2000ApJ...542..870D

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 NH₃(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 HC₃N, N₂H⁺, and CS. The average density toward all lines of sight was 2x10⁴ 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 in the abundance of HC₃N 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).