A&A 466, 667-673 (2007)
DOI: 10.1051/0004-6361:20066647

Vertical abundance stratification in the blue horizontal branch star HD 135485

V. R. Khalack^{1, 2}, F. LeBlanc¹, D. Bohlender³, G. A. Wade², and B. B. Behr<sup>4

1</sup>  Département de Physique et d'Astronomie, Université de Moncton, Moncton, N.-B., E1A 3E9, Canada
    e-mail: khalakv@umoncton.ca
²  Department of Physics, Royal Military College of Canada, PO Box 17000 stn `FORCES', Kingston, Ontario, K7K 4B4, Canada
³  National Research Council of Canada, Herzberg Institute of Astrophysics, 5071 West Saanich Road, Victoria, BC, V9E 2E7, Canada
⁴  Department of Astronomy, University of Texas at Austin, 1 University Station C1400, Austin TX 78712-0259, USA

(Received 26 October 2006 / Accepted 29 January 2007)

Abstract
Context.It is commonly believed that the observed overabundances of many chemical species relative to the expected cluster metallicity in blue horizontal branch (BHB) stars appear as a result of atomic diffusion in the photosphere. The slow rotation of BHB stars (with  K), typically  km s^-1, is consistent with this idea.
Aims.In this work we search for observational evidence of vertical chemical stratification in the atmosphere of HD 135485. If this evidence exists, it will demonstrate the importance of atomic diffusion processes in the atmospheres of BHB stars.
Methods.We undertake an extensive abundance stratification analysis of the atmosphere of HD 135485, based on recently acquired high resolution and S/N CFHT ESPaDOnS spectra and a McDonald-CE spectrum.
Results.Our numerical simulations show that nitrogen and sulfur reveal signatures of vertical abundance stratification in the stellar atmosphere. It appears that the abundances of these elements increase toward the upper atmosphere. This fact cannot be explained by the influence of microturbulent velocity, because oxygen, carbon, neon, argon, titanium and chromium do not show similar behavior and their abundances remain constant throughout the atmosphere. It seems that the iron abundance may increase marginally toward the lower atmosphere. This is the first demonstration of vertical abundance stratification of metals in a BHB star.

Key words: stars: atmospheres -- stars: horizontal-branch -- stars: chemically peculiar -- stars: individual: HD 135485

© ESO 2007

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