SIMBAD references

We compare the results of our JCMT spectral line survey of molecular gas towards ultracompact HII regions with the predictions of models of sulphur chemistry in hot cores. We investigate the range of evolutionary models that are consistent with the observed physical conditions and chemical abundances, and see to what extent it is possible to constrain core ages by comparing abundances with the predictions of chemical models. The observed abundance ratios vary little from source to source, suggesting that all the sources are at a similar evolutionary stage. The models are capable of predicting the observed abundances of H₂S, SO, SO₂, and CS. The models fail to predict the amount of OCS observed, suggesting that an alternative formation route is required. An initial H₂S abundance from grain mantle evaporation of ∼10^–7 is preferred.