Column density ratios. Continuous lines indicate the range of solutions found in the hypothesis of optically thin lines in the range of gas temperature 20−100 K. Dashed lines indicate upper limits from undetected lines. The possibility that CS and H₂ CO lines are optically thick is reflected by the thin dotted lines. When a bar falls outside of the diagram, the upper limit is indicated. Sources are ordered as (1) DG Tau, (2) DG Tau B, (3) HL Tau, (4) Haro 6-13, and (5) IRAS 04302.