The formation of silicon-carbon chain molecules in IRC+10216

Abstract

Linear silicon-carbon chain molecules, SiC$_n$ (n = 3--6), have been detected in the circumstellar envelope of the carbon-rich AGB star IRC+10216. Here, we discuss the mechanisms likely to be at work in the synthesis of chains up to SiC$_8$ and present for the first time a detailed comparison between observations and model calculations. Using a combination of measured and postulated rate coefficients, we show that the column densities of the silicon carbide molecules observed in IRC+10216 can be reproduced to with a factor of four. Furthermore, our model calculations suggest that the column densities of SiC$_7$ and SiC$_8$ are a factor of 2--4 less than that of SiC$_6$. We discuss the influence of neutral-neutral reactions involving parent SiC$_2$ on chain growth and show that their neglect leads to a reduction in column densities of carbides up to a maximum of about six for SiC$_6$ and SiC$_7$. Our results suggest that the detection of SiC$_7$ and SiC$_8$ in IRC+10216 will be challenging but that astronomical searches should be undertaken.