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Data from the Five College Radio Astronomy Observatory CO Mapping Survey ofthe Taurus molecular cloud are combined with extinction data for a sample of292 background field stars to investigate the uptake of CO from the gas to icygrain mantles on dust within the cloud. On the assumption that the reservoir ofCO in the ices is well represented by the combined abundances of solid CO andsolid CO2 (which forms by oxidation of CO on the dust), we find that the totalcolumn density (gas + solid) correlates tightly with visual extinction (Av)over the range 5 < Av < 30 mag, i.e., up to the highest extinctions covered byour sample. The mean depletion of gas-phase CO increases monotonically fromnegligible levels for Av < 5 to approximately 30 percent at Av = 10 and 60percent at Av = 30. As these results refer to line-of-sight averages, they mustbe considered lower limits to the actual depletion at loci deep within thecloud, which may approach 100 percent. We show that it is plausible for suchhigh levels of depletion to be reached in dense cores on timescales of order0.6 Myr, comparable with their expected lifetimes. Dispersal of cores duringstar formation may be effective in maintaining observable levels of gaseous COon the longer timescales estimated for the age of the cloud.

THE UPTAKE OF INTERSTELLAR GASEOUS CO INTO ICY GRAIN MANTLES IN A QUIESCENT DARK CLOUD