2001A&A...366..651V

Observations are reported of IR emission of H₂ from a region of the Orion molecular cloud (OMC 1) between the Becklin-Neugebauer object and IRc2 to the north and the Trapezium stars to the south. Data were obtained using the ESO 3.6 m telescope in the K-band around 2µm with the ADONIS adaptive optics system. Images of H₂ v=1-0 S(1) show a spatial resolution of ∼0.15''. Detailed investigations of the distribution of sizes of structures in our images have been performed by area-perimeter analysis, Fourier analysis and brightness distribution studies. These demonstrate that structure is not fractal but shows a preferred scales of between 3x10^–3 and 4x10^–3pc. In an attempt to estimate the density in observed structures, predictions of both shock models and photodissociation region models have been compared with measured emission brightness in the H₂ v=1-0 S(1) line. Magnetic (C-type) shocks with velocities of 30km/s and pre-shock densities of 10⁶cm^–3 yield the best representation of our data, notwithstanding significant discrepancies for the brightness ratio between v=2-1 S(1) and v=1-0 S(1) lines. Our results show that post-shock densities are several times 10⁷cm^–3. This is sufficiently high that the passage of C-type shocks in Orion yields gravitational instability which may in turn trigger star formation in the post-shock gas.