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 N₂H⁺(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 C<sup>17</sup>O and C<sup>18</sup>O suggests that their emission comes from a shell outside the colder interior traced by the millimeter continuum dust. The C<sup>18</sup>O(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 C<sup>18</sup>O lines, from ~=4×10<sup>4</sup> cm<sup>-3</sup> toward the dust peak to ~=6×10<sup>3</sup> cm<sup>-3</sup> 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<sub>2</sub>H<sup>+</sup> and CO depletion factors (f<sub>D</sub><=6). This can be explained with a simple scenario in which: (1) the TMC-1C core is embedded in a relatively dense environment [n(H<sub>2</sub>)~=10<sup>4</sup> cm<sup>-3</sup>], where CO is mostly in the gas phase and the N<sub>2</sub>H<sup>+</sup> abundance had time to reach equilibrium values; (2) the surrounding material (rich in CO and N<sub>2</sub>H <sup>+</sup>) is accreting onto the dense core nucleus; (3) TMC-1C is older than 3×10<sup>5</sup> yr, to account for the observed abundance of N<sub>2</sub>H <sup>+</sup> across the core (~=10<sup>-10</sup> with respect to H<sub>2</sub> ); and (4) the core nucleus is either much younger (~=10<sup>4</sup> yr) or ``undepleted'' material from the surrounding envelope has fallen toward it in the past 10,000 yr.