New molecules in IRC +10216: confirmation of C5S and tentative identification of MgCCH, NCCP, and SiH3CN

The carbon-star envelope IRC +10216 harbors a rich variety of molecules, with more than 80 detected to date. During the course of a λ 3 mm survey of IRC +10216 carried out with the IRAM 30-m telescope we have detected various weak lines, with antenna temperatures of a few mK, which we assign to rotational transitions of four new molecules. The observation of three lines of C₅S confirms a previous tentative identification of this molecule based on a line at 24.0 GHz. We also report the tentative identification of three molecules not yet observed in space: MgCCH, the first metal acetylide detected in space, and NCCP and SiH₃CN, the phosphorus and silicon analogs of cyanogen (NCCN) and methyl cyanide (CH₃CN). We derive the following column densities: N(C₅S) = (2-14) × 10¹² cm^-2 (depending on the rotational temperature adopted), N(MgCCH) = 2 × 10¹² cm^-2, N(NCCP) = 7 × 10¹¹ cm^-2, and N(SiH₃CN) = 10¹² cm^-2. The S-bearing carbon chain C₅S is less abundant than C₃S, while MgCCH has an abundance in between that of MgNC and those of MgCN and HMgNC. On the other hand, NCCP and SiH₃CN are the least abundant P- and Si-bearing molecules observed to date in IRC +10216. Based on the behavior of similar molecules it is likely that these four species are formed in the outer circumstellar layers of IRC +10216. We discuss possible gas-phase formation routes.

Based on observations carried out with the IRAM 30 m telescope. IRAM is supported by INSU/CNRS (France), MPG (Germany) and IGN (Spain).Appendix A is available in electronic form at http://www.aanda.org