Calculated abundances (left) and abundance ratios (right) as a function of time for a cold dense cloud. Observed values in TMC-1 (see Table 3) are indicated by horizontal dashed lines. The observed HC₂S⁺ abundance and C₂S/HC₂S⁺ ratio are an upper and a lower limit, respectively, and are indicated by vertical arrows. Dotted lines represent the calculated abundances of C₂S and HC₂S⁺ (left panel) and its ratio (right panel) when the reaction O + C₂S is neglected.