nucleus is probably elongated as found in the past (Sekanina, 1988), we show that the relevant measurements were likely contaminated by the contribution of a variable coma.