Thermophysical simulations of comet Hale-Bopp

Abstract

Aims. In this work, we simulate the global behavior of comet Hale-Bopp with our thermophysical model starting with simple, homogeneous conditions, so that dust mantling and the active area develop consistently depending on the properties of the simulated nucleus. We aim to obtain a range of compatibility between our model and the observations, that can be used as constraints on some of the characteristics of cometary nuclei. Methods. Our thermophysical model includes crystallization (and release of trapped CO), sublimation/recondensation, heat and gas transport through the nucleus, and dragged dust release. We run a battery of simulations with different parameter sets selected according to our current knowledge of comets and compare our results with observational data. Initial calculations are performed for a comet radius R0 = 30 km. To match the calculated integrated H2O production to the observed rate, we renormalize to a new R ,w hich must be within 20 and 40 km, that is a range compatible with several estimates. Further selection is performed comparing the simulated water and carbon monoxide production rate profiles with the observational profiles and checking that the observational upper/lower limits of the H2O production are fulfilled. Results. We have found a reasonable agreement between our model and the data for H2O and CO production rates, without the need of distributed sources, for the following initial conditions: the nucleus is composed of water, carbon monoxide, and dust with a moderate dust proportion, tending to be icy, with a dust-to-ice ratio of between 0.5 and 1. The water ice must be initially amorphous with 15 to 20% of trapped carbon monoxide. The icy matrix has a thermal inertia between 100 and 200 J m −2 Ks −1/2 , considering the initial composition with crystalline ice at 140 K. The dust follows an exponential size distribution with particles from 0.1 μ mt o 1m m and leaves the comet dragged by the expelled vapor with a dragging efficiency (dust-to-gas ratio) of 3.