JWST/MIRI detection of suprathermal OH rotational emissions: probing the dissociation of the water by Lyman alpha photons near the protostar HOPS 370

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 $E_U/k=4.65\times 10^4$ K. The relative strengths of the observed OH lines provide a powerful diagnostic of the ultraviolet radiation field in a heavily-extinguished region ($A_V\sim 10-20$) 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 $\tilde{B}-X$ band by ultraviolet radiation produced by fast ($\sim 80\mathrm{km}{\mathrm{s}}^{-1}$) shocks along the jet. The observed dominance of emission from symmetric ($A^{\prime }$) OH states over that from antisymmetric ($A^{\prime \prime }$) states provides a distinctive signature of this particular population mechanism. Moreover, the variation of intensity with rotational quantum number suggests specifically that Ly$\hat{I}\pm$ 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$\hat{I}\pm$ flux to estimate the Ly$\hat{I}\pm$ production rate, we find that $\sim 4\%$ of the Ly$\hat{I}\pm$ photons are absorbed by water. Combined with direct measurements of water emissions in the $\hat{I}{½}_2=1-0$ band, the OH observations promise to provide key constraints on future models for the diffusion of Ly$\hat{I}\pm$ photons in the vicinity of a shock front.

30 pages, 7 figures. Accepted for publication in ApJ Letters