Context. High spectral resolution studies of cool Ap stars reveal conspicuous anomalies of the shape and strength of many absorption lines. This is a signature of large atmospheric chemical gradients ( chemical stratification) produced by the selective radiative levitation and gravitational settling of chemical species.
Aims. Previous observational studies of the chemical stratification in Ap stars were limited to fitting simple parametrized chemical profiles. Here we present a new approach to mapping the vertical chemical structures in stellar atmospheres.
Methods. We have developed a regularized chemical inversion procedure that uses all information available in high-resolution stellar spectra. The new technique for the first time allowed us to recover chemical profiles without making a priori assumptions about the shape of chemical distributions. We have derived average abundances and applied the vertical inversion procedure to the high-resolution VLT UVES spectra of the weakly magnetic, cool Ap star HD133792.
Results. Our spectroscopic analysis yielded improved estimates of the atmospheric parameters of HD133792. We show that this star has negligible v(e) sin i and the mean magnetic field modulus < B > = 1.1 +/- 0.1 kG. We have derived average abundances for 43 ions and obtained vertical distributions of Ca, Si, Mg, Fe, Cr, and Sr. All these elements except Mg show high overabundance in the deep layers and solar or sub-solar composition in the upper atmosphere of HD133792. In contrast, the Mg abundance increases with height.
Conclusions. We find that transition from the metal-enhanced to metal-depleted zones typically occurs in a rather narrow range of depths in the atmosphere of HD133792. Based on the derived photospheric abundances, we conclude that HD133792 belongs to the rare group of evolved cool Ap stars, which possesses very large Fe-peak enhancement, but lacks a prominent overabundance of the rare-earth elements.
2006. Vol. 460, no 3, 831-842 p.
stars : abundances; stars : atmospheres; stars : chemically peculiar; stars : individual : HD 133792