Herschel/HIFI Observations of Hydrogen Fluoride Toward Sagittarius B2(M)
  Herschel/HIFI observations have revealed the presence of widespread
absorption by hydrogen fluoride (HF) J = 1-0 rotational transition, toward a
number of Galactic sources. We present observations of HF J = 1-0 toward the
high-mass star-forming region Sagittarius B2(M). The spectrum obtained shows a
complex pattern of absorption, with numerous features covering a wide range of
local standard of rest velocities (-130 to 100 km -1). An analysis of this
absorption yields HF abundances relative to H2 of ~1.3 {\times}10-8, in most
velocity intervals. This result is in good agreement with estimates from
chemical models, which predict that HF should be the main reservoir of
gas-phase fluorine under a wide variety of interstellar conditions.
Interestingly, we also find velocity intervals in which the HF spectrum shows
strong absorption features that are not present, or are very weak, in spectra
of other molecules, such as 13CO (1-0) and CS (2-1). HF absorption reveals
components of diffuse clouds with small extinction that can be studied for the
first time. Another interesting observation is that water is significantly more
abundant than hydrogen fluoride over a wide range of velocities toward
Sagittarius B2(M), in contrast to the remarkably constant H2O/HF abundance
ratio with average value close to unity measured toward other Galactic sources.
