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Astron. Astrophys. 363, L9-L12 (2000)

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1. Introduction

The envelope expelled by the carbon star CW Leo/IRC +10216 is remarkably rich in reactive species and in refractory compounds. Most of the stable metal-bearing molecules are observed fairly close the star. The reactive species, in particular the linear carbon-chain radicals, are generally confined to a thin shell in the outer envelope. Whereas the stable metal compounds form in the hot stellar atmosphere before being expelled in the envelope, the C-chains must be formed in situ in the cold, rather tenuous circumstellar medium.

Previous to the work here, only four metal-containing radicals have been identified in IRC +10216: SiC, SiN, MgNC and MgCN. Interferometric maps of the fairly strong mm-wave emission of SiC and MgNC indicate that these radicals exist in the same outer shell as the carbon chains. This somewhat unexpected finding may be important to understand the interaction of metals with grains and the depletion of metals in the gas ejected by objects like IRC +10216 into the interstellar medium (see e.g. Guélin et al. 1993, Turner 1994, Glassgold 1996). For this reason, detection of more metal-containing radicals is clearly of interest.

The small number of metallic radicals observed in space is largely due to the lack of accurate spectroscopic data. Recently, four of us succeeded in detecting in a laboratory discharge three silicon-bearing radicals of astrophysical interest: SiCCH, SiCN and SiNC (Apponi et al. 2000, hereafter AMGT). The microwave spectra of the ground states of these radicals are now accurately measured, which prom-pted us to search for the first two species in space. In this Letter, we report the discovery of SiCN in IRC +10216.

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© European Southern Observatory (ESO) 2000

Online publication: December 5, 2000