Excitation conditions in the Orion molecular cloud obtained from observations of ortho- and para-lines of H {2}

International audienceAims: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. Methods: K-band observations of H2 obtained with the Canada-France-Hawaii 3.6 m 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 arcsec (~200 AU). 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. Results: 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 10^5-107 cm-3 and shock velocities lie between 10 and 40 km s-1. 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±2 km s-1 and pre-shock density 1±0.5×106 cm-3 (3sigma) and two with shock speeds of ~36±2 km s-1 and pre-shock densities of 7.5±2.5×104 cm-3