The chemical composition of IRAS 05341+0852: a post-AGB F supergiant with 21μm emission

An abundance analysis of the photosphere of the F-type Post-AGB candidate IRAS 05341+0852 is presented. The analysis of the spectra suggests that the atmospheric model: Teff=6500 K, log g=0.5, ξt=5 km s−1 and [M/H]=1.0, best represents the photosphere of the star. We find that the star is metal-poor ([Fe/H]=−0.9) and carbon-rich (C/O≈1.9). Lithium, carbon, nitrogen, oxygen, aluminum and silicon are found to be overabundant. Most importantly this star has large overabundance of s-process elements which are as follows: [Y/Fe]=1.6, [Ba/Fe]=2.2, [La/Fe]=2.9, [Ce/Fe]=1.6, [Pr/Fe]=1.8, [Nd/Fe]=2.0, and [Sm/Fe]=1.7. These s-process enhancements are of similar magnitude to those seen in FG Sge. The enhancement of Li and Al are expected due to Hot Bottom Burning (HBB), in massive AGB stars. However, the low metallicity, heliocentric radial velocity (vr = 25 km s−1), and the high Galactic latitude (b = −12o) suggest that IRAS 05341+0852 is a low-mass star. Also the [S/Fe]=0.2 indicates that the low Fe abundance is intrinsic and is not due to fractionation. The significant overabundance of lithium (log ≤ 2.5) and aluminum ([Al/Fe]=1.4) in this star is a puzzle. Furthermore we have detected the C2 Phillips and CN Red bands in absorption. We find rotational temperatures, column densities, and expansion velocities which show that these lines are formed in the detached circumstellar dust shell (the AGB ejecta) and we find an expansion velocity of the AGB ejecta of vexp = 10.8± 1.0 km s−1.