Lithium in chemically peculiar CP stars with magnetic fields |
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| Authors: | N S Polosukhina A V Shavrina |
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| Institution: | (1) Scientific Research Institute Crimean Astrophysical Observatory, Ukraine;(2) Main Astronomical Observatory, National Academy of Sciences, Ukraine |
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| Abstract: | The problem of lithium in chemically peculiar Ap-CP stars has been the subject of debate for many years. The main reason for
this is a lack of spectral observations of Ap stars in the neighborhood of the lithium resonance doublet Li I 6708 Å. An international
cooperation project on “Lithium in cool CP stars with magnetic fields” was started in 1996. Systematic observations of CP
stars in spectral regions of the 6708 Å and 6103 Å lines at the ZTSh (CrAO), CAT (ESO), Feros (ESO), and the 74″ telescope
of the Mount Stromlo Observatory (Australia) have been used to analyze spectra of several CP stars studied by the way the
6708 Å lithium line varies with the stars’ rotational phase. Monitoring of the spectra of the oscillating CP stars (group
I) HD 83368, HD 60435, and HD 3980, for which significant Doppler shifts of the Li I 6708 Å line are observed led to the discovery
of “lithium spots” on the surface of these stars whose positions are related to the magnetic field structure. Models of the
surfaces of these stars with the special program “ROTATE” based on the profiles of the Li I 6708 Å line are used to estimate
the size of the spots, their positions on the stars’ surface, and the lithium abundances in these spots. A detailed analysis
and modelling of the spectra of slowly rotating oscillating CP stars with strong, invariant lithium 6708 Å emission, including
blending with lines of the rare earth elements, reveals an enhanced lithium abundance, with the abundance determined from
the lithium 6103 Å line being higher than that determined from the 6708 Å line for all the stars. This may indicate vertical
stratification of lithium in the atmospheres of CP stars with an anomalous isotopic composition (6Li/7Li = 0.2–0.5). HD 101065, an ultraslow rotator (vsini ≈ 1.5) visible from the poles and with powerful oscillations which cause pulsating line broadening in its spectrum, is unique
among these stars. The amount of lithium in the atmosphere of HD 101065 logN(Li) = 3.1 on a scale of logN(H) = 12.0 and the isotope ratio 6Li/7Li ≈ 0.3. The high estimates of 6Li/7Li may be explained by the production of lithium in spallation reactions and the preservation of surface 6Li and 7Li by strong magnetic fields in the upper layers of the atmosphere near the magnetic poles.
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Translated from Astrofizika, Vol. 50, No. 3, pp. 463–492 (August 2007). |
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| Keywords: | stars chemically peculiar CP — stars lithium spots |
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