TMC-1C: An accreting starless core

We have mapped the starless core TMC-1C in a variety of molecular lines with the IRAM 30 m telescope. High-density tracers show clear signs of self-absorption, and subsonic infall asymmetries are present in N ₂H⁺(1-0) and DCO⁺(2-1) lines. The inward velocity profile in N₂H⁺(1-0) is extended over a region about 7000 AU in radius around the dust continuum peak, which is the most extended "infalling" region observed in a starless core with this tracer. The kinetic temperature (∼12 K) measured from C¹⁷O and C¹⁸O suggests that their emission comes from a shell outside the colder interior traced by the millimeter continuum dust. The C ¹⁸O(2-1) excitation temperature drops from 12 to ≃ 10 K away from the center. This is consistent with a volume density drop of the gas traced by the C¹⁸O lines, from ≃4 × 10⁴ cm ^-3 toward the dust peak to ≃6 × 10³ cm ^-3 at a projected distance from the dust peak of 80″ (or 11,000 AU). The column density implied by the gas and dust show similar N ₂H⁺ and CO depletion factors (f_D ≤ 6). This can be explained with a simple scenario in which: (1) the TMC-1C core is embedded in a relatively dense environment [n(H₂) ≃ 10 ⁴ cm^-3], where CO is mostly in the gas phase and the N₂H⁺ abundance had time to reach equilibrium values; (2) the surrounding material (rich in CO and N₂H⁺) is accreting onto the dense core nucleus; (3) TMC-1C is older than 3 × 10⁵ yr, to account for the observed abundance of N₂H ⁺ across the core (≃ 10^-10 with respect to H ₂); and (4) the core nucleus is either much younger (≃10 ⁴ yr) or "undepleted" material from the surrounding envelope has fallen toward it in the past 10,000 yr.