Stellar Coronal Abundances. V. Evidence for the First Ionization Potential Effect in α Centauri

Abstract

We present an analysis of the EUV spectrum of the nearby binary α Cen AB (G2 V, K0 V), observed by the Extreme Ultraviolet Explorer satellite (EUVE). Emission measure curves are constructed for elements with high and low first ionization potential (FIP) in order to investigate the coronal composition and determine whether or not a solar-like compositional fractionation mechanism ("FIP effect") is operating in the corona of α Cen. As pointed out in the recent analysis of the α Cen EUVE spectra by Mewe et al., the emission measure distribution is very much like that of the Sun. It peaks at log T ~ 6.3, has a minimum near log T ~ 5.5, and has a slope between these points of approximately 1.75 (EM ∝ T^7/4). The emission measure results for the different elements are not consistent with the underlying photospheric composition and indicate that the low-FIP elements are overabundant relative to the high-FIP elements by a factor of about 2. This result represents the first evidence of the FIP effect in solar-like stars with similar activity to that of the Sun. We also discuss the recently raised possibility that resonant scattering of line radiation might be important in EUV stellar spectra like that of α Cen. We conclude that this is unlikely and that the results of global plasma model-fitting techniques that have engendered this hypothesis have not taken into account the flux from spectral lines that are missing from current models of plasma radiative loss.