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.
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