concentric, indicating that the mass loss process has been discontinuous and not fully isotropic. The radial distributions of the species mapped reveal subtle differences: while the hydrocarbon radicals have very similar radial distributions, the CN-containing species show more diverse distributions, with HC$_3$N appearing earlier in the expansion and the radical CN extending later than the rest of the species. The observed morphology can be rationalized by a chemical model in which the growth of polyynes is mainly produced by rapid gas-phase chemical reactions of C$_2$H and C$_4$H radicals with unsaturated hydrocarbons, while cyanopolyynes are mainly formed from polyynes in gas-phase reactions with CN and C$_3$N radicals.]{}

Introduction
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Linear acetylenic carbon chains, with a polyyne ($-$C$\equiv$C)$_n$$-$H or cyanopolyyne ($-$C$\equiv$C)$_n$$-$C$\equiv$N backbone, have long been observed in a wide range of astronomical environments, such as cold molecular clouds, star-forming regions, photon-dominated regions, and the ejecta of evolved stars ([@mor1976] 1976; [@win1978] 1978; [@gue1978] 1978; [@bro1978] 1978; [@buj1981] 1981; [@cer1996] 1996; [@par2005] 2005; [@gup2009] 2009; see [@loo2016] 2016 for a recent disproof of HC$_{11}$N detection). The chemical synthesis of carbon chains in these environments has been a matter of debate over the years, with hypotheses based on top-down mechanisms involving very small carbonaceous 