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

Abstract

Aims. New SiO multi-transition millimetre line observations of a sample of carbon stars, including J = 8 → 7 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.

Circumstellar molecular line emission from S-type AGB stars: mass-loss rates and SiO abundances

“…(2) and that the size of the SiO emitting region does not differ significantly between the different chemical types. This is in agreement with the results of Schöier et al (2006b). Figure 6a shows our mass-loss-rate results for the S-type AGB star sample compared with previous results for carbon stars Table 6.…”

“…There exists strong evidence that the circumstellar SiO emission carries information on the region where the mass loss is initiated and where the dust formation takes place (Reid & Moran 1981;Reid & Menten 1997;Schöier et al 2006a) making it a particularly interesting molecule to study. Schöier et al (2006b) modelled circumstellar SiO line observations from a sample of carbon stars and when comparing this to a similar analysis performed by González Delgado et al (2003) for a large sample of M-type AGB stars, they found no apparent distinction between their circumstellar SiO abundance distributions. For the carbon stars, the derived abundances are several orders of magnitude higher than expected from thermal equilibrium stellar atmosphere chemistry.…”

“…We find no correlation between the LRS class and the derived SiO abundance for the S-type stars. The derived circumstellar SiO abundances for the sample of S-type AGB stars are compared to those obtained for carbon stars (Schöier et al 2006b) and M-type AGB stars (González Delgado et al 2003) in Fig. 3.…”

“…The median value is 6×10 −6 . Figure 3 shows a histogram of the circumstellar SiO abundances for the S-type stars compared to those found for M-type stars (González Delgado et al 2003) and carbon stars (Schöier et al 2006b). The spread of the SiO abundance distribution for the S-type stars (as measured by the ratio between the 90th percentile and the 10th percentile) is about a factor of 25.…”

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

“…More generally, Cherchneff (2006) studied the inner wind composition as a function of the carbon-to-oxygen (C/O) ratio of the stellar photosphere and confirmed the formation of O-bearing species in carbon stars and that of C-bearing molecules in O-rich AGB stars as a result of non-equilibrium chemistry induced by periodic shocks. Several observations of high energy rotational transitions of HCN, SiS, CS, and SiO in AGBs and supergiants at mm and submm wavelengths corroborated these results (e.g., Schöier et al 2006Schöier et al , 2007Ziurys et al 2007Ziurys et al , 2009Decin et al 2008).…”