Contour plots of the HC₃N/HC₅N abundance ratio as a function of the O/H and C/H gaseous elemental abundance. The grey zone marks the region where the C/O elemental ratio is ≥1, which is unrealistic (see text). The solid lines mark the loci where the HC₃N abundance is equal to the measured one, namely between 0.5 and 5 × 10^-11. The models were obtained for a gas temperature of 40 K and a H₂ density of 1.2 × 10⁶ cm^-3, valid for the envelope of FIR4 (Ceccarelli et al. 2014). Left panel: solution for FIR4-HC₅N and FIR5, where the the measured HC₃N/HC₅N abundance ratio is 4–12 and ≤6, respectively. The cosmic-ray ionisation rate is 4 × 10^-14 s^-1 and the time is 3 × 10⁴ yr. Right panel: a possible solution for FIR4-lowHC₅N, where the measured HC₃N/HC₅N abundance ratio is 10–30. The cosmic-ray ionisation rate is 1 × 10^-17 s^-1 and the time is 1 × 10⁵ yr.