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. Highdensity tracers show clear signs of self-absorption, and subsonic infall asymmetries are present in N2H + (1Y0) and DCO + (2Y1) lines. The inward velocity profile in N2H + (1Y0) is extended over a regionabout 7000AU in radius around the dust continuum peak, which is the mostextended‘‘infalling’’ regionobservedin a starless core withthis 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(2Y1) 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 peakto ’6 ; 10 3 cm � 3 at aprojecteddistancefrom thedust peakof80 00 (or11,000AU). 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(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 surroundingmaterial(richinCOandN2H + )isaccretingontothedensecorenucleus;(3)TMC-1Cisolderthan3 ; 10 5 yr, to account for the observed abundance of N2H + across the core (’10 � 10 with respect to H2); 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. Subject headingg dust, extinction — stars: formation — submillimeter Online material: color figures