The chemistry of fluorine-bearing molecules in diffuse and dense interstellar gas clouds

We present a theoretical investigation of the chemistry of fluorine-bearing molecules in diffuse and dense interstellar gas clouds, combining recent estimates for the rates of relevant chemical reactions with a self-consistent model for the physical and chemical conditions within gas clouds that are exposed to the interstellar ultraviolet radiation field. The chemistry of interstellar fluorine is qualitatively different from that of any other element, because unlike the neutral atoms of any other element found in diffuse or dense molecular clouds, atomic fluorine undergoes an exothermic reaction with molecular hydrogen. Over a wide range of conditions attained within interstellar gas clouds, the product of that reaction, hydrogen fluoride, is predicted to be the dominant gas-phase reservoir of interstellar fluorine nuclei. Fluorine is the heavy element that shows the greatest tendency toward molecule formation; in diffuse clouds of small extinction, the predicted HF abundance can even exceed that of CO, even though the gas-phase fluorine abundance is 4 orders of magnitude smaller than that of carbon. Our model predicts HF column densities of similar to 10(13) cm(-2) in dark clouds and column densities as large as 1011 cm(-2) in diffuse interstellar gas clouds with total visual extinctions as small as 0.1 mag. Such diffuse clouds will be detectable by means of absorption-line spectroscopy of the J = 1-0 transition at 243.2 mu m using the Stratospheric Observatory for Infrared Astronomy (SOFIA) and the Herschel Space Observatory (HSO). The CF+ ion is predicted to be the second most abundant fluorine-bearing molecule, with typical column densities a factor of similar to 10(2) below those of HF; with its lowest two rotational transitions in the millimeter-wave spectral region, CF+ may be detectable from ground-based observatories. HF absorption in quasar spectra is a potential probe of molecular gas at high redshift, providing a possible bridge between the UV/optical observations capable of probing H-2 in low column density systems and the radio/millimeter-wavelength observations that probe intervening molecular clouds of high extinction and large molecular fraction; at redshifts beyond similar to 0.3, HF is potentially detectable from ground-based submillimeter observatories in several atmospheric transmission windows.