The Orion Fingers: H 2 Temperatures and Excitation in an Explosive Outflow

Abstract

We measure H 2  temperatures and column densities across the Orion Becklin-Neugebauer/Kleinmann-Low (BN/KL) explosive outflow from a set of 13 near-infrared (IR) H 2  rovibrational emission lines observed with the TripleSpec spectrograph on Apache Point Observatory’s 3.5 m 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 × 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 ultraviolet (UV) pumping models to look for signatures of H 2  fluorescence from H i  Ly α  pumping. Dissociative (J-type) shocks and nebular emission from the foreground Orion H ii  region are considered as possible Ly α  sources. From our radiative transfer models, we predict that signatures of Ly α  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.