Interstellar SiO as a tracer of high-temperature chemistry

The J = 2-1 transition of SiO has been searched for toward both hot and cold molecular gas. SiO was not detected toward the dark clouds TMC-1, L134 N, and B335, down to column density upper limits of N < 2-4 x 10(10) cm-2. The species, however, has been observed toward all sources with a kinetic temperature greater than or equal to 30 K, with the largest column densities (N approximately 10(13)-10(17) cm-2) measured in the warmest (TK > or = 100 K) material. The abundance of SiO, relative to HCN, is found to be approximately 0.1-1 in the massive star-forming regions toward Orion and NGC 7538; toward the dark clouds, the upper limits to this ratio is less than 0.0002-0.004. A similar enhancement in the warmer regions is reflected in the SiO/H2 ratio as well. A linear relation was found between the natural log of the SiO concentration and 1/TK, suggesting that the species' formation involves a chemically specific process that contains an activation barrier of approximately 90 K. SiO was also found to be underabundant with respect to SO in cold clouds, with SiO/SO < 1/1000, versus SiO/SO > or =, measured in Orion-KL. The formation of SiO is therefore linked closely to the local gas kinetic temperature, rather than the oxygen abundance, and its synthesis is likely to involve high-temperature gas-phase reactions. The species thus may serve as an unambiguous indicator of high-temperature or "shock" chemistry.