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The chemistry of interstellar molecules containing the halogen elements

Although they are only minor constituents of the interstellar medium, halogen-containing molecules are of special interest because of their unique thermochemistry. Here, we present a theoretical study of the chemistry of interstellar molecules containing the halogen elements chlorine and fluorine. We have modeled both diffuse and dense molecular clouds, making use of updated estimates for the rates of several key chemical processes. We present predictions for the abundances of the three halogen molecules that have been detected to date in the interstellar medium: HF, CF+ and HCl. As in our previous study of fluorine-bearing interstellar molecules, we predict HF to be the dominant gas-phase reservoir of fluorine within both diffuse and dense molecular clouds; we expect the Herschel Space Observatory to detect widespread absorption in the HF J=1-0 transition. Our updated model now overpredicts the CF+ abundance by a factor ~10 relative to observations of the Orion Bar; this discrepancy has widened because we now adopt a laboratory measurement of the CF+ dissociative recombination rate that is smaller than the estimate we adopted previously. This disagreement suggests that the reaction of C+ with HF proceeds more slowly than the capture rate assumed in our model; a laboratory measurement of this reaction rate would be very desirable. Our model predicts diffuse cloud HCl abundances that are similar to those predicted previously and detected tentatively toward zeta Oph. Two additional species are potentially detectable from photodissociation regions: the H2Cl+ and HCl+ molecular ions. Ortho-H2Cl+ has its lowest-lying transition in the millimeter spectral region observable from the ground, and the lowest rotational transition of HCl+ is observable with Herschel's HIFI instrument.