Thermal Water Vapor Emission from Shocked Regions in Orion

Abstract

Using the Long Wavelength Spectrometer on board the Infrared Space Observatory, we have observed thermal water vapor emission from a roughly circular field of view approximately 75'' in diameter centered on the Orion BN-KL region. The Fabry-Perot line strengths, line widths, and spectral line shifts observed in eight transitions between 71 and 125 μm show good agreement with models of thermal emission arising from a molecular cloud subjected to a magnetohydrodynamic C-type shock. Both the breadth and the relative strengths of the observed lines argue for emission from a shock rather than from warm quiescent gas in the Orion core. Although one of the eight transitions appears anomalously strong and may be subject to the effects of radiative pumping, the other seven indicate an H₂O/H₂ abundance ratio on the order of 5×10⁻⁴ and a corresponding gas-phase oxygen-to-hydrogen abundance ratio on the order of 4×10⁻⁴. Given current estimates of the interstellar, gas-phase, oxygen and carbon abundances in the solar vicinity, this value is consistent with theoretical shock models that predict the conversion into water of all the gas-phase oxygen that is not bound as CO. The overall cooling provided by rotational transitions of H₂O in this region appears to be comparable to the cooling through rotational lines of CO but is an order of magnitude lower than cooling through H₂ emission. However, the model that best fits our observations shows cooling by H₂O and CO dominant in that portion of the postshock region where temperatures are below ~800 K and neither vibrational nor rotational radiative cooling by H₂ is appreciable.