MOLECULAR GAS IN THE GALACTIC-CENTER REGION - III - PROBING SHOCKS INMOLECULAR CORES

Multiline observations of (CO)-O-18 and SiO isotopomers toward 33 mole cular peaks in the Galactic center region, taken at the SEST, JCMT and HHT telescopes, are presented. The (CO)-O-18 presumably traces the to tal 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 a ll Galactic center sources observed. A comparison of the strength of t he 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 o f (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 valu e. 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, kn own 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. Assu ming 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 a t higher density that is (presently) relatively cool. The relative abu ndance of SiO is typically 10(-9), but differs significantly between i ndividual 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) y ears to the large scale kinematics in the inner similar to 500 PC Of t he Galaxy.