Long-term optical spectrophotometric monitoring of comet C/1995 O1 (Hale-Bopp)

We observed comet C/1995 O1 (Hale-Bopp) at 4.6–2.9 AU pre-perihelion and 2.8–12.8 AU post-perihelion with optical long-slit spectroscopy. Emission bands of CN, C3, C2 and NH2 have been covered. Emission of C3 was detected up to 7.0 AU, and CN could be followed up to 9.8 AU post-perihelion. Spatial column density profiles of the radicals have been used to derive effective parent Haser scale lengths for heliocentric distances beyond 3 AU. Production rates were derived based on these Haser scale lengths. The observations of CN are in agreement with HCN as the major parent molecule of this radical at large distances from the Sun (i.e. beyond ~3 AU). We compare the measured CN production rate to sublimation rates of HCN from a simple nucleus sublimation model. The variation of CN production rates with changing heliocentric distance gives no indication for sublimation from the interior and is consistent with very little thermal lag of the nucleus