Chemical constraints and microstructure in TMC-1 core D

Citation
Tw. Hartquist et al., Chemical constraints and microstructure in TMC-1 core D, ASTRON ASTR, 369(2), 2001, pp. 605-610
Citations number
36
Language
INGLESE
art.tipo
Article
Categorie Soggetti
Space Sciences
Journal title
ASTRONOMY AND ASTROPHYSICS
ISSN journal
0004-6361 → ACNP
Volume
369
Issue
2
Year of publication
2001
Pages
605 - 610
Database
ISI
SICI code
0004-6361(200104)369:2<605:CCAMIT>2.0.ZU;2-F
Abstract
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 ch emical Variations in TMC-1 in an effort to determine whether chemical const raints require the microstructure to be confined or replenished for times m uch longer than the sound crossing timescale. We explore here a chemical mo del which, though consistent with the assumption of an age of B 10(4) yr an d a number density of H nuclei of 2 10(5) cm(-3), shows the richness in bot h molecular variety and abundances observed in Core D. In particular, the c omputed HC3N fractional abundance relative to H-2 is 6 10(-8), in agreement , with the latest observations of Ohishi & Kaifu (1998). Apparently, the ch emistry 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 io nization rate is consistently larger than that usually assumed for dark reg ional carbon atoms and hydrocarbons that strike grains are rapidly hydrogen ated 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 wi th 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, coll apse of Core D must have been triggered by a stellar wind-driven shock of s everal kms This speed is low enough that magnetic moderation of the shock w ould have prevented the activation of a high temperature chemistry. The mod el 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 thi s 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 yo uth of Core D.