The inner wind of IRC+10216 revisited: new exotic chemistry and diagnostic for dust condensation in carbon stars^⋆

Departement Physik, Universität Basel, Klingelbergstrasse 82, 4056 Basel, Switzerland
e-mail: isabelle.cherchneff@unibas.ch

Received: 29 November 2011
Accepted: 26 April 2012

Abstract

Aims. We model the chemistry of the inner wind of the carbon star IRC+10216 and consider the effects of periodic shocks induced by the stellar pulsation on the gas to follow the non-equilibrium chemistry in the shocked gas layers. We consider a very complete set of chemical families, including hydrocarbons and aromatics, hydrides, halogens, and phosphorous-bearing species. Our derived abundances are compared to those for the latest observational data from large surveys and the Herschel telescope.

Methods. A semi-analytical formalism based on parameterised fluid equations is used to describe the gas density, velocity, and temperature from 1 R_⋆ to 5 R_⋆. The chemistry is described using a chemical kinetic network of reactions and a set of stiff, ordinary, coupled differential equations is solved.

Results. The shocks induce an active non-equilibrium chemistry in the dust formation zone of IRC+10216 where the collision destruction of CO in the post-shock gas triggers the formation of O-bearing species such as H₂O and SiO. Most of the modelled molecular abundances agree very well with the latest values derived from Herschel data on IRC+10216. The hydrides form a family of abundant species that are expelled into the intermediate envelope. In particular, HF traps all the atomic fluorine in the dust formation zone. The halogens are also abundant and their chemistry is independent of the C/O ratio of the star. Therefore, HCl and other Cl-bearing species should also be present in the inner wind of O-rich AGB or supergiant stars. We identify a specific region ranging from 2.5 R_⋆ to 4 R_⋆, where polycyclic aromatic hydrocarbons form and grow. The estimated carbon dust-to-gas mass ratio derived from the mass of aromatics formed ranges from 1.2 × 10^-3 to 5.8 × 10^-3 and agrees well with existing values deduced from observations. This aromatic formation region is situated outside hot layers where SiC₂ is produced as a bi-product of silicon carbide dust synthesis. The MgS grains can form from the gas phase but in lower quantities than those necessary to reproduce the strength of the 30 μm emission band. Finally, we predict that some molecular lines will show a flux variation with pulsation phase and time (e.g., H₂O), while other species will not (e.g., CO). These variations merely reflect the non-equilibrium chemistry that destroys and reforms molecules over a pulsation period in the shocked gas of the dust formation zone.