HD 172481: A super lithium-rich metal-deficient post-AGB binary with a red AGB companion

Abstract

We present in this paper a study on the peculiar supergiant HD 172481. Its spectral type (F2Ia), high galactic latitude (b = -10 fraction of degree37), circumstellar dust, high radial velocity and moderate metal deficiency ([Fe/H] = -0.55) confirm the post-AGB character of this object. A detailed chemical analysis shows slight but real s-process overabundances, however no CNO-enhancement was detected. Furthermore, the spectral energy distribution and the TiO bands in the red part of the spectrum reveal a red luminous companion. The luminosity ratio of the hot F type component and this cool M type companion is derived for a reddening of () and indicates that the companion must also be strongly evolved and probably evolving along the AGB. Neither our photometric data-set, nor our radial velocity monitoring show evidence for orbital variability which may indicate that the period is too large for direct binary interaction. Most interestingly, a strong lithium resonance line is detected, which yields an abundance of = 3.6. Several explanations for this large lithium content are explored.