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It is commonly believed that the observed overabundances of many chemicalspecies relative to the expected cluster metallicity in blue horizontal branch(BHB) stars appear as a result of atomic diffusion in the photosphere. The slowrotation of BHB stars (with T_eff > 11,500K), typically v sin{i} < 10 km/s, isconsistent with this idea. In this work we search for observational evidence ofvertical chemical stratification in the atmosphere of HD135485. If thisevidence exists, it will demonstrate the importance of atomic diffusionprocesses in the atmospheres of BHB stars. We undertake an extensive abundancestratification analysis of the atmosphere of HD135485, based on recentlyacquired high resolution and S/N CFHT ESPaDOnS spectra and a McDonald-CEspectrum. Our numerical simulations show that nitrogen and sulfur revealsignatures of vertical abundance stratification in the stellar atmosphere. Itappears that the abundances of these elements increase toward the upperatmosphere. This fact cannot be explained by the influence of microturbulentvelocity, because oxygen, carbon, neon, argon, titanium and chromium do notshow similar behavior and their abundances remain constant throughout theatmosphere. It seems that the iron abundance may increase marginally toward thelower atmosphere. This is the first demonstration of vertical abundancestratification of metals in a BHB star.Comment: 8 pages, 5 figures, accepted to A&

Vertical abundance stratification in the blue horizontal branch star HD 135485