JWST/MIRI detection of suprathermal OH rotational emissions: probing the dissociation of the water by Lyman alpha photons near the protostar HOPS 370
David A. Neufeld, P. Manoj, Himanshu Tyagi, Mayank Narang, Dan M. Watson, S. Thomas Megeath, Ewine F. Van Dishoeck, Robert A. Gutermuth, Thomas Stanke, Yao-Lun Yang, Adam E. Rubinstein, Guillem Anglada, Henrik Beuther, Alessio Caratti o Garatti, Neal J. Evans II, Samuel Federman, William J. Fischer, Joel Green, Pamela Klaassen, Leslie W. Looney, Mayra Osorio, Pooneh Nazari, John J. Tobin, Lukasz Tychoniec, Scott Wolk
arXiv:2404.07299v1 Announce Type: new
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 ($A^{primeprime}$) 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.arXiv:2404.07299v1 Announce Type: new
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 ($A^{primeprime}$) 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.