H2infrared emission and the formation of dense structures in the Orion molecular cloud

Abstract

Observations are reported of IR emission of H 2 from a region of the Orion molecular cloud (OMC1) 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 2 μ m with the ADONIS adaptive optics system. Images of ${\rm H}_2 v=1{-}0 S$(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 3 10 -3 and 4 10 -3 pc. 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 ${\rm H}_2 v=1{-}0 S$(1) line. Magnetic (C-type) shocks with velocities of 30 km s -1 and pre-shock densities of 10 6 cm -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 10 7 cm -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.