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

1. Introduction

The molecular environment in the inner of the Galaxy differs drastically from that in the Galactic disk. Large scale surveys of CO isotopomers (e.g. Bally et al. 1987, Heiligman 1987, Jackson et al. 1996, Dahmen et al. 1997a (Paper I), Bitran et al. 1997) show that the cloud and the intercloud medium in the gaseous bulge of the Milky Way are molecular. The gas is characterized by large linewidths, indicating a high degree of turbulence. As shown by Dahmen et al. 1998 (Paper II), there is no simple relationship between CO line intensities and H₂ column densities and thus no global (CO) conversion factor.

Except for a few extraordinary regions such as Sgr A and Sgr B2, little evidence for ongoing massive star formation in Galactic center Giant Molecular Clouds (GMCs) is found, as is demonstrated by a general lack of strong FIR or radio continuum point sources associated with these clouds (Odenwald & Fazio 1984, Güsten 1989). Ambient dust temperatures are fairly low at K (e.g. Cox & Laurijs 1989). Based on ISO data, this is also a typical in the sources studied in this paper.

In the Galactic disk, quasithermal SiO emission is tightly correlated with high temperature regions (e.g. Ziurys et al. 1989). A close association with outflows strongly suggests that grain disruption by shocks is the major mechanism for releasing SiO into the gas phase (Martín-Pintado et al. 1992), although high temperature gas phase chemistry (Langer & Glassgold 1990) may play a minor role. In the Galactic center region, SiO is much more widespread (see e.g. the survey in the transition by Martín-Pintado et al. 1997), which was interpreted as evidence for large scale or ubiquitous (fast) shocks.

In this paper, and SiO data are presented for molecular cloud cores of the Bally et al. (1987) CS survey. Our measurements allow to (1) trace the H₂ column density, (2) determine or constrain density and temperature structure, (3) estimate SiO abundances and (4) obtain information about silicon isotope ratios. Since our sources are selected on the basis of their intensity in CS, a general high density tracer, the sample is not a priori biased toward strong SiO emission.

NH₃ data (Hüttemeister et al. 1993b) show that at least two phases of different kinetic temperatures are present within all cloud cores without massive star formation: A cool component with of 20 - 30 K, close to the temperature of the dust in the Galactic center and a warm component with K. K is considered by many authors, (e.g. the reviews by Morris & Serabyn 1996, Mezger et al. 1996) as `typical'. This is, however, just the average over the hot and cool component and has no meaning as a distinct physical component.

A major aim of this study is to determine the physical parameters, distribution and origin of these phases and to decide which of them, cool or warm, is associated with the bulk of the SiO emission.

© European Southern Observatory (ESO) 1998

Online publication: May 15, 1998

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