Long-term Evolution of the Outgassing of C/1995 O1 (Hale-Bopp) from Radio Observations

N. Biver, D. Bockelée-Morvan, P. Colom, J. Crovisier, E. Gérard, E. Lellouch, D. Gautier (Obs. Paris), J.K. Davies, W.R.F. Dent (JAC, Hawaii), R. Moreno, G. Paubert, J. Wink (IRAM), D. Despois (Obs. Bordeaux), D.C. Lis, D. Mehringer, D. Benford, M. Gardner, T.G. Phillips (Caltech), H. Rauer (DLR, Berlin)

C/1995 O1 (Hale-Bopp) has been observed on a regular basis since August 1995 with the Nançay, IRAM, JCMT and CSO radio telescopes. We observed the onset of outgassing of nine molecular species (OH, CO, HCN, CH tex2html_wrap_inline13 OH, H tex2html_wrap_inline15 CO, H tex2html_wrap_inline15 S, CS, CH tex2html_wrap_inline13 CN, HNC) and monitored their production rates as a function of heliocentric distance ( tex2html_wrap_inline21 ). As comet Hale-Bopp approached the Sun, these species displayed different behaviours. Far from the Sun, the most volatile species were found in general relatively more abundant in the coma, but there was no precise correlation between the overabundances and the equilibrium sublimation temperatures of the species. The change from a CO-driven coma to an H tex2html_wrap_inline15 O-driven coma occured at about 3 AU. The HNC/HCN abundance ratio increased with decreasing tex2html_wrap_inline21 , questioning its true value in cometary nuclei. At perihelion, the relative production rates approached those found in other comets near 1 AU.

The expansion velocity of the gaseous species, derived from the line shapes, increased with decreasing tex2html_wrap_inline21 with a law close to tex2html_wrap_inline29 . The kinetic temperature of the coma, estimated from the relative intensities of the CH tex2html_wrap_inline13 OH and CO lines, also increased, from about 10 K at 7 AU to 120 K at perihelion. The different spectral shifts observed for the OH and CO lines at tex2html_wrap_inline21 > 3.5 AU are taken as evidence that H tex2html_wrap_inline15 O was then mostly sublimating from icy grains, in contrast to the more volatile CO species, which was mainly outgassed from the nucleus.