A critical study of molecular photodissociation and CHON grain sources for cometary C-2

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 photo-dissociation 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 al 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 H2CO and CO (similar to 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 NH2) than r(2) which is found for C-2.