Neutral cometary atmospheres. V - C2 and CN in comets

Abstract

Brightness profiles of C2 in comets Bennett 1970 II and Kohoutek 1973 XII are presented. Model analysis of these profiles yields radial Haser scale lengths for production and destruction of C2 which, when combined with other scale length determinations in the literature, are shown to vary as the square of the heliocentric distance. This is consistent with photochemical production and destruction. Also presented is an updated compilation of CN scale lengths, which shows that the mean parent scale length law varies as r+15. A rea- nalysis of published cometary photometry, using the new scale length laws, yields a C2 to CN ratio which is independent of heliocentric distance. The previously documented drop-off in C2 production rate relative to other neutral species for heliocentric distances larger than ~1.5 AU was a simple artifact of the previously assumed scale length variations. Analysis of the sunward-tailward distortion of the brightness profiles with a Monte Carlo particle-trajectory model shows that C2 is released from its parent molecule with an ejection speed of ~0.5 km s_1, owing to the excess photolysis energy. This result also implies that the photochemical lifetimes for the C2 parent and C2 respectively are 3.1 x 104 s and 1.2 x 105 s at 1 AU. Subject