Neutral and Ionized Hydrides in Star-forming Regions -- Observations
  with Herschel/HIFI
  The cosmic abundance of hydrides depends critically on high-energy UV, X-ray,
and particle irradiation. Here we study hydrides in star-forming regions where
irradiation by the young stellar object can be substantial, and density and
temperature can be much enhanced over interstellar values. Lines of OH, CH, NH,
SH and their ions OH+, CH+, NH+, SH+, H2O+, and H3O+ were observed in
star-forming regions by the HIFI spectrometer onboard the Herschel Space
Observatory. Molecular column densities are derived from observed ground-state
lines, models, or rotational diagrams. We report here on two prototypical
high-mass regions, AFGL 2591 and W3 IRS5, and compare them to chemical
calculations making assumptions on the high-energy irradiation. A model
assuming no ionizing protostellar emission is compared with (i) a model
assuming strong protostellar X-ray emission and (ii) a two-dimensional (2D)
model including emission in the far UV (FUV, 6 -- 13.6 eV) irradiating the
outflow walls that separate the outflowing gas and infalling envelope material.
We confirm that the effect of FUV in two dimensional models with enlarged
irradiated surfaces is clearly noticeable. A molecule that is very sensitive to
FUV irradiation is CH+, enhanced in abundance by more than 5 orders of
magnitude. The HIFI observations of CH+ lines agree with the two-dimensional
FUV model by Bruderer et al. which computes abundances, non-LTE excitation and
line radiative transfer.{Ref 20} It is concluded that CH+ is a good FUV tracer
in star-forming regions. The effect of potential X-ray irradiation is not
excluded, but cannot be demonstrated by the present data.
