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Water in Star-forming regions with Herschel (WISH) is a Herschel Key Programinvestigating the water chemistry in young stellar objects (YSOs) duringprotostellar evolution. Hydroxyl (OH) is one of the reactants in the chemicalnetwork most closely linked to the formation and destruction of H2O.High-temperature chemistry connects OH and H2O through the OH + H2 <-> H2O + Hreactions. Formation of H2O from OH is efficient in the high-temperature regimefound in shocks and the innermost part of protostellar envelopes. Moreover, inthe presence of UV photons, OH can be produced from the photo-dissociation ofH2O. High-resolution spectroscopy of the OH 163.12 micron triplet towards HH 46and NGC 1333 IRAS 2A was carried out with the Heterodyne Instrument for the FarInfrared (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 wereobserved with the Photodetector Array Camera and Spectrometer (PACS) in fourtransitions of OH and two [OI] lines. The OH transitions at 79, 84, 119, and163 micron and [OI] emission at 63 and 145 micron were detected with PACStowards the class I low-mass YSOs as well as the intermediate-mass and class IHerbig Ae sources. No OH emission was detected from the class 0 YSO NGC 1333IRAS 2A, though the 119 micron was detected in absorption. With HIFI, the163.12 micron was not detected from HH 46 and only tentatively detected fromNGC 1333 IRAS 2A. The combination of the PACS and HIFI results for HH 46constrains the line width (FWHM > 11 km/s) and indicates that the OH emissionlikely originates from shocked gas. This scenario is supported by trends of theOH flux increasing with the [OI] flux and the bolometric luminosity. Similar OHline ratios for most sources suggest that OH has comparable excitationtemperatures despite the different physical properties of the sources.

Herschel observations of the hydroxyl radical (OH) in young stellar objects