Discovery of Superthermal Hydroxyl (OH) in the HH 211 Outflow

We present a 5-37 μm infrared spectrum obtained with the Spitzer Space Telescope toward the southeastern lobe of the young protostellar outflow HH 211. The spectrum shows an extraordinary sequence of OH emission lines arising in highly excited rotational levels up to an energy E/k~28,200 K above the ground level. This is, to our knowledge, by far the highest rotational excitation of OH observed in the interstellar medium. The spectrum also contains several pure rotational transitions of H2O (v=0), H2 (v=0) S(0)-S(7), HD (v=0) R(3)-R(6), and atomic fine-structure lines of [Fe II], [Si II], [Ne II], [S I], and [Cl I]. The origin of the highly excited OH emission is most likely the photodissociation of H2O by the UV radiation generated in the terminal outflow shock of HH 211.