Medium and far infrared observation of molecules in AGB and post-AGB C-rich stars

J. Cernicharo ¹, A. Heras ², R. Waters ³, E. González-Alfonso ¹, I. Yamamura ³, M. Guélin ⁴, R. Neri ⁴, S. Pérez-Martínez ¹, P. Cox ⁵, T. de Jong ⁶, A.G.G.M. Tielens ⁷, & P. Najarro ¹

¹ Consejo Superior de Investigaciones Cientificas. Instituto de Estructura de la Materia. Dpto Fisica Molecular. C/Serrano 121. 28006 Madrid. Spain

² ISO Science Operations Centre, Space Science Department of ESA, Villafranca, P. O. Box 50727, 28080, Madrid, Spain

³ Astronomical Institute Anton Pannekoek, Amsterdan, The Netherlands

⁴ IRAM, Grenoble, France

⁵ IAS, Orsay, France

⁶ University of Amsterdan, Amsterdam, The Netherlands

⁷ Kapteyn Institute, Groningen, The Netherlands

We present the last ISO results concerning the infrared emission of AGB and post-AGB C-rich stars. In AGB stars like IRC+10216 the spectrum is dominated by the absorption bands of C₂H₂ and HCN, although some HCN bands appear in emission (see also poster by E. Gonález-Alfonso and Cernicharo). Many others molecular bands are seen in IRC+10216 that remain unidentified. We discuss the possible carriers of these species. In proto-planetary nebula like CRL618 the infrared data from ISO shows an incredible abundance of carbon chains and small hydrocarbons. The interpretation of the infrared spectrum of this object require an important help from laboratory data because the IR spectrum is particularly rich in absorption bands of gas phase molecules. Millimeter wave observations of the same object indicate that the emission arises from the innermost shocked region resulting from the interaction of the extremely fast wind from the central star and the remmant envelope of the AGB phase of CRL618. Among the new molecular species detected for the first time in the CRL618, and also in the interstellar space, are C₄H₂ and C₆H₂, and perhaps some small hydrocarbons like CH₃CH₃ and CH₃CH₂CH₃, and for the first time in circumstellar envelopes CH₃CCH and tentatively CH₃CCCCH. Many absorption bands, that we believe arise from gas phase molecules, are also seen at the frequencies of some of the unidentified infrared bands. Some of the molecules quoted above are also detected in the ISO/SWS spectrum of CRL2688, another C-rich protoplanetary nebula. The spectrum of NGC7027 is completely dominated by the emission bands of large carbon-rich species. A detailed comparison of the IR spectrum of IRC+10216, CRL2688, CRL618 and NGC7027 is provided.