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 different projection conditions over a period of 142 days, are compared with the theoretical orientations computed on the basis of two different models: the Spin Model, in which the SS is assumed to be a linear dust jet from an active source close to the cometary pole, pointing about parallel to the nucleus spin axis preceding in the space around a precession axis throughout the relevant time; the Burst Model, in which the observed shapes and orientations of the SS are assumed to be the result of the keplerian motion of dust particles ejected from an active spot located somewhere on the sun-faced nucleus hemisphere during a burst occurred some time before the observations. The best fit of the position angles of the SS linear axis provides two alternative scenarios: the most data consistent Spin Model is characterized by a spin axis preceding from the starting obliquity Ii = 90 o and argument Φi = 110 o to the final If = 120 o andΦf = 80 o around the precession axis of Ip = 74 o and Φp = 62 o with a precession period of about 2.5 years; the most data consistent Burst Model is characterized by a burst from an active spot located at a cometographic latitude and longitude of −10 and 36, respectively, occurred about 150 days before perihelion (corresponding to a sun distance of 2.1 AU) with a dust ejection velocity of about 10 m s.