SIMBAD references

We present observations of H₂S, SO and SO₂ towards a sample of Class 0 and Class I sources. The H₂S abundances rule out its formation in the gas phase and we assume that it is released from grain mantles. Applying a model of the evolution of the sulphur chemistry first used in the study of hot cores, we show that this model can reproduce the results of our observations even for the relatively low temperatures and densities of these sources. We demonstrate that the chemical evolution of sulphur bearing species is a potentially valuable probe of chemical timescales in low mass star forming regions. Overall, the model indicates that Class I sources are at a later chemical evolutionary stage than their Class 0 counterparts, with an average difference in chemical age of at least 3x10³ years and an upper limit on the age of the Class I sources of a few times 10⁵ years.