The Alpha Centauri binary system : Atmospheric parameters and element abundances

• Published in: Astronomy and astrophysics (Berlin) 2008-09, Vol.488 (2), p.653-666
• Main Authors: PORTO DE MELLO, G. F, LYRA, W, KELLER, G. R
• Format: Article
• Language: English
• Subjects: Astronomy; Earth, ocean, space; Exact sciences and technology
• ISSN: 0004-6361; 1432-0746
• DOI: 10.1051/0004-6361:200810031

Context. The alpha Centauri binary system, owing to its duplicity, proximity and brightness, and its components' likeness to the Sun, is a fundamental calibrating object for the theory of stellar structure and evolution and the determination of stellar atmospheric parameters. This role, however, is hindered by a considerable disagreement in the published analyses of its atmospheric parameters and abundances. Aims. We report a new spectroscopic analysis of both components of the alpha Centauri system, compare published analyses of the system, and attempt to quantify the discrepancies still extant in the determinations of the atmospheric parameters and abundances of these stars. Methods. The analysis is differential with respect to the Sun, based on spectra with R = 35 000 and signal-to-noise ratio geq1000, and employed spectroscopic and photometric methods to obtain as many independent T_{\rm eff} determinations as possible. We also check the atmospheric parameters for consistency against the results of the dynamical analysis and the positions of the components in a theoretical HR diagram. Results. The spectroscopic atmospheric parameters of the system are found to be T_{\rm eff} = (5847 pm 27) K, [Fe/H] = +0.24 pm 0.03, log g = 4.34 pm 0.12, and xi_t = 1.46 pm 0.03 km s super(-1), for alpha Cen A, and T_{\rm eff} = (5316 pm 28) K, [Fe/H] = +0.25 pm 0.04, log g = 4.44 pm 0.15, and xi_t = 1.28 pm 0.15 km s{\rm-1} for alpha Cen B. The parameters were derived from the simultaneous excitation & ionization equilibria of Fe I and Fe II lines. T_{\rm eff}s were also obtained by fitting theoretical profiles to the H\alpha line and from photometric calibrations. Conclusions. We reached good agreement between the three criteria for alpha Cen A. For alpha Cen B the spectroscopic T_{\rm eff} is similar to 140 K higher than the other two determinations. We discuss possible origins of this inconsistency, concluding that the presence of non-local thermodynamic equilibrium effects is a probable candidate, but we note that there is as yet no consensus on the existence and cause of an offset between the spectroscopic and photometric T_{\rm eff} scales of cool dwarfs. The spectroscopic surface gravities also agree with those derived from directly measured masses and radii. An average of three independent T_{\rm eff} criteria leads to T_{\rm eff} (A) = (5824 pm 26) K and T_{\rm eff} (B) = (5223 pm 62) K. The abundances of Na, Mg, Si, Mn, Co, and Ni and, possibly, Cu are signif