The Dust Environment of Comet 46P/Wirtanen at Perihelion: A Period of Decreasing Activity?

Abstract

The inverse dust tail model (M. Fulle 1989, Astron. Astrophys. 217, 283–297) is applied to a dust tail image of short-period Comet 46P/Wirtanen obtained by K. Jockers et al. (1998, Astron. Astrophys. 335, L56–L59) to provide information on the dust environment which will be probed by the ESA Rosetta space mission. The obtained results are consistent with those coming from dust coma photometry during 2 months before perihelion, so that we provide a dust environment model consistent with all available data concerning 46P/Wirtanen dust. Although the dust coma brightness was increasing up to perihelion, we show that most probably during the month preceding the comet perihelion the dust mass loss rate decreased from about 50 to 20 kg s −, for an assumed dust albedo of 4%. This is due to a probable time evolution of the dust size distribution power index γ, dropping from values close to −3 a month before perihelion, to values close to −4 at perihelion. These results point out how dangerous are arbitrary interpretations of the results of dust coma photometry, which, on the other hand, provide first-order quality constraints to test all dust environment models. These, however, remain indispensable to correctly interpret the collected data. The results of this paper are compared with those obtained by L. Colangeli et al. (1998, Icarus 134, 35–46) applying the same tail model to ISOCAM data, so that we obtain an upper limit of the dust albedo equal to 4% and we provide parametric expressions of the time evolution of the dust ejection velocity, mass loss rate, and size distribution over all the comet orbit section relevant for the Rosetta mission (i.e., from 2.5 to 1.0 AU inward). These expressions closely agree with those derived by M. Fulle et al. (1997a, Astron. Astrophys. Suppl. Ser. 126, 183–195) from the results of hydrodynamical models of the 46P/Wirtanen coma (J. F. Crifo and A. V. Rodionov 1997, Icarus 127, 319–353), but the perihelion decrease of the mass loss rate.