Self-consistent treatment of dynamics and chemistry in the winds from carbon-rich AGB stars^*

I. Tests of the equilibrium and kinetic chemical codes

¹ N. Copernicus Astronomical Center, Rabiańska 8, 87–100 Toruń, Poland e-mail: mara@ncac.torun.pl
² Institute of Astronomy, Russian Academy of Sciences, Pyatnitskaya 48, Moscow, Russian Federation

Received: 17 October 2006
Accepted: 27 February 2007

Abstract

Aims.The main aim of this paper was to test our (chemical and kinetic) codes, which will be used during self-consistent modelling of dynamics and chemistry in the winds from C-rich AGB stars.

Methods.We used the thermodynamical equilibrium code to test the different databases of dissociation constants. We also calculated the equilibrium content of the gas using the kinetic code that includes the chemical network of neutral-neutral reactions. The influence of reaction rates updated using the UMIST database for Astrochemistry 2005 (UDFA05) was tested.

Results. The local thermodynamical equilibrium calculations show that the NIST database reproduces equilibrium concentrations fairly well in comparison with previous computations, while consistency for the other, commonly used, dissociation constants is worse.
The most important finding is that the steady state solution obtained with the kinetic code for the reactions network is different from the thermodynamical equilibrium solution. In particular, the important opacity sources CN and C₂ are underabundant relative to thermodynamical equilibrium, while O-bearing molecules (like SiO, H₂O, and OH) are overabundant. After updating the reaction rates by data from the UDFA05 database, the consistency in O-bearing species becomes much better, however the disagreement in C-bearing species is still present.