The nearby solar analogue and K dwarf binary alpha Cen AB is the brightest coronal EUV source in the sky as viewed from earth. It is extremely important because it contains two stars with well-known parameters and solar-like coronal activity, and thus forms the best observational link between solar and stellar coronae. However, the stars are too close together (18 arcsec) to be completely resolved by EUVE.
Nevertheless, a recent EUVE observation of Alpha Cen AB designed to maximally separate the two components in the medium wavelength spectroscopic imaging direction, has enabled CEA scientist Dr. J.J. Drake to estimate the relative brightnesses of the two components in lines of He II 304, Mg VII/IX and Fe from charge states IX-XVI. The stars are approximately of equal brightness at temperatures up to log T ~ 6. At higher temperatures Alpha Cen B is brighter than Alpha Cen A by about a factor of two. This work now enables the coronae of the G and K stars to be studied comparatively, providing the first opportunity to probe the change in coronal structure in the spectral sequence G-K for stars of solar-like coronal activity.
VIEWGRAPH: The ratio of EUV fluxes from the A and B components of Alpha Cen, expressed as the flux from A over that from B, F_A/F_B, for spectral lines formed at different temperatures. At temperatures up to about log T ~ 6, the two stars are approximately the same brightness. At hotter temperatures, B is brighter than A by about a factor of 2.