| Issue |
A&A
Volume 469, Number 2, July II 2007
|
|
|---|---|---|
| Page(s) | 561 - 574 | |
| Section | Interstellar and circumstellar matter | |
| DOI | https://doi.org/10.1051/0004-6361:20065786 | |
| Published online | 13 March 2007 | |
Excitation conditions in the Orion molecular cloud obtained
from observations of ortho- and para-lines of H
*,**
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
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 
–0 S(1) ortho- and –0 S(0)
para-lines at a spatial resolution of 0
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 –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 
km s-1 and
pre-shock density 
cm-3 (3σ) and two
with shock speeds of ~
km s-1 and pre-shock densities of
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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