1999ApJ...523L.165C

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 n_d∼10⁵ cm^–3. The corresponding radius of the ``depleted region'' is r_d∼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 n_d_. We conclude from our data that the core is probably young (∼10⁴ 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.