A study of the physics and chemistry of L134N

We have carried out a comprehensive and self-consistent study of the physic al and chemical state of the core of the dark cloud L134N (L183), whose mol ecular abundances provide a standard against which chemical models may be c ompared. We used observations of the NH3(1, 1) and (2, 2) rotation-inversio n transitions to estimate the kinetic temperature, which was found to be co nsistent with 10 K and not varying with position. Densities were determined from multitransition statistical equilibrium calculations for HC3N, N2H+, and CS. The average density toward all lines of sight was 2 x 10(4) cm(-3). As found by previous studies, the emission from various molecular species peaks in different positions: SO and SO2 peak west of the central position, which is the location of the strongest emission from (e.g.) N2H+ and CH3OH , with a second peak occurring for NH3 and HC3N to the north of the center. The most striking abundance variations occur in a north-south cut through the core center for HC3N, C2H, CS, SO, and SO2. A north to south decrease i n the abundance of HC3N and CS and a dramatic change in the CS/SO ratio, wh ich 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 nort hwest end of the ridge).