χ² as afunction of H₂ volume density and column density of each of the three ¹³C isotopologues of HC₃N in TMC-1 CP. Contours correspond to 1, 2, and 3 σ levels. The three maps have the same scale in the x and y axes to facilitate the comparison. The column density of HCC¹³CN is clearly higher than those of H¹³CCCN and HC¹³CCN. The volume density of H₂ is constrained to a very narrow range, (0.9–1.1) × 10⁴ cm⁻³, by the three ¹³C isotopologues of HC₃N.