Direct Determination of the HF/H2 Abundance Ratio in Interstellar Gas
  We report the first detection of the v=1--0, R(0) ro-vibrational transition
of HF at 2.499385 microns arising from interstellar gas. The line is seen in
absorption toward 3 background sources---HD 154368, Elias 29, and AFGL 2136 IRS
1---all of which have reported H2 column densities determined from observations
of H2. This allows for the first direct determination of the HF/H2 abundance
ratio. We find values of N(HF)/N(H2)=1.15*10^-8 and 0.69*10^-8 for HD 154368
and Elias 29, respectively. The sight line toward AFGL 2136 IRS 1 also shows
absorption from the v=1--0, R(1) transition of HF, indicating warm, dense (n_H
> 10^9 cm^-3) gas, likely very close to the central protostar. Ascribing
portions of the HF absorption to warm and cold gas, we find
N(HF)/N(H2)=(1.7--2.9)*10^-8 and (0.33--0.58)*10^-8 for the two components,
respectively. Except for the warm component toward AFGL 2136 IRS 1, all
observed HF/H2 ratios are well below N(HF)/N(H2)=3.6*10^-8, the value predicted
if nearly all gas-phase fluorine is in the form of HF. Models of fluorine
chemistry that account for depletion onto grains are able to reproduce the
results toward HD 154368, but not in the cold, dense gas toward AFGL 2136 IRS 1
and Elias 29. Most likely, some combination of simplifying assumptions made in
the chemical models are responsible for these discrepancies.
