First hyperfine resolved far-infrared OH spectrum from a star-forming
  region
  OH is an important molecule in the H2O chemistry and the cooling budget of
star-forming regions. The goal of the Herschel key program `Water in
Star-forming regions with Herschel' (WISH) is to study H2O and related species
during protostellar evolution. Our aim in this letter is to assess the origin
of the OH emission from star-forming regions and constrain the properties of
the emitting gas. High-resolution observations of the OH 2Pi1/2 J = 3/2-1/2
triplet at 1837.8 GHz (163.1 micron) towards the high-mass star-forming region
W3 IRS 5 with the Heterodyne Instrument for the Far-Infrared (HIFI) on Herschel
reveal the first hyperfine velocity-resolved OH far-infrared spectrum of a
star-forming region. The line profile of the OH emission shows two components:
a narrow component (FWHM approx. 4-5 km/s) with partially resolved hyperfine
structure resides on top of a broad (FWHM approx. 30 km/s) component. The
narrow emission agrees well with results from radiative transfer calculations
of a spherical envelope model for W3 IRS 5 with a constant OH abundance of
approx. 8e-9. Comparison with H2O yields OH/H2O abundance ratios of around 1e-3
for T > 100 K and around unity for T < 100K, consistent with the current
picture of the dense cloud chemistry with freeze-out and photodesorption. The
broad component is attributed to outflow emission. An abundance ratio of OH/H2O
> 0.028 in the outflow is derived from comparison with results of water line
modeling. This ratio can be explained by a fast J-type shock or a slower
UV-irradiated C-type shock.
