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Astron. Astrophys. 334, 646-658 (1998)

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Molecular gas in the Galactic center region

III. Probing shocks in molecular cores *

S. Hüttemeister 1, 2, G. Dahmen 3, 4, R. Mauersberger 5, C. Henkel 4, T.L. Wilson 4, 6 and J. Martín-Pintado 7

1 Radioastronomisches Institut der Universität Bonn, Auf dem Hügel 71, D-53121 Bonn, Germany
2 Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, MA 02138, USA
3 Physics Department, Queen Mary & Westfield College, University of London, Mile End Road, London E1 4NS, UK
4 Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, D-53121 Bonn, Germany
5 Steward Observatory, The University of Arizona, Tucson, AZ 85721, USA
6 Submillimeter Telescope Observatory, The University of Arizona, Tucson, AZ 85721, USA
7 Centro Astronómico de Yebes, Apartado 148, E-19080 Guadalajara, Spain

Received 1 December 1997 / Accepted 24 February 1998

Abstract

Multiline observations of C18 O and SiO isotopomers toward 33 molecular peaks in the Galactic center region, taken at the SEST, JCMT and HHT telescopes, are presented. The C18 O presumably traces the total H2 column density, while the SiO traces gas affected by shocks and high temperature chemistry. The [FORMULA] line of SiO is seen only in few regions of the Galactic disk. This line is easily detected in all Galactic center sources observed. A comparison of the strength of the rare isotopomers 29 SiO and 30 SiO to the strength of the main isotopomer 28 SiO implies that the [FORMULA] transition of 28 SiO is optically thick. The 29 Si [FORMULA] Si isotope ratio of 1.6 in the Galactic center clouds is consistent with the terrestrial value. Large Velocity Gradient models show that the dense component ([FORMULA] [FORMULA]) in typical molecular cores in the Galactic center is cool ([FORMULA] [FORMULA] K), contrary to what is usually found in Giant Molecular Clouds in the disk, where the densest cores are the hottest. High kinetic temperatures, [FORMULA] K, known to exist from NH3 studies, are only present at lower gas densities of a few 103 [FORMULA], where SiO is highly subthermally excited. Assuming that [FORMULA] traces all of the molecular gas, it is found that in all cases but one, SiO emission is compatible with arising in gas at higher density that is (presently) relatively cool. The relative abundance of SiO is typically [FORMULA], but differs significantly between individual sources. It shows a dependence on the position of the source within the Galactic center region. High abundances are found in those regions for which bar potential models predict a high likelihood for cloud-cloud collisions. These results can be used to relate the amount of gas that has encountered shocks within the last [FORMULA] years to the large scale kinematics in the inner [FORMULA] 500 pc of the Galaxy.

Key words: ISM: clouds – ISM: molecules – ISM: structure – Galaxy: center – radio Lines: ISM

* Based on observations obtained at the Swedish-ESO Submillimeter Telescope (SEST, Project C-0518, 1996), the James Clark Maxwell Telescope (JCMT, operated by the Joint Astronomy Centre on behalf of the Particle Physics and Astronomy Research Council of the United Kingdom, the Netherlands Organisation for Scientific Research, and the National Research Council of Canada) and the Heinrich-Hertz-Telescope (HHT, operated by the the Submillimeter Telescope Observatory).

Send offprint requests to: S. Hüttemeister, RAIUB

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© European Southern Observatory (ESO) 1998

Online publication: May 15, 1998

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