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Astron. Astrophys. 357, 920-930 (2000)

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1. Introduction

All the information concerning the Li abundance in stellar atmospheres is extremely important for testing the stellar internal structure and evolution theory. In fact Li is destroyed in the internal stellar layers because of nuclear reactions of lithium with hydrogen at relatively low temperatures of about two million degrees, and surface Li dilution occurs because of convective motions. Hence the presence of lithium in the stellar spectra is the evidence of slowed circulation between the external cool layers and the internal hot ones. The interpretation of the observed lithium abundance in the stellar atmospheres is one enigma of astrophysics. In fact, until the present time the physical processes responsible for the variety in the abundance of this element among stars with similar physical parameters, are not clear.

During the evolution along the main sequence, a normal star with noticeable initial lithium abundance is expected to lose it due to the fragility of the Li nucleus and to various mechanisms of mixing, and to come to the end of its evolution having almost fully exhausted the reservoir of this element. However, among the evolved stars (red giants) large differences of lithium abundance have been observed. This fact shows that in the process of evolution, there are not only processes of lithium destruction, but other possible causes, which brake this Li depletion. In the cases of large lithium abundance, various hypotheses about lithium synthesis were suggested (Wallerstein & Conti, 1969). The magnetic field may be one of the causes, braking the mixing of star matter and the convective motions. Therefore in the papers of Wallerstein & Conti (1969), Wallerstein & Sneden (1982) and of Lambert & Ries (1981), and Lambert & Sawyer (1984) it is hypothesized that the Li-rich giants in their evolution changes have passed the stage of chemically peculiar star with magnetic field and that lithium synthesis is the result of a spallation reaction, which takes place on the stellar surfaces with strong magnetic fields, which would accelerate protons and alpha particles.

With respect to the lithium abundance in chemically peculiar stars there are some contradictory data (Faraggiana et al., 1996; Hack et al., 1997). The chemically peculiar stars possess unusual individual characteristics, first of all, such chemical anomalies as high abundances of heavy elements, particularly of rare elements, rather strong magnetic fields, non homogeneous distribution of chemical elements on the stellar surface. The method of Doppler imaging, applied to some stars, shows that chemical anomalies are distributed in spots or rings, connected apparently with the magnetic field structure (Hatzes, 1991). Some of the roAp stars are characterized by non-radial pulsations on time scales of minutes to tens of minutes (Kurtz, 1990). The large range of the lithium line intensity in the spectra of CP stars (Faraggiana et al., 1996; Hack et al., 1997) is evidence of the complexity of the physical nature of these stars. Until now there has been no theory which can explain this phenomenon, assuming that the 6708 Å feature can indeed be interpreted as the Li I doublet, rather than as an unidentified line of some strongly overabundant rare earth element. Although spallation reactions in local flashes on the stellar surface have been suggested, this idea has never been thoroughly developed. On the other hand, the radiative diffusion theory (Babel & Michaud 1991a, 1991b) seems successful in explaining the abundance anomalies of many species, though no specific study on Li in Ap stars has been published. Since Li is mostly ionized in these stars, it is expected to sink into the deep atmospheric layers, because its electronic structure is, then, similar to that of He, an element known - and predicted - to be deficient in the atmospheres of Ap stars. On the other hand, Babel (1993) suggested that ambipolar diffusion of hydrogen may cause Li enrichment near the magnetic poles, but unfortunately no detailed study of this possibility was published.

To clarify the problem of the identification of the 6708 Å feature and answer the question of the diversity of the Li abundances among Ap stars (if the 6708 Å feature is indeed due to Li), it is necessary to increase the number of spectroscopic observations in the Li line regions. The high resolution echelle spectrographs and CCD cameras allowing high S/N ratios, the modern methods of spectral analysis (atmospheric models and computations of synthetic spectra), the use of refined atomic data, especially for rare earth elements, give the possibility to study in more detail than ever before, the spectrum in the regions of the Li lines and thus to approach the solution to this problem.

In most of the works connected with lithium observations, the resonance lithium doublet at 6708 Å is used and, very rarely, the 6103 Å line, which is in the wing of a strong Fe II line. To observe the lines of neutral lithium is difficult; usually its abundance is not large, because Li I is easily ionized (the ionization potential [FORMULA] eV). The neutral lithium can be observed only in the surface layers of the stellar atmospheres. Besides, the resonance doublet at 6708 Å is the isotopic blend of 6Li and 7Li. For most stars the isotopic ratio [FORMULA]Li/7Li is small, [FORMULA] (Andersen et al., 1984).

The first results of the observations of the roAp star HD 83368 in the spectral region of Li I 6708 Å, in the frame of the international project "Lithium in the chemically peculiar stars", were presented in the paper of North et al. (1998). In this paper the detailed description of the observations, obtained at ESO with the CAT telescope in 1996, and the interpretation of the profile changes of Li I 6708 Å with the rotational phase by the "spotted" star model, are given (see Table 1 and Fig. 1 and Fig. 2 of the quoted paper). In the present paper we continue the analysis of the spectra of this star, assuming that the 6708 Å feature can be attributed to Li I , which seems probable according to Polosukhina et al. (1999). This work is the first attempt to analyse the physical properties of HD 83368, using the method of spectral synthesis for the atmospheric model. HD 83368 is the best photometrically studied example of rapidly oscillating Ap star, belonging to the SrCrEu peculiarity type with [FORMULA] and with radius [FORMULA] (North et al., 1998). The effective magnetic field changes in intensity with an amplitude of [FORMULA] (Mathys, 1991). Mathys yields for this star a very high value for the quadratic magnetic field - 11 kG (Mathys, 1995; Mathys & Hubrig, 1997). The star has a single mode of pulsation. The maximum photometric amplitude of the pulsations occurs when a magnetic pole is located on the line of sight; therefore, the interval between two maxima of pulsation corresponds to half of the rotation period [FORMULA].


Table 1. List of observations. Spectral range: A = 6675-6735 Å, B = 6120-6180 Å, C = 6615-6670 Å.
a: HD 101065 (Przybylski's star), [FORMULA] = 0 km s-1 (Cowley & Mathys, 1998)

[FIGURE] Fig. 1. The best curve fitting the observed longitudinal magnetic field variations (by the least square method, Eq. (4). The Thompson's data (Thompson, 1983) are indicated by circles and Mathys's data (Mathys, 1991) by asterisks, (Mathys & Hubrig, 1997) by squares, and the excluded data of Mathys (1991) by triangles. Vertical bars indicate the errors of the measurements.

[FIGURE] Fig. 2. Variation of the mean surface magnetic field with the rotational phase of HD 83368 calculated for [FORMULA] and [FORMULA] = 2730 G.

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Online publication: June 5, 2000