1997A&A...323..469B

We present optical high-resolution spectra of a sample of sixteen post-AGB stars and IRC +10216. Of the post-AGB stars, ten show C₂ Phillips (A¹{PI}_u-X¹{SIGMA}⁺_g) and Swan (d³{PI}_g-a³{PI}_u) and CN Red System (A²{PI}-X²{SIGMA}⁺) absorption, one CH⁺ (A¹{PI}-X¹{SIGMA}⁺) emission, one CH⁺ absorption, and four without any molecules. We find typically T_rot ∼43-399, 155-202, and 18-50K, logN∼14.90-15.57, 14.35, and 15.03-16.47cm^–2 for C₂, CH⁺, and CN respectively, and 0.6≤N(CN)/N(C₂)≤11.2. We did not detect isotopic lines, which places a lower limit on the isotope ratio of ¹²C/¹³C>20. The presence of C₂ and CN absorption is correlated with cold dust (T_dust≤300K) and the presence of CH⁺ with hot dust (T_dust≥300K). All objects with the unidentified 21µm emission feature exhibit C₂ and CN absorption, but not all objects with C₂ and CN detections exhibit a 21µm feature. The derived expansion velocity, ranging from 5 to 44km/s, is the same as that derived from CO millimeter line emission. This unambiguously proves that these lines are of circumstellar origin and are formed in the AGB ejecta (circumstellar shell expelled during the preceding AGB phase). Furthermore there seems to be a relation between the C₂ molecular column density and the expansion velocity, which is attributed to the fact that a higher carbon abundance of the dust leads to a more efficient acceleration of the AGB wind. Using simple assumptions for the location of the molecular lines and molecular abundances, mass-loss rates have been derived from the molecular absorption lines and are comparable to those obtained from CO emission lines and the infrared excess.