SWS spectroscopy of the Orion Peak 1 outflow: H₂, HD

D. Rosenthal ¹, F. Bertoldi ¹, S. Drapatz ¹, & R. Timmermann ²

¹ Max-Planck-Institut für extraterrestrische Physik, 85740 Garching, Germany

² Universität zu Köln, Zülpicher Str.77, 50937 Köln, Germany

We have obtained near- and mid-infrared spectra of the Orion Peak 1 outflow, the brightest H₂ emission region in the sky. Using the Short-Wavelength-Spectrometer on board ISO we obtained a full-scan spectrum from 2.4 to , and numerous deeper AOT02 spectra at selected wavelengths. A wealth of emission and absorption features was detected, including some 80 rotation-vibrational and pure rotational lines, latter ranging from 0-0 S(1) to 0-0 S(25). Emission of hydrogen recombination, ionic fine structure, and molecular lines of CO and , as well as PAH features were detected. Besides the prominent water ice and silicate features at and , and possibly are found in absorption. For some of the lines, these observations provide the first detection in the interstellar medium.

For the first time an infrared transition of HD was detected in the ISM, =0-0 R(5) at 19.4305 m. Assuming similar excitation conditions for H₂ and HD, we were able to derive each molecules total column density in the warm, shocked gas, resulting in an abundance ratio in the Orion outflow. The apparent deuterium abundance is lower by a factor five from the cosmic value, which can in part be due to chemical depletion of HD relative to H₂ in dissociative C-shocks, and in part also to subthermal excitation of the observed level provided the gas density is lower than . Accounting approximately for these effects, we derive a deuterium abundance of , consistent with recent DI and HI observations.