Chemical constraints and microstructure in TMC-1 core D

Microstructure has been detected in Core D of TMC-1. Unless it is confined or replenished by an as vet unexamined mechanism, this microstructure will dissipate on the sound-crossing timescale, which is less than 10(5) yr. We reconsider the large number of models that have been proposed to explain chemical Variations in TMC-1 in an effort to determine whether chemical constraints require the microstructure to be confined or replenished for times much longer than the sound crossing timescale. We explore here a chemical model which, though consistent with the assumption of an age of B 10(4) yr and a number density of H nuclei of 2 10(5) cm(-3), shows the richness in both molecular variety and abundances observed in Core D. In particular, the computed HC3N fractional abundance relative to H-2 is 6 10(-8), in agreement, with the latest observations of Ohishi & Kaifu (1998). Apparently, the chemistry of TMC-1 cannot be used to discard the possibility that TMC-1 Core D is young. This model has the following characteristics: the cosmic ray ionization rate is consistently larger than that usually assumed for dark regional carbon atoms and hydrocarbons that strike grains are rapidly hydrogenated and promptly returned to the gas phase as methane; CO and N-2 striking grains are immediately returned to the gas phase unaltered; other chemical species containing at least one atom more massive than helium colliding with dust grains remain frozen on their surfaces;;ind the material other than hydrogen was initially in atomic form. For such a model to be viable, collapse of Core D must have been triggered by a stellar wind-driven shock of several kms This speed is low enough that magnetic moderation of the shock would have prevented the activation of a high temperature chemistry. The model results indicate that Core D would have fractional abundances of H2O and O-2 at a core age of g 10(4) yr consistent with the upper limits placed by very recent observations of Core D made with SWAS. The implications of this study are (1) that hydrogenation of atoms at the surfaces of dust grains may be a significant contributor to the chemistry of dark clouds; (2) that the special chemical nature of TMC-1 is due primarily to the exceptional youth of Core D.