Theoretical HC₃N (9–8)/HC₅N (31–30) line intensity ratio assuming LTE and optically thin conditions as a function of the gas temperature for different values of the HC₃N/HC₅N abundance ratio as marked (black curves). The range of values observed within a 2σ error bar towards the three regions listed in Table 1 are showed as dashed areas: FIR4-HC₅N (blue), FIR4-lowHC₅N (green) and FIR5 (red). We note that the temperature of FIR4 was estimated to be ~40 K (Ceccarelli et al. 2014) while no estimates for FIR5 exist.