Density-Enhanced Gas and Dust Shells in a New Chemical Model for Irc+10216

Abstract

A new chemical model is presented for the carbon-rich circumstellar envelope of the AGB star IRC+10216. The model includes shells of matter with densities that are enhanced relative to the surrounding circumstellar medium. The chemical model uses an updated reaction network including reactions from the RATE06 database and a more detailed anion chemistry. In particular, new mechanisms are considered for the formation of CN − , C 3 N − and C 2 H − , and for the reactions of hydrocarbon anions with atomic nitrogen and with the most abundant cations in the circumstellar envelope. New reactions involving H − are included which result in the production of significant amounts of C 2 H − and CN − in the inner envelope. The calculated radial molecular abundance profiles for the hydrocarbons C 2 H, C 4 H and C 6 H and the cyanopolyynes HC 3 N and HC 5 N show narrow peaks which are in better agreement with observations than previous models. Thus, the narrow rings observed in molecular microwave emission surrounding IRC+10216 are interpreted as arising in regions of the envelope where the gas and dust densities are greater than the surrounding circumstellar medium. Our models show that CN − and C 2 H − may be detectable in IRC+10216 despite the very low theorised radiative electron attachment rates of their parent neutral species. We also show that magnesium isocyanide (MgNC) can be formed in the outer envelope through radiative association involving Mg + and the cyanopolyyne species.

Astronomical identification of CN-, the smallest observed molecular anion

“…The physical parameters of the envelope were taken from Agúndez (2009). The rate constants and branching ratios of the reactions of anions with H, O, and N atoms, studied in the laboratory by Eichelberger et al (2007), were updated according to the values used by Cordiner & Millar (2009) and Walsh et al (2009) 1 . Photodetachment rates of molecular anions were assumed by Millar et al (2007) to depend on the electron affinity of the neutral counterpart.…”

“…2 is the calculated radial distribu-tion of the abundances of CN − (black thin line) and some other molecular anions. CN − is predicted to form at a much greater radius than C 4 H − , C 6 H − , C 3 N − , and C 5 N − , because, unlike the other anions, it is not formed directly from the radical CN but by means of the reactions of the anions C − n (n = 5−10) with N atoms (see also Cordiner & Millar 2009). Since CN is a small molecule, the rate constant for the reaction of radiative electron attachment is likely to be very small.…”

The UMIST database for astrochemistry 2012

“…IRC+10216 has therefore been the subject of intensive ongoing astrochemical study (e.g. Bieging & Rieu 1988;Guélin et al 1999;Cordiner & Millar 2009;De Beck et al 2012;Agúndez et al 2012), with over 80 different molecules detected in this source to date. We assume the central star loses mass at a uniform rate of 1.5 × 10 −5 M yr −1 (De Beck et al 2012).…”

A line confusion limited millimeter survey of Orion KL I. Sulfur carbon chains

“…That the C 2 S/C 3 S abundance ratio is similar to that of dark clouds or evolved stars may indicate that these species formed in the gas phase. Gas phase chemical models predict C 2 S/C 3 S 2 and 0.3 in TMC-1 and IRC10216, respectively (Walsh et al 2009;Cordiner & Millar 2009). …”