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

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 H 2 O 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 SiC 2 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., H 2 O), 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.

Dust formation in the oxygen-rich AGB star IK Tauri

“…To describe the physical conditions experienced by the upper atmosphere of IK Tau, lifted by the recurrent passage of shocks induced by the stellar pulsation, we use the semianalytical model of the inner wind of AGB stars developed by Cherchneff et al (1992) and Cherchneff (1996), and used by Willacy & Cherchneff (1998), Duari et al (1999), Cau (2002), and Cherchneff (2006Cherchneff ( , 2011Cherchneff ( , 2012. The post-shock gas consists of (1) a narrow layer in the immediate back of the shock front where the highly endothermic, collision-induced dissociation of molecular hydrogen provides the gas cooling through the reaction H 2 + H → H + H + H; and (2) a region where the gas cools by adiabatic expansion, expands to larger radii, and falls back to its initial position owing to the stellar gravity.…”

“…2. We direct the reader to the papers by Cherchneff et al (1992), Cherchneff (1996), Willacy & Cherchneff (1998), Duari et al (1999), Cau (2002) and Cherchneff (2006Cherchneff ( , 2011Cherchneff ( , 2012 for full details of the formalism and model we use to describe the inner wind of AGB stars. This simple analytical model does not represent a fully consistent picture of the shocked upper stellar atmosphere and does not aim to reproduce mass-loss rates in AGB stars.…”

“…Following Cherchneff (2011aCherchneff ( , 2012, the implementation of rates for reverse reactions was changed to accommodate available new rates measured in combustion and aerosol chemistry. The rate of a specific reverse process was not calculated from the equilibrium constant by assuming detailed balance as in Duari et al (1999) and Cherchneff (2006), but directly entered in the chemical network with its available measured, calculated, or estimated value.…”

“…This new, purely kinetic approach has been tested for the case of the carbon star IRC+10216 and provides better results when compared to observational data. Specifically, the formation of water and other relevant molecules in carbon stars was successfully explained by this revised shock-induced chemistry applied to the inner stellar wind (Cherchneff 2011a(Cherchneff , 2012.…”

“…This molecule traps all oxygen (carbon) atoms in a carbon-rich (oxygen-rich) gas. The partial dissociation of CO by the shocks releases oxygen atoms in carbon-rich environments and carbon atoms in oxygen-rich environments, leading to a dual chemistry that forms O-bearing species like water, H 2 O, in the wind of carbon stars, and C-bearing species in the wind of O-rich AGBs (Willacy & Cherchneff 1998;Duari et al 1999;Cherchneff 2006Cherchneff , 2011aCherchneff , 2012. The widespread occurrence of warm water in several carbon stars has been highlighted with Herschel (Neufeld et al 2011) whereas hydrogen cyanide, HCN, was detected close to the stellar surface in O-rich Miras, again with Herschel (Justtanont et al 2012).…”