B of [@agu2006] 2006 and Figures 1 and 4 in [@cor2009] 2009). For the purpose of reproducing the observed radial location of carbon chains, we have adopted a $N_{\rm H}$ / $A_V$ ratio 1.5 times lower than the canonical interstellar value of [@boh1978] (1978), regardless of the implicit assumption on the dust opacity. The adopted cosmic-ray ionization rate of H$_2$ is $1.2\times10^{-17}$ s$^{-1}$, a value typical of the local interstellar medium that is well suited for IRC+10216, according to observations of HCO$^+$ ([@agu2006] 2006; [@pul2011] 2011). We use a large network of gas-phase chemical reactions, whose rate constants have been taken from the literature on gas-phase chemical kinetics and from the UMIST and KIDA databases ([@mce2013] 2013; [@wak2015] 2015). More details on the reactions important for the chemistry of carbon chains in IRC+10216 and on the rate constants are given below.

![image](fig_scheme_polyynes){width="\columnwidth"} ![image](fig_scheme_cyanopolyynes){width="\columnwidth"} ![image](fig_abundances_polyynes){width="\columnwidth"} ![image](fig_abundances_cyanopolyynes){width="\columnwidth"}

Fig. \[fig:chemistry\] summarizes some salient features of the chemical model in relation to the growth of carbon chains in IRC+10216. According to the model, the formation of carbon chains begins when the parent molecules C$_2$H$_2$ and HCN are photodissociated, in the region of the outer envelope where $A_V$$\sim$1 mag, producing the radicals C$_2$H and CN. 