1 Instituto de Ciencia de Materiales de Madrid, CSIC, C/ Sor Juana Inés de la Cruz 3, 28049 Cantoblanco, Spain
2 Institut de Radioastronomie Millimétrique, 300 rue de la Piscine, 38406 St.-Martin-d’Hères, France
3 Université de Toulouse, UPS-OMS, IRAP, 31000 Toulouse, France
4 CNRS, IRAP, 9 Av. Colonel Roche, BP 44346, 31028 Toulouse Cedex 4, France
5 Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, MA 02138, USA
6 School of Engineering and Applied Sciences, Harvard University, Cambridge, MA 02138, USA
Received: 17 December 2016
Accepted: 13 February 2017
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 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 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 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.
Key words: astrochemistry / molecular processes / techniques: interferometric / stars: AGB and post-AGB / circumstellar matter / radio lines: stars
Based on observations carried out with ALMA and the IRAM 30 m Telescope. ALMA is a partnership of ESO (representing its member states), NSF (USA) and NINS (Japan), together with NRC (Canada) and NSC and ASIAA (Taiwan), in cooperation with the Republic of Chile. The Joint ALMA Observatory is operated by ESO, AUI/NRAO and NAOJ. IRAM is supported by INSU/CNRS (France), MPG (Germany) and IGN (Spain). This paper makes use of the following ALMA data: ADS/JAO.ALMA#2013.1.00432.S.
Data cube (FITS file) is available at the CDS via anonymous ftp to cdsarc.u-strasbg.fr (220.127.116.11) or via http://cdsarc.u-strasbg.fr/viz-bin/qcat?J/A+A/601/A4
© ESO, 2017