THE CHEMICAL-COMPOSITION OF THE HIGH-VELOCITY POST AGB STAR HD-56126(F51)

HD 56126 (F 5I) is an IRAS source with detached cold dust shell with characteristics similar to the dust shells around planetary nebuale. From an analysis of high resolution, high signal-to-noise ratio spectra using model atmospheres, metal and CNO abundances have been determined. It is found that in HD56126 [C/H] congruent-to -0.01, [N/H] congruent-to +0.17, [O/H] = -0.02, [S/H] congruent-to +0.01 and [Fe/H] less-than-or-equal-to -1.O. The C/N/O abundance ratios and CNO abundances relative to Fe suggest that HD 56126 and related stars are evolved and in some of these stars the material processed by triple alpha, CN and ON cycles has reached the surface.From the high resolution spectra we derive the radial velocity of HD 56126 to be +105.0 +/- 1.0 km s-1 which suggests that HD 56126 is a high velocity star belonging to the old disk.The abundance ratios [C/Fe], [N/Fe], [O/Fe] and [S/Fe] in HD 56126 and related post AGB stars are large and cannot be easily explained by nucleosynthesis and/or mixing. The large ratios of [C/Fe], [N/Fe], [O/Fe] and [S/Fe] are due to depletion of Fe but not of C, N, O and S elements. The depletion of refractory elements and the lack of depletions in CNO and S in the atmosphere of these post AGB stars is similar to that observed in the interstellar medium. Since most of these stars have circumstellar dust shells the depleted refractory elements appear to be locked up in dust grains. The results suggest that in the recent past (possibly during the OH IR stars stage) the outer atmospheres of these stars expanded and cooled to the limit of the condensation temperature of refractory elements. Formation of cores of dust grains very close to the stars and the resulting dust-driven mass loss may be able to explain the observed abundances peculiarities in these post AGB stars.