SiO in C-rich circumstellar envelopes of AGB stars: effects of non-LTE chemistry and grain adsorption

¹ Stockholm Observatory, AlbaNova University Center, 106 91 Stockholm, Sweden e-mail: fredrik@astro.su.se
² Onsala Space Observatory, 439 92 Onsala, Sweden
³ European Southern Observatory, Casilla 19001, Santiago 19, Chile

Received: 31 December 2005
Accepted: 10 April 2006

Abstract

Aims.New SiO multi-transition millimetre line observations of a sample of carbon stars, including J =  observations with the APEX telescope, are used to probe the role of non-equilibrium chemistry and the influence of grains in circumstellar envelopes of carbon stars.

Methods.A detailed radiative transfer modelling, including the effect of dust emission in the excitation analysis, of the observed SiO line emission is performed. A combination of low- and high-energy lines are important in constraining the abundance distribution.

Results.It is found that the fractional abundance of SiO in these C-rich environments can be several orders of magnitude higher than predicted by equilibrium stellar atmosphere chemistry. In fact, the SiO abundance distribution of carbon stars closely mimic that of M-type (O-rich) AGB stars. A possible explanation for this behaviour is a shock-induced chemistry, but also the influence of dust grains, both as a source for depletion as well as production of SiO, needs to be further investigated. As observed for M-type AGB stars, a clear trend that the SiO fractional abundance decreases as the mass-loss rate of the star increases is found for the carbon stars. This indicates that SiO is accreted onto dust grains in the circumstellar envelopes.