Growth of carbon chains in IRC +10216 mapped with ALMA

Abstract

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 λ 3 mm rotational lines of the hydrocarbon radicals C 2 H, C 4 H, and C 6 H, and the CN-containing species CN, C 3 N, HC 3 N, and HC 5 N 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 subshells that 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 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.

Determining the effects of clumping and porosity on the chemistry in a non-uniform AGB outflow

“…Highly porous models withṀ = 10 −5 M yr −1 have a peak value close to the observed abundance, both for a one-and a two-component model. HC 3 N is located in a molecular shell around 3 × 10 16 cm in the outflow of IRC+10216 (Agúndez et al 2017). The location We note that models with f vol = 0.4 have the same porosity length h * = 2.5 × 10 13 cm as the one-component models with f vol = 0.2, l * = 5 × 10 12 cm and f vol = 0.4, l * = 1 × 10 13 cm.…”

The Maser-emitting Structure and Time Variability of the SiS Lines J=14−13 and 15-14 in IRC+10216

“…These scenarios could be compatible with the complex structure composed of thin arcs found in the envelope of IRC+10216 (Mauron & Huggins 1999, 2000Leão et al 2006;Decin et al 2015;Quintana-Lacaci et al 2016;Guélin et al 2018). These structures are predicted to appear naturally in envelopes surrounding single stars by Woitke (2006) and in binary systems by Mastrodemos & Morris (1999), and/or with the peculiar formation of molecules such as H 2 O, C 2 H 4 , C 4 H 2 , and carbon chains close to the star (Agúndez et al 2010(Agúndez et al , 2017Fonfría et al 2017a,b).…”

AGB outflows as tests of chemical kinetics and radiative transfer models

“…Its outflow is not smooth: density-enhanced shells are visible throughout the outflow. These shells are moreover distinctly clumpy (Agúndez et al 2017). Despite several studies, the relative distribution of the cyanopolyynes, HC 2n+1 N, and hydrocarbon radicals, C n H (n = 1, 2, 3...), within its CSE is not yet understood.…”

“…The cyanopolyynes follow such a radial distribution (Millar & Herbst 1994;Millar et al 2000;Agúndez et al 2017). The hydrocarbon radicals, however, are observed to be cospatial (Guélin et al 1993(Guélin et al , 1999Agúndez et al 2017). Several chemical models have attempted to explain this behaviour.…”