Spatially extended OH+emission from the Orion Bar and Ridge

Abstract

We report the first detection of a Galactic source of OH+ line emission: the Orion Bar, a bright nearby photon-dominated region. Line emission is detected over ~1' (0.12 pc), tracing the Bar itself as well as the Southern tip of the Orion Ridge. The line width of ~4 km/s suggests an origin of the OH+ emission close to the PDR surface, at a depth of A_V ~0.3-0.5 into the cloud where most hydrogen is in atomic form. Steady-state collisional and radiative excitation models require unrealistically high OH+ column densities to match the observed line intensity, indicating that the formation of OH+ in the Bar is rapid enough to influence its excitation. Our best-fit OH+ column density of ~1x10^14 cm^-2 is similar to that in previous absorption line studies, while our limits on the ratios of OH+/H2O+ (>~40) and OH+/H3O+ (>~15) are higher than seen before. The column density of OH+ is consistent with estimates from a thermo-chemical model for parameters applicable to the Orion Bar, given the current uncertainties in the local gas pressure and the spectral shape of the ionizing radiation field. The unusually high OH+/H2O+ and OH+/H3O+ ratios are probably due to the high UV radiation field and electron density in this object. In the Bar, photodissociation and electron recombination are more effective destroyers of OH+ than the reaction with H2, which limits the production of H2O+. The appearance of the OH+ lines in emission is the result of the high density of electrons and H atoms in the Orion Bar, since for these species, inelastic collisions with OH+ are faster than reactive ones. In addition, chemical pumping, far-infrared pumping by local dust, and near-UV pumping by Trapezium starlight contribute to the OH+ excitation. Similar conditions may apply to extragalactic nuclei where OH+ lines are seen in emission.