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

Authors :
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
Wolk, Scott
Publication Year :
2024

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.<br />Comment: 30 pages, 7 figures. Accepted for publication in ApJ Letters

Details

Database :
arXiv
Publication Type :
Report
Accession number :
edsarx.2404.07299
Document Type :
Working Paper