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Spectroscopic Study on the Beryllium Abundances of Red Giant Stars

An extensive spectroscopic study was carried out for the beryllium abundances of 200 red giants (mostly of late G and early K type), which were determined from the near-UV Be II 3131.066 line based on high-dispersion spectra obtained by Subaru/HDS, with an aim of investigating the nature of surface Be contents in these evolved giants; e.g., dependence upon stellar parameters, degree of peculiarity along with its origin and build-up timing. We found that Be is considerably deficient (to widely different degree from star to star) in the photosphere of these evolved giants by ~1-3 dex (or more) compared to the initial abundance. While the resulting Be abundances (A(Be)) appear to weakly depend upon T_eff, log g, [Fe/H], M, age, and v_sin i, this may be attributed to the metallicity dependence of A(Be) coupled with the mutual correlation between these stellar parameters, since such tendencies almost disappear in the metallicity-scaled Be abundance ([Be/Fe]). By comparing the Be abundances (as well as their correlations with Li and C) to the recent theoretical predictions based on sophisticated stellar evolution calculations, we concluded that such a considerable extent/diversity of Be deficit is difficult to explain only by the standard theory of first dredge-up in the envelope of red giants, and that some extra mixing process (such as rotational or thermohaline mixing) must be responsible, which presumably starts to operate already in the main-sequence phase. This view is supported by the fact that appreciable Be depletion is seen in less evolved intermediate-mass B-A type stars near to the main sequence.