Hubble Space TelescopeBeryllium Abundances in the α Centauri System

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Abstract
High signal-to-noise ratio Hubble Space Telescope Goddard High-Resolution Spectrograph spectra of α Centauri A (spectral type G2 V) and α Centauri B (spectral type K1 V) have been analyzed in the Be II λ3130 spectral region. Both stars offer an excellent opportunity for testing predictions of 9Be destruction since they are nearby, have a well-determined orbit and parallax, and thus have very well known physical parameters. A detailed spectrum synthesis has been made using a line list that has been carefully tested by comparison with the Sun and with metal-poor stars of different temperature and metallicity. Our analysis gives [Be/H] = +0.06 ± 0.09 dex for α Cen A and -0.54 ± 0.28 dex for α Cen B, using a model atmosphere with a metallicity [M/H] = +0.10. The implications of the new beryllium abundances and the previous lithium studies for models of stellar light-element depletion are then studied. Both the Sun and α Cen A are more highly depleted in lithium than younger stars; this is not consistent with standard stellar models. The Sun, α Cen A, and α Cen B have photospheric 9Be abundances lower than the current accepted solar meteoritic value. Because the initial 9Be abundance for the α Cen system is not known, and the depletion is much smaller than that for lithium, it is difficult to determine precise beryllium depletion factors for this system. However, because neither star is expected to deplete beryllium in standard models, the lower relative abundance of α Cen B might be evidence for main-sequence beryllium depletion. The theoretical implications of beryllium depletion are briefly discussed.