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 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 (similar to 12 K) measured from (CO)-O-17 and (CO)-O-18 suggests that their emission comes from a shell outside the colder interior traced by the millimeter continuum dust. The (CO)-O-18(2-1) excitation temperature drops from 12 to similar or equal to 10 K away from the center. This is consistent with a volume density drop of the gas traced by the C18O lines, from similar or equal to 4x10(4) cm(-3) toward the dust peak to similar or equal to 6x10(3) cm(-3) at a projected distance from the dust peak of 80 '' (or 11,000AU). The column density implied by the gas and dust show similar N2H+ and CO depletion factors (fD <= 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) similar or equal to 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 3x10(5) yr, to account for the observed abundance of N2H+ across the core (similar or equal to 10(-10) with respect to H-2); and (4) the core nucleus is either much younger (similar or equal to 10(4) yr) or "undepleted'' material from the surrounding envelope has fallen toward it in the past 10,000 yr.