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Issue A&A
Volume 469, Number 2, July II 2007
Page(s) 561 - 574
Section Interstellar and circumstellar matter
DOI http://dx.doi.org/10.1051/0004-6361:20065786



A&A 469, 561-574 (2007)
DOI: 10.1051/0004-6361:20065786

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

L. E. Kristensen1, T. L. Ravkilde2, D. Field2, J. L. Lemaire1, and G. Pineau des Forêts3, 1

1  Observatoire de Paris & Université de Cergy-Pontoise, LERMA, UMR 8112 du CNRS, 92195 Meudon Cedex, France
    e-mail: lars.kristensen@u-cergy.fr
2  Department of Physics and Astronomy, University of Aarhus, 8000 Aarhus C, Denmark
3  Institut d'Astrophysique Spatiale, UMR 8617 du CNRS, Université de Paris Sud, 91405 Orsay, France

(Received 8 June 2006 / Accepted 6 March 2007)

Abstract
Aims.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$\farcs$45 (~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 105-107 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 $18\pm2$ km s-1 and pre-shock density $1\pm0.5\times10^6$ cm-3 (3$\sigma$) and two with shock speeds of ~$36\pm2$ km s-1 and pre-shock densities of $7.5\pm2.5\times10^4$ cm-3.


Key words: ISM: individual objects: OMC1 -- ISM: kinematics and dynamics -- ISM: molecules -- shock waves -- ISM: lines and bands



© ESO 2007


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