Abundances of CH₃CCH derived from chemical modelling for the warm-up stage of a hot core type model. Each panel represents a model with a different final collapse density, in the range n_H = 1.6 × 10⁷–6 × 10¹⁰ cm⁻³. Solid lines denote gas-phase abundances, while dotted lines indicate grain-surface abundances. The red and blue dashed lines show the observational abundances in IRAS 16293A and B, respectively.