A Critical Study of Molecular Photodissociation and CHON Grain Sources for Cometary C₂

The spatial distribution of C 2 in the very inner comae of comets (<10 4 km) often shows a profile which is flatter than can be reproduced by standard two-generation models. Because of this it has been suspected that a source other than the typical two-generation photodissociation is responsible. Previous investigators have suggested that either grains, most likely CHON grains, or a three-generation photodissociation may the source for cometary C 2 . We have performed an analysis of a set of spatial profiles of C 2 in comet P/Halley to investigate this question. A three-generation dissociation model that unlike previous attempts can include the ejection velocity of daughter fragments at each dissociation is compared with published results of a purely radial three-generation model. We find that the addition of ejection velocities has a similar effect on three-generation models as has been shown already in two-generations by the vectoral and Monte Carlo models. Specifically, the addition of typical ejection velocities which are usually on the order of 1 km s -1 for a heavy species essentially fills in the "hole" in the inner coma which would otherwise be left by three-generation models that are purely radial, yielding profiles that are not easily distinguishable from two-generation model (Haser, vectorial, or Monte Carlo). We have also constructed a CHON Grain Halo (CGH) model that also produces flat inner profiles. The inferred size of the CGH is 88,000 km at 1 AU. This is consistent with some estimates of the depletion of small grains in comet Halley but is much larger than the extended source for H 2 CO and CO (~10 4 km). The strict r 2 variation for the heliocentric distance dependence for the C 2 source is better explained by a direct CGH source. A combination of either grandparent-parent molecule scale lengths or grandparent CHON grains and an intermediate molecule would yield parent scale length variations closer to r 1.5 (more like CN and NH 2 ) than r 2 which is found for C 2 .