The Orion fingers: H${}_2$ temperatures and excitation in an explosive outflow

Abstract

We measure H${}_2$ temperatures and column densities across the Orion BN/KL explosive outflow from a set of thirteen near-IR H${}_2$ rovibrational emission lines observed with the TripleSpec spectrograph on Apache Point Observatory's 3.5-meter telescope. We find that most of the region is well-characterized by a single temperature (~2000-2500 K), which may be influenced by the limited range of upper energy levels (6000-20,000 K) probed by our data set. The H${}_2$ column density maps indicate that warm H${}_2$ comprises 10${}^{-5}$ - 10${}^{-3}$ of the total H${}_2$ column density near the center of the outflow. Combining column density measurements for co-spatial H${}_2$ and CO at T = 2500 K, we measure a CO/H${}_2$ fractional abundance of 2$\times$10${}^{-3}$, and discuss possible reasons why this value is in excess of the canonical 10${}^{-4}$ value, including dust attenuation, incorrect assumptions on co-spatiality of the H${}_2$ and CO emission, and chemical processing in an extreme environment. We model the radiative transfer of H${}_2$ in this region with UV pumping models to look for signatures of H${}_2$ fluorescence from H I Ly$\hat{I}\pm$ pumping. Dissociative (J-type) shocks and nebular emission from the foreground Orion H II region are considered as possible Ly$\hat{I}\pm$ sources. From our radiative transfer models, we predict that signatures of Ly$\hat{I}\pm$ pumping should be detectable in near-IR line ratios given a sufficiently strong source, but such a source is not present in the BN/KL outflow. The data are consistent with shocks as the H${}_2$ heating source.

24 pages, 8 figures, 3 tables, accepted to ApJ