Photospheric composition of the carbon-rich 21 micron post-AGB stars IRAS 22223+4327 and IRAS 04296+3429

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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 21 μm feature in their IR-spectra. The spectroscopic indicators provide accurate atmospheric parameters of Teff=6500 K, log g=1.0 and ξₜ = 5.5 km/s for IRAS 2223+4327 and Teff=7000 K, log g=1.0 and ξₜ = 4.0 km/s for IRAS 04296+3429. 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. The mean abundance of all the measured s-process-elements is [s/Fe]=+1.0 for IRAS 2223+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 = 0.2 mbarn⁻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.