TOMOGRAPHY OF A SUNWARD STRUCTURE IN THE DUST TAIL OF COMET 19P BORRELLY/

The observed orientations of a sunward structure (SS) in the dust tail of comet 19P/Borrelly, as it appears in 20 images taken under differe nt projection conditions over a period of 142 days, are compared with the theoretical orientations computed on the basis of two different mo dels: the Spin Model, in which the SS is assumed to be a linear dust j et from an active source close to the cometary pole, pointing about pa rallel to the nucleus spin axis preceding in the space around a preces sion axis throughout the relevant time; the Burst Model, in which the observed shapes and orientations of the SS are assumed to be the resul t of the keplerian motion of dust particles ejected from an active spo t located somewhere on the sun-faced nucleus hemisphere during a burst occurred some time before the observations. The best fit of the posit ion angles of the SS linear axis provides two alternative scenarios: t he most data consistent Spin Model is characterized by a spin axis pre ceding from the starting obliquity I-i = 90 degrees and argument Phi(i ) = 110 degrees to the final I-f = 120 degrees and Phi(f) = 80 degrees around the precession axis of I-p = 74 degrees and Phi(p) = 62 degree s with a precession period of about 2.5 years; the most data consisten t Burst Model is characterized by a burst from an active spot located at a cometographic latitude and longitude of -10 degrees and 36 degree s, respectively, occurred about 150 days before perihelion (correspond ing to a sun distance of 2.1 AU) with a dust ejection velocity of abou t 10 m s(-1).