Long-term Monitoring of the Outgassing of Comet C/1995 O1 (Hale-Bopp) at Radio Wavelengths

C/1995 O1 (Hale-Bopp) has been observed on a regular basis since August 1995 with the Nançay, IRAM, JCMT, CSO and SEST radio telescopes. The lines of nine molecular species: OH, CO, HCN, CH_3OH, H_2CO, H_2S, CS, CH_3CN and HNC were monitored. Observations are still going on at SEST, and in July 1998, CO, HCN and CH_3OH were still detected at 5.9 AU from the Sun. The molecular production rates displayed different behaviours as a function of heliocentric distance (r_h). The change from a CO to a H_2O-driven coma was observed around 3 AU inbound. HNC, H_2CO and CS displayed much steeper evolution of their production rates with r_h than CO, HCN, CH_3OH and H_2S. The HNC/HCN, H_2CO/HCN and CS/HCN abundance ratios followed r_h(-2.5) , r_h(-2.1) and r_h(-0.7) respectively. This is somehow in contradiction with their respective volatilities, questionning the true abundance of these molecules in cometary nuclei. The kinetic temperature of the coma, estimated from the relative intensities of CH_3OH and CO lines, displayed a mean evolution as 100 r_h(-1.1) K. The mean expansion velocity of the gas, derived from the line shapes, also increased with decreasing r_h, from 0.5 km s(-1) at 6 AU to 1.05 km s(-1) at perihelion. Differences between pre- and post-perihelion suggest a slower decrease of the temperature and expansion velocity outbound, and smaller production rates of the minor species outbound beyond 3 AU. An analysis of the evolution of the CO lines Doppler shifts versus the position of the comet along its orbit coarsely constrains the direction of the rotation axis of the nucleus (alpha ~ 320(deg) , delta ~ -60(deg) ).