Gas-grain Fluorine and Chlorine Chemistry in the Interstellar Medium

We have studied the formation of fluorine- and chlorine-bearing species for a variety of dense interstellar conditions using a gas-grain network. Our homogeneous models have been constructed for low-temperature dense clouds, as well as warm-up regions. In addition to the observed species HF, CF+, HCl, HCl+, and H2Cl+, we have included a number of additional halogen-containing molecules, and explored their gas-phase and grain-surface chemistry. These molecules include neutral species such as CI2, CIO, CCl, and HCCl, as well as the carbon-halogen species CH2CI and CH3CI, and ionic species such as CCl+, ClO+, CH3ClH+, HF+, SiF+, and H2F+. Predictions are made for the abundances of these species as functions of time, and comparisons are made with the observed abundances obtained for halogen species in dense regions, which include HF, HCl, CH3Cl, and CF+. The peak fractional abundance of the newly detected gas-phase CH3CI is predicted to be approximate to 10(-10)-5 x 10(-8) in our warm-up simulations, depending upon density and the age of the pre-warm-up phase after which warm-up begins. These values can be compared with the observed abundance of methyl chloride in the hot corino IRAS 16293-2422 if the abundance of methanol is known.