(a) Map (colour image) of the rotational temperature obtained from the rotation diagram of the CH₃CCH molecule. Contours represent the same map of the 3 mm continuum emission as in Fig. 1c. (b) Map of the relative error on the rotation temperature. (c) Same as panel a, for the CH₃CCH column density. (d) Same as panel b, for the CH₃CCH column density. The synthesised beam is shown in the bottom-right corner.