Abstract
The basic parameters and detailed chemical compositions of three asymptotic giant branch stars with similar effective temperatures and surface gravities have been determined using CCD spectra obtained with the echelle spectrometers of the SAO 6-m telescope. The metallicity and chemical composition of the optical counterpart of the OH/IR star IRAS 18123 + 0511 have been derived for the first time. The abundance [X/H]⊙ of the iron group elements (V, Cr, Fe) is −0.45 dex. An overabundance of oxygen, [O/Fe]⊙=1.44 dex, is detected in the atmosphere of this star. The abundances of s-process heavy elements are not enhanced, and are instead underabundant with respect to the metallicity: the average value of [X/Fe]⊙ for Y, Zr, Ba, La, Ce, Pr, Nd is −0.25. The derived abundances confirm that IRAS 18123 + 0511 is in the AGB stage of its evolution. The metallicity of the object, together with its radial velocity V r=78.0 km/s and Galactic latitude |b|=11°, suggest that it belongs to the old disk population. The expansion velocity of the circumstellar envelope, V exp≈21 km/s, is derived from the positions of circumstellar absorption bands. The set of parameters obtained for the low-metallicity, highlatitude supergiants BD + 18° 2757 and BD + 18° 2890 (with iron abundances [Fe/H]⊙=2.10 and −1.48, respectively) confirm that they are evolved halo stars, and probably UU Her-type stars.
Published Version
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