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

5. Conclusions

We have explored a new technique to study the physical and chemical conditions of the AGB ejecta by looking at molecular absorption and emission lines in the optical spectra of post-AGB stars. We find that all stars exhibiting the unidentified 21 µm feature have C₂ and CN absorption. Stars which show C₂ and CN do not show CH absorption. The presence of C₂ and CN is correlated with the presence of cold dust (  K), while CH is correlated with the presence of hot dust (  K).

The expansion velocities determined from the molecular absorption lines are in very good agreement with the expansion velocities derived from CO millimeter line emission. This proves that the molecular absorption lines are formed in the AGB ejecta. The absolute heliocentric velocity of the AGB ejecta in the line-of-sight can be determined very accurately thanks to the large number of molecular lines available in the optical. We find a typical error of  km s^-1 in our observations.

From the observed equivalent widths (see App. A, only at CDS) of the absorption lines the rotational temperatures and column densities can be determined. We find that the rotational temperature of C₂ is significantly higher than that of CN. C₂ is super-thermally excited, whereas CN is sub-thermally excited. This is consistent with the fact that C₂ is a homonuclear molecule (with ) and CN a heteronuclear molecule, while the primary excitation mechanism of C₂ is optical pumping by the stellar radiation field. A more detailed analysis of the excitation of these species can lead to better constraints on the physical parameters in the AGB ejecta and to an independent determination of the mass-loss rate. These points will be further investigated in a subsequent paper (Paper III in preparation).

Interestingly, we found that the molecular column densities increase with expansion velocity. This is interpreted as due to the fact that carbon-rich dust is accelerated to higher velocities by the stellar radiation field. The observed column densities are an indicator of the molecular abundance. Mass-loss rates are computed which are of the same order of magnitude as those found from the IR excess and from CO emission lines. In view of the important assumption made to be able to compute the mass-loss rate, we stress that these rates should be cited cautiously.

© European Southern Observatory (ESO) 1997

Online publication: June 5, 1998

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