THE OH DISTRIBUTION IN COMETARY ATMOSPHERES - A COLLISIONAL MONTE-CARLO MODEL FOR HEAVY SPECIES

An extension of the cometary atmosphere Monte Carlo particle trajectory model formalism has been made which makes it both physically correct for heavy species and yet still computationally reasonable. The derivation accounts for the collision path and scattering redirection of a heavy radical (i.e., a radical with mass comparable to H2O) traveling through a fluid coma with a given radial distribution in outflow speed and temperature. A modification of the standard '' rejection method '' of choosing an appropriate target molecule for scattering is presented for the relative flux calculation. The revised model verifies that the earlier fast-H atom approximations used in earlier work are valid, while at the same time it is applied to a case where the heavy radical formalism is necessary: the OH distribution. The OH distribution in comets has been studied in the context of this revised model, and analysis of the IUE observations of OH in comet Halley has also been compared with the standard vectorial model. The kinetic distribution of OH in the coma has in addition been studied with regard to collisions between molecules. The variation of the outflow speed of water molecules in the coma with nucleocentric and heliocentric distances, which had been shown to be important for understanding velocity-resolved data, has important consequences for the calculation of water production rates from OH observations. Specifically, we find that a steeper variation of water production rate with heliocentric distance is required for a water coma which is consistent with the velocity-resolved observations of comet P/Halley (and with our hybrid dusty gasdynamic/Monte Carlo model). For the H2O production rate from Halley we find that the changing outflow speed causes the power-law dependence in heliocentric distance (r) to be steeper by a factor of about r-0.5 compared with the standard vectorial model analysis having a constant 1 km s-1 outflow speed. Future applications of the revised model are also suggested.