1998A&A...332..928D

We present a detailed chemical analysis on the basis of high-resolution, high signal-to-noise optical spectra of two post-AGB objects IRAS 22223+4327 and IRAS 04296+3429. Both display the unidentified 21µm feature in their IR-spectra. The analysis is performed using HR 1865(F0Ib) and HR 1017(F5Iab) as reference stars. The spectroscopic indicators provide accurate atmospheric parameters of T_eff=6500K, logg=1.0 and ξ_t=5.5km/s for IRAS 22223+4327 and T_eff=7000K, logg=1.0 and ξ_t=4.0km/s for IRAS 04296+3429. Our high-resolution data are inconsistent with the significantly lower temperatures deduced from spectral-type determinations in the literature based on low resolution spectra and highlight the need of high-resolution spectroscopy for the determination of accurate fundamental parameters of chemically peculiar supergiants. Both photospheres are found to be metal-deficient with [Fe/H]=-0.4 and -0.7 respectively. C and N are found to be overabundant. Useful O-lines were only detected in the brighter IRAS 22223+4327 and the O abundance is found to follow the Fe deficiency: the C/O photospheric abundance is about 1.3, but due to the lack of oxygen lines it is difficult to determine accurately. This corroborates the fact that the carriers of the 21µm feature are formed in a carbon rich circumstellar chemistry. Moreover, these IRAS-stars have large overabundances of s-process-elements. The mean abundance of all the measured s-process-elements is [s/Fe]=+1.0 for IRAS 22223+4327 and +1.4 for IRAS 04296+3429. The distribution of the s-process elements can best be described as due to a distribution of neutron exposures with a low mean neutron exposure of τ₀=0.2mbarn^–1. The 21 µm stars form an interesting sub-group in the total post-AGB sample of stars, not only for their IR characteristics, but also in a broader context of stellar (chemical) evolution theory. They show, in contrast to other post-AGB stars, that the 3rd dredge-up has been efficient during their AGB evolution. The mean neutron exposure is lower than expected for their metallicity. The spectroscopic parameters found for the massive spectral analogues (T_eff=7500K, logg=2.0 and ξ_t=3.0km/s for HR 1865 and T_eff=6500K, logg=2.0 and ξ_t=3.5km/s for HR 1017) agree well with the values found in the literature. Both stars display an enrichment in Na, which is commonly observed in massive supergiants but theoretically not well understood.