Outer atmospheres of cool stars. III. IUE spectra and transition region models for alf CEN A and B.

Abstract

We describe lUE ultraviolet spectra of two nearby dwarf stars, α Centauri A (G2 V) and B (K1 V). These data include high-resolution profiles of the Mg II h and k features and lower- resolution integrated fluxes of lines from the following species: H I, C I-IV, N V, O I, A1 II, Si II-IV, and Fe II. We find that surface fluxes in chromospheric and transition-region lines of α Cen A and B are nearly identical to those of the quiet Sun. In addition, the measured stellar line fluxes are in good agreement with predictions of a transition-region scaling law based on conductive heating and pressures estimated from chromospheric models of α Cen A and B. While this agreement does not verify the conductive heating hypothesis, it does suggest that the basic physical processes that control the structure and energy balance in the chromospheres and transition regions of α Cen A and B and the Sun are, on a gross scale, very likely the same.