Chemical composition of a sample of candidate post-asymptotic giant branch stars

Abstract

We have derived elemental abundances for a sample of nine IRAS sources with colours similar to those of post-asymptotic giant branch (post-AGB) stars. For IRAS 01259+6823, IRAS 05208−2035, IRAS 04535+3747 and IRAS 08187−1905, this is the first detailed abundance analysis based upon high-resolution spectra. Mild indication of s-processing for IRAS 01259+6823, IRAS 05208−2035 and IRAS 08187−1905 has been found and a more comprehensive study of s-process-enhanced objects IRAS 17279−1119 and IRAS 22223+4327 has been carried out.

We have also made a contemporary abundance analysis of the high Galactic latitude supergiants BD+39°4926 and HD 107369. The former is heavily depleted in refractories and estimated [Zn/H] of −0.7 dex most likely gives initial metallicity of the star. For HD 107369 the abundances of α and Fe-peak elements are similar to those of halo objects and moderate deficiency of s-process elements is seen. IRAS 07140−2321, despite being a short-period binary with a circumstellar shell, does not exhibit selective depletion of refractory elements.

We have compiled the stellar parameters and abundances for post-AGB stars with s-process enhancement, those showing significant depletion of condensable elements and those showing neither. The compilation shows that the s-process-enhanced group contains a very small number of binaries, and observed [α/Fe] are generally similar to thick-disc values. It is likely that they represent AGB evolution of single stars.

The compilation of the depleted group contains a larger fraction of binaries and generally supports the hypothesis of dusty discs surrounding binary post-AGB stars inferred via the shape of their spectral energy distribution and mid-infrared interferometry. IRAS 07140−2321 and BD+39°4926 are difficult to explain with this scenario and indicate the existence of an additional parameter/condition needed to explain the depletion phenomenon. However, the conditions for discernible depletion, minimum temperature of 5000 K and initial metallicity larger than −1.0 dex found from our earlier work still serve as useful criteria.