Astron. Astrophys. 323, 469-487 (1997)

1. Introduction

The first study of molecules in the optical spectrum of a post-AGB star concerned the presence of C₃ absorption and C₂ emission in the reflected light of the lobes of the Cygnus Egg Nebula (Crampton et al. 1975). Renewed interest was triggered by the discovery by Waelkens et al. (1992), Balm & Jura (1992), and Hall et al. (1992) of CH emission in the optical spectrum of the famous Red Rectangle. Recently, Waelkens et al. (1995) reported on the presence of CH absorption in HD 213985. In this paper (which we will refer to as Paper II) we will study the presence of C₂, CN, and CH in the optical spectrum of thirteen (post-)AGB stars.

Bakker et al. (1996c, Paper I) were the first to analyze the molecular bands of C₂ and CN in the spectrum of the post-AGB star HD 56126 and showed that the expansion velocity and the excitation conditions are consistent with the lines being formed in the AGB ejecta. Extending this work, Bakker et al. (1995) showed that the same is valid for the molecular absorption lines in four other post-AGB stars (IRAS 04296+3429, IRAS 05113+1347, IRAS 08005-2356, and AFGL 2688) and argued on the basis of the relation between CO (or OH for IRAS 08005-2356) and C₂ or CN expansion velocity that these molecular absorption lines are formed in the AGB ejecta and are therefore of circumstellar origin. Hrivnak (1995) discussed the presence of C₂ and C₃ in the low-resolution spectra of nine post-AGB stars.

Theoretical models of the formation and dissociation of molecules in the extended envelope of the carbon-rich AGB star IRC +10216 by Cherchneff et al. (1993) showed that, e.g., C₂ and CN are only present in a thin shell of material within the extended envelope. Close to the star carbon and nitrogen are locked up in complex stable molecules such as C₂ H₂ and HCN, while at larger distances the interstellar ultraviolet radiation field photodissociates these molecules to C₂ and CN. At even larger distances the UV radiation field photodissociates simple molecules into their constituent atoms and ions. The net effect is that simple molecules exist only in a thin shell of material.

Interstellar C₂ has been discussed extensively by van Dishoeck & Black (1982). They have shown that the excitation of C₂ is a sensitive balance between photoexcitation and collisional (de-)excitation. By modeling the excitation, the relative population over the rotational energy levels of the C₂ ground state can be used to determine the particle density, radiation field, and kinetic temperature of the line-forming region. In a separate paper (Paper III in preparation) we will apply this model to the stars studied, while an earlier account of this work can be found in Bakker et al. (1995).

In Sect. 2 we discuss the criteria used in selecting our sample of mainly carbon-rich post-AGB stars, and describe the observations and data reduction. Sect. 3 describes the method used in identifying molecular bands and determination of the expansion velocities of the AGB ejecta. Rotational diagrams are used to derive molecular rotational temperatures, column densities, and mass-loss rates. The results are discussed (Sect. 4) for the sample as a whole and for each star separately. We will finish with a short conclusion (Sect. 5).

© European Southern Observatory (ESO) 1997

Online publication: June 5, 1998

helpdesk.link@springer.de