SO and CS observations of molecular clouds - II. Analysis and modelling ofthe abundance ratios - probing O-2/CO with SO/CS?

We here analyse the observational SO and CS data presented in Nilsson ct al . (2000). The SO/CS integrated intensity ratio maps are presented for 19 mo lecular clouds, together with tables of relevant ratios at strategic positi ons, where we have also observed (SO)-S-34 and/or (CS)-S-34. The SO/CS abun dance ratio as calculated from an LTE analysis is highly varying within and between the sources. Our isotopomer observations and Monte Carlo simulatio ns verify that this is not an artifact due to optical depth problems. The v ariation of the maximum SO/CS abundance ratio between the clouds is 0.2-7. The largest variations within a cloud are found for the most nearby objects , possibly indicating resolution effects. We have also performed time depen dent chemical simulations. We compare the simulations with our observed SO/ CS abundance ratios and suggest a varying oxygen to carbon initial abundanc e, differing temporal evolution, density differences and X-ray sources asso ciated with young stellar objects as possible explanations to the variation s. In particular, the observed variation of the maximum SO/CS abundance rat io between the clouds can be explained by using initial O/C+ abundance rati os in the range 1.3-2.5. We finally derive a relationship between the SO/CS and O-2/CO abundance ratios, which may be used as a guide to find the most promising interstellar O-2 search targets.