  Linear carbon chains are common in various types of astronomical molecular
sources. Possible formation mechanisms involve both bottom-up and top-down
routes. We have carried out a combined observational and modeling study of the
formation of carbon chains in the C-star envelope IRC+10216, where the
polymerization of acetylene and hydrogen cyanide induced by ultraviolet photons
can drive the formation of linear carbon chains of increasing length. We have
used ALMA to map the emission of 3 mm rotational lines of the hydrocarbon
radicals C2H, C4H, and C6H, and the CN-containing species CN, C3N, HC3N, and
HC5N with an angular resolution of 1". The spatial distribution of all these
species is a hollow, 5-10" wide, spherical shell located at a radius of 10-20"
from the star, with no appreciable emission close to the star. Our observations
resolve the broad shell of carbon chains into thinner sub-shells which are 1-2"
wide and not fully 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 HC3N 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 C2H and C4H radicals with
unsaturated hydrocarbons, while cyanopolyynes are mainly formed from polyynes
in gas-phase reactions with CN and C3N radicals.
