Evolution of the chromospheric and coronal activity of intermediate mass stars

Abstract

Recent ultraviolet and X-ray observations pertaining to the outer atmospheric structure of intermediate mass (4-6 M₀) stars and the evolution of their structure are presented. A distance-limited (d <= 200 pc) IUE ultraviolet survey of early K bright giants shows that C IV emission commonly is present. These stars are almost evenly split between stars showing hybrid-chromospheric and coronal outer atmospheric structures. EXOSAT observations have been obtained for three hybrid stars, of which only α TrA, the nearest, is detected. The temperature of the emitting plasma is likely to be ~10⁶ K. Observations of six K II stars made with the Einstein satellite show no detections. The general conclusion from the available X-ray data is that early K bright giants are not strong X-ray sources.