SILICON CHEMISTRY IN INTERSTELLAR CLOUDS

Interstellar SiO was discovered shortly after CO but it has been detected mainly in high-density and high-temperature regions associated with outflow sources. We present a new model of interstellar silicon chemistry that explains the lack of SiO detections in cold clouds and contains an exponential temperature dependence for the SiO abundance. A key aspect of the model is the sensitivity of SiO production by neutral silicon reactions to density and temperature, which arises from the dependence of the rate coefficients on the population of the excited fine-structure levels of the silicon atom. This effect was originally pointed out in the context of neutral reactions of carbon and oxygen by Graff, who noted that the leading term in neutral atom-molecule interactions involves the quadrupole moment of the atom. Similar to the case of carbon, the requirement that Si has a quadrupole moment requires population of the J = 1 level, which lies 111 K above the J = 0 ground state and has a critical density ncr ≥ 106 cm-3. The SiO abundance then has a temperature dependence proportional to exp (-111/T) and a quadratic density dependence for n < ncr. As part of the explanation of the lack of SiO detections at low temperatures and densities, our model also emphasizes the small efficiencies of the production routes and the correspondingly long times needed to reach equilibrium. Measurements of the abundance of SiO, in conjunction with theory, can provide information on the physical properties of interstellar clouds such as the abundances of oxygen bearing molecules and the depletion of interstellar silicon.