The Orion Fingers: H2 Temperatures and Excitation in an Explosive Outflow.

We measure H₂ temperatures and column densities across the Orion Becklin-Neugebauer/Kleinmann-Low (BN/KL) explosive outflow from a set of 13 near-infrared (IR) H₂ 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 H₂ column density maps indicate that warm H₂ comprises 10⁻⁵–10⁻³ of the total H₂ column density near the center of the outflow. Combining column density measurements for co-spatial H₂ and CO at T = 2500 K, we measure a CO/H₂ 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 H₂ and CO emission, and chemical processing in an extreme environment. We model the radiative transfer of H₂ in this region with ultraviolet (UV) pumping models to look for signatures of H₂ fluorescence from H i Lyα pumping. Dissociative (J-type) shocks and nebular emission from the foreground Orion H ii region are considered as possible Lyα sources. From our radiative transfer models, we predict that signatures of Lyα 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 H₂ heating source. [ABSTRACT FROM AUTHOR]