  Water in Star-forming regions with Herschel (WISH) is a Herschel Key Program
investigating the water chemistry in young stellar objects (YSOs) during
protostellar evolution. Hydroxyl (OH) is one of the reactants in the chemical
network most closely linked to the formation and destruction of H2O.
High-temperature chemistry connects OH and H2O through the OH + H2 <-> H2O + H
reactions. Formation of H2O from OH is efficient in the high-temperature regime
found in shocks and the innermost part of protostellar envelopes. Moreover, in
the presence of UV photons, OH can be produced from the photo-dissociation of
H2O. High-resolution spectroscopy of the OH 163.12 micron triplet towards HH 46
and NGC 1333 IRAS 2A was carried out with the Heterodyne Instrument for the Far
Infrared (HIFI) on board Herschel. The low- and intermediate-mass YSOs HH 46,
TMR 1, IRAS 15398-3359, DK Cha, NGC 7129 FIRS 2, and NGC 1333 IRAS 2A were
observed with the Photodetector Array Camera and Spectrometer (PACS) in four
transitions of OH and two [OI] lines. The OH transitions at 79, 84, 119, and
163 micron and [OI] emission at 63 and 145 micron were detected with PACS
towards the class I low-mass YSOs as well as the intermediate-mass and class I
Herbig Ae sources. No OH emission was detected from the class 0 YSO NGC 1333
IRAS 2A, though the 119 micron was detected in absorption. With HIFI, the
163.12 micron was not detected from HH 46 and only tentatively detected from
NGC 1333 IRAS 2A. The combination of the PACS and HIFI results for HH 46
constrains the line width (FWHM > 11 km/s) and indicates that the OH emission
likely originates from shocked gas. This scenario is supported by trends of the
OH flux increasing with the [OI] flux and the bolometric luminosity. Similar OH
line ratios for most sources suggest that OH has comparable excitation
temperatures despite the different physical properties of the sources.
