5 Conclusions

We have carried out a frequency sweep of the carbon-rich star IRC+10216 between 129 and 172.5 GHz. 380 lines were detected, of which 317 have been identified and assigned to 30 different molecular species.

The most recurrent molecule in IRC+10216's 2 mm spectrum is C₄H, whose different isotopomers and vibrationally excited states give rise to 60 lines. It is followed by SiC₂, with 44 lines from the three stable silicon isotopomers and from Si¹³CC. NaCN with 23 lines is also a major polluant of the spectrum.

HC₃N and its isotopomers account for 21 lines. Other carbon chains like HCCN, C₃H, CH₃CN, HC₅N, C₃N, H₂C₄ (Cernicharo et al. [1991b]), C₅H and C₆H account for 75 lines. Altogether, the above carbon chain molecules give rise to 36% of lines observed in the 2-mm window, a proportion which rises to 41% if the C_nS chains are included.

The main characteristics of the IRC+10216 molecular envelope, compared to warm and dense interstellar clouds with similar physical conditions, are i) the large number of carbon chain molecules and radicals, as well as of silicon, sulfur and metallic compounds, and ii) the small number of saturated species and oxygen compounds. As a result, the 2-mm spectra of IRC+10216 and Orion A (see Ziurys & McGonagle 1993) look almost orthogonal, i.e. show a remarkably small number of common lines.

Acknowledgements

The present survey includes spectra taken in collaboration with J. Gómez-González, S. Saito, S. Yamamoto and L. Ziurys, who we would like to thank. This work has been partially supported by the Spanish DGICYT under grants PB90-408, PB93-048, PB96-0883 and PNIE98-1351E. J. Cernicharo was at CNRS-Observatoire de Grenoble when the survey was initiated and stayed at the Observatorio Astronómico Nacional (OAN) for several years while working on the data.