2007A&A...469..561K

We seek to study excitation mechanisms in the inner region of the Orion Molecular Cloud by comparing observations of ortho- and para-lines of H₂ with theoretical models of slow shocks and photodissociation regions. K-band observations of H₂ obtained with the Canada-France-Hawaii 3.6m telescope using the PUEO adaptive optics system are reported. Data were centered on the Becklin-Neugebauer object northwest of the Trapezium stars. Narrow-band filters were used to isolate emission from the v=1-0 S(1) ortho- and v=1-0 S(0) para-lines at a spatial resolution of 0.45" (∼200AU). We are able to combine their intensity to obtain the column densities of rovibrationally excited ortho and para H₂ levels of the molecular gas at high spatial resolution. The resulting line ratios show variations between 2 and the statistical equilibrium value of 6. We find 4 different classes of emission, characterised by the ratio of the v=1-0 S(1) and S(0) line brightness and the absolute line brightness. Shock models are used to estimate the physical properties of pre-shock density and shock velocity for these 4 classes. We find that the pre-shock density is in the range of 10⁵-10⁷cm^–3 and shock velocities lie between 10 and 40km/s. Studies of individual objects, using additional constraints of shock velocity and width, allow quite precise physical conditions to be specified in three prominent bow shocks, one with a shock speed of 18±2km/s and pre-shock density 1±0.5x10⁶cm^–3 (3σ) and two with shock speeds of ∼36±2km/s and pre-shock densities of 7.5±2.5x10⁴cm^–3.