2017ApJ...850..105A

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 Cl₂, ClO, CCl, and HCCl, as well as the carbon-halogen species CH₂Cl and CH₃Cl, 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 CH₃Cl is predicted to be ~10^–10–5×10^–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.