Production and kinematics of CO in comet C/1995 O1 (Hale-Bopp) at large post-perihelion distances

Radio observations of different molecular species in comet C/1995 O1(Hale-Bopp) have been carried out regularly since August 1995. We present an analysis of carbon monoxide spectra observed at SEST. A detailed picture of the nature of the post-perihelion outgassing at large heliocentric distances is shown. The data starts at 2.9 AU, with active water sublimation, and stretch outside 10 AU, where CO-driven activity is still detected. The activity is studied using a coma model which separates the outgassing into two different sources, one insolation-dependent source, peaking at the subsolar point, and one isotropically outgassing source. Gas velocities and production rates of the two sources are derived for two different cases. In the first case the isotropic source is assumed to be inside the nucleus, and in the second case this source is assumed to be CO-rich material in the coma. Under both assumptions, the results indicate that the two component sources evolve independently. At heliocentric distances around 3 AU, acceleration of the CO molecules in the innermost region of the coma is required for the model to fit the observations. This acceleration must occur at a slower rate than that predicted by hydrodynamic simulations. From 6 to 10 AU, the derived parameters appear remarkably constant. The nuclear outgassing velocity is generally unchanged with heliocentric distance during this wide interval. Observations of molecules other than CO are included in the SEST dataset, and a comparison is made to the modelling results. HCN and CH 3OH show signs of being produced in the same way as the CO up to a heliocentric distance of 6 AU.