SIMBAD references

A set of 32 carbon stars envelopes in a large range of spectral types has been modelled. The models were performed with two kinds of grains : SiC and amorphous carbon. Contrary to our previous study each kind of grain may be formed now at different distances from the central star. Correlations between SiC/A.C. ratio abundance and extinction opacity as well as SiC/A.C. ratio and period of luminosity were found. These correlations may indicate that the quantity of SiC grains relative to amorphous carbon grains decreases with carbon star evolution. A modification in SiC feature with increasing extinction opacity seems also to be connected to this evolution.