CO Depletion in the Starless Cloud Core L1544

Abstract

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 observations are consistent with a model in which CO is condensed out onto dust 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 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 (HCO+)-O-18 and (DCO+)-C-13, show double-peaked profiles which suggest that we are observing superposed emission from the foreground and background undepleted layers with density below n(d). We conclude from our data that the core is probably young (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 dense cloud cores.