ISOCAM IMAGING OF COMETS 65P GUNN AND 46P/WIRTANEN/

In this paper we present images of Comets 65P/Gunn and 46P/Wirtanen ob tained by means of ISOCAM in the broadband filters at 9.62 (LW7) and 1 5.00 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 t he coma and tail dust environment of the targets. The application of t ail models to the comet images allowed us to derive several interestin g parameters about the dust ejection and dynamics. The tail of 46P/Wir tanen is symmetric with respect to the prolonged radius vector and a s trong 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 diff erential size distribution (between -3.5 and -4), and the mass loss ra te (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-t o-gas ratio is close to one, within a heliocentric distance from 2.5 t o 2.0 AU. The dust tail of 65P/ Gunn is strongly asymmetric. The fitti ng 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 differenti al size distribution (around a value of -4), and the dust mass loss ra te (between 100 and 300 kg s(-1)) have been determined. Dust mass loss rates determined by IR tail models depend on the scattering efficienc y and the temperature of grains, but are independent of poorly known p arameters, such as dust bulk density and albedo. (C) 1998 Academic Pre ss