THE PHYSICS AND CHEMISTRY OF SMALL TRANSLUCENT MOLECULAR CLOUDS - X -SIO

We have made a survey of SiO in 29 of our standard sample of 11 cirrus cores and 27 Clemens-Barvainis translucent cores the structures and c hemistry of which have been studied earlier in this series. SiO is det ected in six objects, favoring those with large products of the local radiation field I-UV and the column density. The fractional abundance of SiO is 1 x 10(-10) in these six objects, and 1 x 10(-11) over the r emaining 23 searched objects for which an average of the observations yielded a detection. These SiO abundances are intermediate between tho se found in energetic star-forming regions (similar to 1 x 10(-8)) and upper limits of similar to 2 x 10(-12) found in cold dense clouds. We have analyzed the gas-phase chemistry of SiO and find that the temper ature-dependent reactions Si + (OH, O-2) --> SiO cannot explain the st rong dependence of SiO abundance upon cloud temperature, contrary to t he conclusion of Langer & Glassgold. We have analyzed the SiO abundanc e characteristics in terms of accretion on and photodesorption from gr ains. Cosmic-ray desorption is found to be negligible. The same analys is applied to many other species studied in the translucent cloud seri es points to a uniquely strong binding of Si atoms onto grains, unlike the situation for other elements (C, N, O, S). We find this by determ ining a photodesorption yield factor for each molecular species which is consistent with the observations of the many species in both transl ucent and cold dense clouds, which agrees with experimental values for C, N, O, and S species, and which is much smaller for Si than for the other elements. Special binding mechanisms for Si onto grains are dis cussed.