H₂ infrared emission and the formation of dense structures in the Orion molecular cloud

¹ Observatoire de Paris-Meudon, 92195 Meudon Principal Cedex, France
² Université de Cergy-Pontoise UMR 8588 du CNRS, 95806 Cergy Cedex, France
³ Institute of Physics and Astronomy, University of Aarhus, 8000 Aarhus C, Denmark and Observatoire de Paris-Meudon, UMR 8588 du CNRS, France
⁴ Theoretical Astrophysics Centre, University of Aarhus, 8000 Aarhus C, Denmark

Corresponding author: D. Field, dfield@ifa.au.dk

Received: 21 June 2000
Accepted: 9 November 2000

Abstract

Observations are reported of IR emission of H₂ 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 μm with the ADONIS adaptive optics system. Images of (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 (1) line. Magnetic (C-type) shocks with velocities of 30 km s^-1 and pre-shock densities of 10⁶ cm^-3 yield the best representation of our data, notwithstanding significant discrepancies for the brightness ratio between (1) and (1) lines. Our results show that post-shock densities are several times 10⁷ 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.