New molecules in IRC+10216: confirmation of C$_5$S and tentative
  identification of MgCCH, NCCP, and SiH$_3$CN
  The C-star envelope IRC+10216 harbors a rich variety of molecules, with more
than 80 detected to date. During the course of a $\lambda$ 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, that we assign to rotational
transitions of four new molecules. The observation of three lines of C$_5$S
confirms a previous tentative identification of this molecule by Bell et al.
(1993) 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 SiH$_3$CN, the phosphorus and silicon analogs
of cyanogen (NCCN) and methyl cyanide (CH$_3$CN). We derive the following
column densities: $N$(C$_5$S) = (2-14) $\times$ 10$^{12}$ cm$^{-2}$ (depending
on the rotational temperature adopted), $N$(MgCCH) = 2 $\times$ 10$^{12}$
cm$^{-2}$, $N$(NCCP) = 7 $\times$ 10$^{11}$ cm$^{-2}$, and $N$(SiH$_3$CN) =
10$^{12}$ cm$^{-2}$. The S-bearing carbon chain C$_5$S is less abundant than
C$_3$S, while MgCCH has an abundance in between that of MgNC and those of MgCN
and HMgNC. On the other hand, NCCP and SiH$_3$CN 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.
