SIMBAD references

The presence of depletion (freeze-out) of CO around low-mass protostars is well established. Here we observe 84 candidate young high-mass sources in the rare isotopologues C¹⁷O and C¹⁸O to investigate whether there is evidence for depletion towards these objects. Observations of the J=2->1 transitions of C¹⁸O and C¹⁷O are used to derive the column densities of gas towards the sources and these are compared with those derived from submillimetre continuum observations. The derived fractional abundance suggests that the CO species show a range of degrees of depletion towards the objects. We then use the radiative transfer code RATRAN to model a selection of the sources to confirm that the spread of abundances is not a result of assumptions made when calculating the column densities. We find a range of abundances of C¹⁷O that cannot be accounted for by global variations in either the temperature or dust properties and so must reflect source to source variations. The most likely explanation is that different sources show different degrees of depletion of the CO. Comparison of the C¹⁷O linewidths of our sources with those of CS presented by other authors reveal a division of the sources into two groups. Sources with a CS linewidth >3km/s have low abundances of C¹⁷O while sources with narrower CS lines have typically higher C¹⁷O abundances. We suggest that this represents an evolutionary trend. Depletion towards these objects shows that the gas remains cold and dense for long enough for the trace species to deplete. The range of depletion measured suggests that these objects have lifetimes of 2-4x10⁵-years.