First results on the evaluation of Haser scale lengths in comet Hale-Bopp (C/1995 O1) at R >= 3 AU

Due to its exceptional brightness, comet Hale-Bopp offered the possibility for unprecedented long-term monitoring. Medium resolution long-slit spectroscopic data and images were obtained using the Danish Faint Object Spectrograph (DFOSC) at the 1.54m Danish telescope and the Boller and Chivens spectrograph at the 1.52m ESO telescope, La Silla, Chile. Pre-perihelion, comet Hale-Bopp was monitored from 4.6 AU to 2.9 AU and post-perihelion, the monitoring yields data from 3 AU up to a heliocentric distance of 5 AU. Production rates are often derived by using a simple Haser model to approximate the coma density distribution. This requires knowledge of the destruction scale lengths of the observed daughter species and its parents. As the brightness of most comets diminishes quickly with heliocentric distance, scale lengths have been determined only in comets around 1 AU. Due to the lack of data for large heliocentric distances, production rates in distant comets can only be derived by extrapolating these scale lengths. In this preliminary analysis of our data we evaluate the scale-lengths of CN, C_3 C_2, and NH_2 at heliocentric distances greater than 3 AU post-perihelion. The differences of production rates derived by using these directly determined and the extrapolated scale lengths are discussed