Rotational diagram of HC₃N for the broad velocity component towards G328.2551-0.5321. The rotational diagram when we assume that HC₃N lines are optically thin is shown in blue. We represent the rotational diagram corrected for the opacity of each line in red. Rotational temperatures and column densities are indicated with the same colour code. The beam dilution was calculated for each transition for a source size of 4″. The error bars represent a lσ- statistical noise plus a 20% calibration error. To calculate the uncertainties on the rotational temperature and the column density, we used the Monte Carlo method and assumed a uniform distribution of the error for each data point.