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Astron. Astrophys. 342, 756-762 (1999)

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1. Introduction

In our previous work devoted to NLTE analysis of the carbon lines in the spectrum of hot B star [FORMULA] Peg we have shown that this element is slightly deficient in the atmosphere of program star: [FORMULA]. Such a deficiency could be a sign of some dynamical processes involving the upper atmosphere layers and leading to an accumulation in the superficial region of the main sequence star the CN-processed material. In particular, Maeder (1987) discussed a turbulent diffusion induced, for example, by the stellar rotation as such a mechanism.

If the turbulent diffusion is really responsible for observational carbon deficiency, then it is absolutely clear that we should detect the corresponding nitrogen overabundance. This scenario resembles that described by Iben (1966a,b,c) for yellow supergiants. Accordingly to standard evolutionary scenario, an atmospheric carbon abundance for such stars must be slightly reduced after the star passed the red supergiant phase, where so-called first dredge-up occurs, while nitrogen becomes overabundant. In particular, the observations of Cepheids confirm the theory predictions (Luck & Lambert, 1985; Luck & Bond, 1989; Andrievsky et al., 1996, Andrievsky & Kovtyukh, 1996; Kovtyukh et al., 1996).

It is quite plausible that rapidly rotating B stars of the main sequence can possess some kind of the large-scale mixing that causes the superficial CN anomalies similar to those observed in Cepheids.

In order to check this supposition, we have undertaken an attempt to carry out a detailed NLTE calculations of nitrogen abundance in the main-sequence B star [FORMULA] Peg (B2 IV). This star possesses the well expressed sharp lines in its spectrum indicating small [FORMULA] value. It makes this rather bright star a very attractive object for investigation of the NLTE effects in the hot stellar atmospheres.

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© European Southern Observatory (ESO) 1999

Online publication: February 23, 1999