1. JWST/MIRI detection of suprathermal OH rotational emissions: probing the dissociation of the water by Lyman alpha photons near the protostar HOPS 370
- Author
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Neufeld, David A., Manoj, P., Tyagi, Himanshu, Narang, Mayank, Watson, Dan M., Megeath, S. Thomas, Van Dishoeck, Ewine F., Gutermuth, Robert A., Stanke, Thomas, Yang, Yao-Lun, Rubinstein, Adam E., Anglada, Guillem, Beuther, Henrik, Garatti, Alessio Caratti o, Evans II, Neal J., Federman, Samuel, Fischer, William J., Green, Joel, Klaassen, Pamela, Looney, Leslie W., Osorio, Mayra, Nazari, Pooneh, Tobin, John J., Tychoniec, Lukasz, and Wolk, Scott
- Subjects
Astrophysics - Astrophysics of Galaxies - 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 ($\sim 80\,\rm km\,s^{-1}$) 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$\alpha$ 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 $\sim 4\%$ 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., Comment: 30 pages, 7 figures. Accepted for publication in ApJ Letters
- Published
- 2024