James Webb Space Telescope Feed Post

Arxiv: JWST/MIRI detection of suprathermal OH rotational emissions: probing the dissociation of the water by Lyman alpha photons near the protostar HOPS 370 Published: 4/10/2024 9:48:33 PM Updated: 4/10/2024 9:48:33 PM

Paper abstract: Using the MIRI/MRS spectrometer on JWST, we have detected pure rotational,suprathermal OH emissions from the vicinity of the intermediate-mass protostarHOPS 370 (OMC2/FIR3). These emissions are observed from shocked knots in ajet/outflow, and originate in states of rotational quantum number as high as 46that possess excitation energies as large as E_U/k = 4.65 \times 10^4 K. Therelative strengths of the observed OH lines provide a powerful diagnostic ofthe ultraviolet radiation field in a heavily-extinguished region (A_V ~ 10- 20) where direct UV observations are impossible. To high precision, the OHline strengths are consistent with a picture in which the suprathermal OHstates are populated following the photodissociation of water in its \tilde B- X band by ultraviolet radiation produced by fast (~ 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}) statesprovides a distinctive signature of this particular population mechanism.Moreover, the variation of intensity with rotational quantum number suggestsspecifically that Ly\alpha radiation is responsible for the photodissociationof water, an alternative model with photodissociation by a 10^4 K blackbodybeing disfavored at a high level of significance. Using measurements of theBr\alpha flux to estimate the Ly\alpha production rate, we find that ~4\% of the Ly\alpha photons are absorbed by water. Combined with directmeasurements of water emissions in the \nu_2 = 1 -0 band, the OH observationspromise to provide key constraints on future models for the diffusion ofLy\alpha photons in the vicinity of a shock front.