Abstract
Multiline observations of (CO)-O-18 and SiO isotopomers toward 33 molecular peaks in the Galactic center region, taken at the SEST, JCMT and HHT telescopes, are presented. The (CO)-O-18 presumably traces the total Hz column density, while the SiO traces gas affected by shocks and high temperature chemistry. The J = 2 --> 1 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 (SiO)-Si-29 and (SiO)-Si-30 to the strength of the main isotopomer (SiO)-Si-28 implies that the J = 2 --> 1 transition of (SiO)-Si-28 is optically thick. The Si-29/Si-30 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 (n(H2), greater than or equal to 10(4) cm(-3)) in typical molecular cores in the Galactic center is cool (T-kin approximate to 25 K), contrary to what is usually found in Giant Molecular Clouds in the disk, where the densest cores are the hottest. High kinetic temperatures, > 100 K, known to exist from NH3 studies, are only present at lower gas densities of a few 10(3) cm(-3), where SiO is highly subthermally excited. Assuming that (CO)-O-18 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 10(-9), 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 similar to 10(6) years to the large scale kinematics in the inner similar to 500 PC Of the Galaxy.