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

An extension of the cometary atmosphere Monte Carlo particle trajector y model formalism has been made which makes it both physically correct for heavy species and yet still computationally reasonable. The deriv ation 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 present ed 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 radica l formalism is necessary: the OH distribution. The OH distribution in comets has been studied in the context of this revised model, and anal ysis of the IUE observations of OH in comet Halley has also been compa red with the standard vectorial model. The kinetic distribution of OH in the coma has in addition been studied with regard to collisions bet ween molecules. The variation of the outflow speed of water molecules in the coma with nucleocentric and heliocentric distances, which had b een shown to be important for understanding velocity-resolved data, ha s 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 m odel). For the H2O production rate from Halley we find that the changi ng outflow speed causes the power-law dependence in heliocentric dista nce (r) to be steeper by a factor of about r-0.5 compared with the sta ndard vectorial model analysis having a constant 1 km s-1 outflow spee d. Future applications of the revised model are also suggested.