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Abstract

Using the MIRI/MRS spectrometer on JWST, we have detected pure rotational, suprathermal OH emissions from the vicinity of the intermediate-mass protostar HOPS 370 (OMC2/FIR3). These emissions are observed from shocked knots in a jet/outflow, and originate in states of rotational quantum number as high as 46 that possess excitation energies as large as E-U/k=4.65x10(4) K. The relative strengths of the observed OH lines provide a powerful diagnostic of the ultraviolet radiation field in a heavily-extinguished region (A(V)similar to 10-20) where direct UV observations are impossible. To high precision, the OH line strengths are consistent with a picture in which the suprathermal OH states are populated following the photodissociation of water in its (B) over bar - X band by ultraviolet radiation produced by fast (similar to 80kms(-1)) shocks along the jet. The observed dominance of emission from symmetric (A ') OH states over that from antisymmetric (A '') states provides a distinctive signature of this particular population mechanism. Moreover, the variation of intensity with rotational quantum number suggests specifically that Ly alpha radiation is responsible for the photodissociation of water, an alternative model with photodissociation by a 10(4) K blackbody being disfavored at a high level of significance. Using measurements of the Br alpha flux to estimate the Ly alpha production rate, we find that similar to 4% of the Ly alpha photons are absorbed by water. Combined with direct measurements of water emissions in the nu(2)=1-0 band, the OH observations promise to provide key constraints on future models for the diffusion of Ly alpha photons in the vicinity of a shock front.