a) Observed HC₃N/N₂H⁺ abundance ratio as a function of H₂ column density for the cores in Cha I Centre (black, filled squares), Cha I South (black, empty squares), Cha I West (black diamond), Cha I South lower limits (black, empty triangles), Cha III (pink asterisks), and Cha III lower limits (pink, empty triangle). b) Observed HC₃N/N₂H⁺ abundance ratios for Cha I (circles) and lower limits (triangles) overplotted on the 870 μm dust continuum emission obtained with LABOCA (Belloche et al. 2011a). The contour levels correspond to a, 2a, 4a, 8a, 16a, 32a, with a = 48 mJy/21′′-beam (4σ).