Evolution of the outgassing of comet Hale-Bopp (C/1995 O1) from radio observations.

Nicolas Biver, Dominique Bockelée-Morvan, Pierre Colom, Jacques Crovisier, John K. Davies, William R.F. Dent, Didier Despois, Eric Gérard, Emmanuel Lellouch, Heike Rauer, Raphael Moreno, Gabriel Paubert
Abstract: Since soon after its discovery at 7 AU from the Sun, comet Hale-Bopp has been studied by radio spectroscopy to investigate the gas species associated with its activity. We present an extensive monitoring performed with the Nançay, IRAM-30m and JCMT radio telescopes, showing the progressive release of CO, CH OH, HCN, OH, H S, CS, H CO, CH CN and HNC as the comet approached the Sun from 6.9 to 1.4 AU. These data provide clues to the chemical and physical state of cometary ices and allow us to investigate the fractionation processes that accompany their sublimation upon solar heating. Simultaneous observations of the relative intensities and line shapes of several transitions are used to measure the expansion velocity and temperature of the cometary atmosphere and their evolution with heliocentric distance, as well as to study possible sublimation from icy grains. The more volatile species are relatively more abundant in the coma far from the Sun, but there is no direct correlation between overabundance and volatility. Evidences for H O sublimation from icy grains are seen beyond 3.5 AU from the Sun. The change from a CO-driven coma to a H O-driven coma occured at about 3 AU. The gas outflow velocity and temperature increased with decreasing heliocentric distance.

Published in Science, 275, 1915 (1997).