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Astron. Astrophys. 356, 695-704 (2000)

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Non-equilibrium H2 ortho-to-para ratio in two molecular clouds of the Galactic Center *

N.J. Rodríguez-Fernández 1, J. Martín-Pintado 1, P. de Vicente 1, A. Fuente 1, S. Hüttemeister 2, T.L. Wilson 3,4 and D. Kunze 5

1 Observatorio Astronómico Nacional (IGN), Apartado 1143, 28800 Alcalá de Henares, Spain (nemesio; martin; vicente; fuente@oan.es)
2 Astronomische Institute, Auf dem Hügel 71, 53121 Bonn 1, Germany (huette@astro.uni-bonn.de)
3 Max-Planck-Institut für Radioastronomie, Postfach 2024, 53010 Bonn, Germany
4 Sub-mm Telescope Observatory, Steward Observatory, The University of Arizona, Tucson, AZ 85728, USA (twilson@as.arizona.edu)
5 MPE, Giesenbachstrasse 1, 85748 Garching, Germany (kunze@mpe.mpg.de)

Received 21 October 1999 / Accepted 4 February 2000

Abstract

We present ISO observations of the S(0), S(1), S(2), and S(3) rotational lines of molecular hydrogen from two molecular clouds near the Galactic Center (GC). We have also measured continuum dust emission at infrared wavelengths with ISO and the rotational radio lines J=1-0 of 13CO and C18O and J=2-1 of C18O with the IRAM-30m telescope. Using the dust continuum spectra and the CO lines we derive a total visual extinction of [FORMULA] 15-20 magnitudes toward these GC clouds. After correcting the H2 data for extinction, the gas temperatures are [FORMULA] 250 K and the column densities of warm gas are [FORMULA] cm-2. This is the first direct measure of the H2 column densities of the warm component; with this, we estimate an NH3 abundance in the warm gas of [FORMULA]. The column density of warm gas is, at least, a factor of 100 larger than the corresponding column densities derived from the warm dust. The observed ortho-to-para ratio (OTPR) is [FORMULA] 1, clearly below the local thermodynamical equilibrium (LTE) OTPR for gas at 250 K of [FORMULA]. Low velocity shocks ([FORMULA] 10 km s-1) are the most likely explanation for the column densities of warm gas and dust and the non-LTE H2 OTPR.

Key words: Galaxy: center – infrared: ISM: continuum – infrared: ISM: lines and bands – ISM: molecules – ISM: clouds – ISM: dust, extinction

* ISO is an ESA project with instruments funded by ESA Member States (especially the PI countries: France, Germany, the Netherlands and the United Kingdom) and with the participation of ISAS and NASA.

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© European Southern Observatory (ESO) 2000

Online publication: April 10, 2000
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