ISOCAM imaging of comets 65P/Gunn and 46P/Wirtanen

In this paper we present images of Comets 65P/Gunn and 46P/Wirtanen obtained by means of ISOCAM in the broadband filters at 9.62 (LW7) and 15.00 mu m (LW3). The observations were performed on 23 March 1996 and on 9 November 1996, respectively. The aim of this work is to analyze the coma and tail dust environment of the targets. The application of tail models to the comet images allowed us to derive several interesting parameters about the dust ejection and dynamics. The tail of 46P/Wirtanen is symmetric with respect to the prolonged radius vector and a strong anisotropy characterizes the dust emission (most probable value of the power index describing the size dependence of the dust ejection velocity u > -1/2). Estimates of the dust ejection velocity (from 14 +/- 3 m s(-1) at a heliocentric distance R = 2.5 AU to 23 +/- 2 m s(-1) at R = 2.0 AU, for 0.1-mm-sized grains), the power index of the differential size distribution (between -3.5 and -4), and the mass loss rate (from 1.5 +/- 0.5 kg s(-1) at R = 2.5 AU to 2 +/- 1 kg s(-1) at R = 2.0 AU) have been obtained. We conclude that the most probable dust-to-gas ratio is close to one, within a heliocentric distance from 2.5 to 2.0 AU. The dust tail of 65P/ Gunn is strongly asymmetric. The fitting process indicates a dust ejection cone axis defined by the argument Phi = 110 degrees and the obliquity I = 80 degrees. The dust ejection velocity (from 16 +/- 1 m s(-1) at R = 2.9 AU to 24 +/- 2 m s(-1) at R = 2.6 AU for 0.1-mm-sized grains), the power index of the differential size distribution (around a value of -4), and the dust mass loss rate (between 100 and 300 kg s(-1)) have been determined. Dust mass loss rates determined by IR tail models depend on the scattering efficiency and the temperature of grains, but are independent of poorly known parameters, such as dust bulk density and albedo. (C) 1998 Academic Press