α Centauri: coronal temperature structure and abundances from ASCA observations

Abstract

We have analyzed the X-ray spectrum of the nearby binary α Cen AB (G2V+K1V) that has been obtained from observations with ASCA. The coronal temperature structure and abundances have been derived from multi-temperature fitting and confirmed by a differential emission measure analysis. The corona as seen by ASCA is essentially isothermal with a temperature around 0.3 keV, consistent with the evolutionary picture of coronae of aging solar-type stars. A comparison between the measurements from various instruments indicates a source variability in the coronal flux (which precludes the joint fitting of data from different instruments taken at different epochs) and temperature structure consistent with that discovered in a series of ROSAT observations. The elemental abundances agree with solar photospheric abundances for Ne, Si, and Fe at 1σ level, while O appears to be underabundant by a factor of about 3 relative to solar photospheric values, and Mg overabundant by a factor of a few. The abundance ratios with respect to Fe are better determined: [O/Fe]=0.4±0.14 (× solar, etc.), [Mg/Fe]=4±1, [Ne/Fe]=1±0.3, and [Si/Fe]=6±4.