CO Depletion in the Starless Cloud Core L1544

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Abstract
We present evidence for CO depletion toward the starless cloud core L1544. A comparison between C17O and the 1.3 mm continuum dust emission shows that CO is depleted by a factor of ~10 at the dust peak. Our observations are consistent with a model in which CO is condensed out onto dust grains at densities above nd ~ 105 cm-3. The corresponding radius of the "depleted region" is rd ~ 6500 AU, and we find that this depletion causes 2.3 M☉ of gas to be lost to view in molecular line emission. Optically thin high-density tracers, such as HC18O+ and D13CO+, show double-peaked profiles which suggest that we are observing superposed emission from the foreground and background undepleted layers with density below nd. We conclude from our data that the core is probably young (~104 yr old) and collapsing. For the component at VLSR = 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 dense cloud cores.