A Study of the Physics and Chemistry of L134N

We have carried out a comprehensive and self-consistent study of the physical and chemical state of the core of the dark cloud L134N (L183), whose molecular abundances provide a standard against which chemical models may be compared. We used observations of the NH 3 (1, 1) and (2, 2) rotation-inversion transitions to estimate the kinetic temperature, which was found to be consistent with 10 K and not varying with position. Densities were determined from multitransition statistical equilibrium calculations for HC 3 N, N 2 H + , and CS. The average density toward all lines of sight was 2 × 10 4 cm -3 . As found by previous studies, the emission from various molecular species peaks in different positions: SO and SO 2 peak west of the central position, which is the location of the strongest emission from (e.g.) N 2 H + and CH 3 OH, with a second peak occurring for NH 3 and HC 3 N to the north of the center. The most striking abundance variations occur in a north-south cut through the core center for HC 3 N, C 2 H, CS, SO, and SO 2 . A north to south decrease in the abundance of HC 3 N and CS and a dramatic change in the CS/SO ratio, which has been shown to be a sensitive tracer of chemical evolutionary state, suggests that the north is at a younger evolutionary state than the south. Despite the "youth" of the N position, the CS/SO ratio suggests that it is still as "old" as or older than the most evolved region in TMC-1 (the northwest end of the ridge).