Search for vertical stratification of metals in atmospheres of blue horizontal-branch stars

Context. The observed abundance peculiarities of many chemical species relative to the expected cluster metallicity in blue horizontal-branch (BHB) stars presumably appear as a result of atomic diffusion in the photosphere. The slow rotation (typically v sin i < 10 km s) of BHB stars with effective temperatures Teff > 11,500 K supports this idea since the diffusion mechanism is only effective in a stable stellar atmosphere. Aims. In this work we search for observational evidence of vertical chemical stratification in the atmospheres of six hot BHB stars: B84, B267 and B279 in M15 and WF2-2541, WF4-3085 and WF4-3485 in M13. Methods. We undertake an abundance stratification analysis of the stellar atmospheres of the aforementioned stars, based on acquired Keck HIRES spectra. Results. We have found from our numerical simulations that three stars (B267, B279 and WF2-2541) show clear signatures of the vertical stratification of iron whose abundance increases toward the lower atmosphere, while the other two stars (B84 and WF4-3485) do not. For WF4-3085 the iron stratification results are inconclusive. B267 also shows a signature of titanium stratification. Our estimates for radial velocity, v sin i and overall iron, titanium and phosphorus abundances agree with previously published data for these stars after taking the measurement errors into account. The results support the hypothesis regarding the efficiency of atomic diffusion in the stellar atmospheres of BHB stars with Teff > 11,500 K.