TMC-1C: An Accreting Starless Core

Abstract

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 N_2H^+(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^(17)O and C^(18)O suggests that their emission comes from a shell outside the colder interior traced by the millimeter continuum dust. The C^(18)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^(18)O lines, from ≃ 4 × 10^4 cm^(−3) toward the dust peak to ≃ 6 × 10^3 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(H_2) ≃ 10^4 cm^(−3)], where CO is mostly in the gas phase and the N_2H^+ abundance had time to reach equilibrium values; (2) the surrounding material (rich in CO and N_2H^+) is accreting onto the dense core nucleus; (3) TMC-1C is older than 3 × 10^5 yr, to account for the observed abundance of N_2H^+ 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.