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Astron. Astrophys. 319, 637-647 (1997)

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1. Introduction

During many years, starting with 1963 (Faraggiana & Hack, 1963), some investigators on CP stars started to pay attention to the problem of lithium. The identification of Li doublet was doubtful because of the relatively low spectral resolution and [FORMULA] ratio. Moreover the problem is complicated by some effects typical of CP stars, like the presence of strong magnetic fields, inhomogeneity in chemical composition at the surface, and different Doppler shifts of lines of different ions with the rotation of the star, and even with rapid, non-radial oscillations on time scales of minutes (Kurtz et al., 1994a, 1994b, 1994c). The technique of Doppler imaging has been applied to several of these stars in order to map the repartition of some elements on their surface; it has been shown that the abundance anomalies are distributed in spots or rings, whose location is related with the magnetic field structure (Hatzes, 1991a, 1991b).

The more recent observations of Li in CP stars, obtained with modern equipments indicate that Li doublet is possibly present in the spectra of some CP stars. However, the presence of several lines of Gd and Ce and of some unidentified features in the same spectral region suggests the possibility that the main component of the Li blend may be due to some unidentified line, and the problem is still open.

The observations of Ap stars in the Li region with modern detectors were carried out by Faraggiana et al. (1986) and concerned a dozen of Ap stars of different types and effective temperatures, including a few stars with [FORMULA]   [FORMULA]  K, where Li would be unobservable, unless it is enormously overabundant. This survey, supplemented by Gerbaldi & Faraggiana (1991), showed that some Ap stars are Li poor, while others may be strongly Li rich. However, only few observations per star were made, and so the behavior of the Li feature versus the rotational phase was not examined. This type of observations began at the Crimean Observatory for some Ap stars and suggested that this element may be overabundant in [FORMULA]  CrB and [FORMULA] Equ (Polosukhina & Lyubimkov,1995). A shift of [FORMULA] 6708 Å  correlated with the phase of rotation was observed in [FORMULA]  CrB and the hypothesis of a different distribution of the Li abundance over the surface of the star may be advanced.

We analyzed the 6693-6721 Å  region of [FORMULA]  CrB in order 1) to obtain a correct identification of the lines, and 2) to derive abundances. We have used the synthetic spectrum method. Atmospheric models were computed with the ATLAS9 code (Kurucz, 1993a) and synthetic spectra were computed with the SYNTHE code (Kurucz, 1993b). Line lists are mostly based on the Kurucz data (Kurucz,1993b).

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© European Southern Observatory (ESO) 1997

Online publication: July 3, 1998