2000ApJ...542..367T

We have mapped the Heiles cloud 2 region in the Taurus molecular cloud complex with H¹³CO⁺ (J=1-0), CH₃OH (J_K=2₀-1₀ A⁺), and C³⁴S (J=2-1) lines. Dense gas traced by the mapped lines with critical densities higher than 10⁴ cm^–3 is concentrated in four condensations, that is, the TMC 1 and TMC 1C filaments, L1527, and TMC 1A. We have found that the three emission lines have remarkably different spatial distributions. The H¹³CO⁺ emission traces well dense cores harboring protostars, while the CH₃OH emission is weak toward the protostars and is rather enhanced toward cores without protostars. We found that there are two starless cores with enhanced CH₃OH emission at the northwestern ends of the TMC 1 and TMC 1C filaments, toward which the H¹³CO⁺ emission is barely seen. On the basis of the analyses using the large velocity gradient (LVG) model, we show that the CH₃OH abundance relative to H¹³CO⁺ is enhanced by up to 1 order of magnitude in the cores without protostars. The C³⁴S abundance relative to H¹³CO⁺ also shows a similar trend to that of CH₃OH. Such an abundance variation between H¹³CO⁺ and CH₃OH and C³⁴S can be explained in the scheme of time-dependent gas-phase chemical evolution, which predicts that CH₃OH and C³⁴S are abundant in the early stages of chemical evolution and become deficient in the later stages. A comparison of the spatial-velocity structures in TMC 1 observed with the three molecular lines suggests that this cloud consists of multiple components with different velocities and different chemical compositions along the line of sight.