CO depletion in the starless cloud core L1544

We present evidence for CO depletion toward the starless cloud core L1544. A comparison between (CO)-O-17 and the 1.3 mm continuum dust emission shows that CO is depleted by a factor of similar to 10 at the dust peak. Our obs ervations are consistent with a model in which CO is condensed out onto dus t grains at densities above n(d) similar to 10(5) cm(-3). The corresponding radius of the "depleted region" is r(d) similar to 6500 AU, and we find th at this depletion causes 2.3 M. of gas to be lost to view in molecular line emission. Optically thin high-density tracers, such as (HCO+)-O-18 and (DC O+)-C-13, show double-peaked profiles which suggest that we are observing s uperposed emission from the foreground and background undepleted layers wit h density below n(d). We conclude from our data that the core is probably y oung (similar to 10(4) yr old) and collapsing. For the component at V-LSR = 7.1 km s(-1) in this line of sight, we estimate [DCO+]/[HCO+] = 0.12 +/- 0 .02, which is larger by a factor of order 2 than values derived in other de nse cloud cores.