THE DISTANT FUTURE OF SOLAR-ACTIVITY - A CASE-STUDY OF BETA-HYDRI .2. CHROMOSPHERIC ACTIVITY AND VARIABILITY

The secular evolution of solar-type chromospheres is studied through a detailed comparison of the present Sun (G2 V) with the very old (congruent-to 9.5 Gyr) star beta Hyi (G2 IV). In such old stars one might observe the lowest possible levels of stellar activity. Detailed studies of various atmospheric layers in beta Hyi have been made, and this Paper II in the series treats chromospheric features and their time variability. High-resolution Ca II H and K profiles show the emission about half that for the Sun, but with the same sense of violet-red asymmetry (the shortward peak the strongest). The emission's wavelength width is slightly broader, consistent with the Wilson-Bappu relation for this slightly more luminous star. Mg II h and k profiles also show an emission weaker than in the Sun, but with the opposite sense of asymmetry, probably altered by absorption in a nearby interstellar cloud. The Mg II variability was monitored from the IUE satellite during about 12 yr, recording some 100 high-resolution fine profiles. Accurate spectrophotometry is assured by recently developed IUE image processing techniques. The emission variations are small (less than or similar to 30% in Mg II h and k) and are characterized by smooth and systematic changes in the line profiles from year to year, suggesting continuous changes in the chromospheric structure, rather than the sudden emergence or growth of active regions. These 12 yr embrace both an activity minimum and a maximum, consistent with an activity cycle somewhat longer (congruent-to 15-18 yr) than the solar one. Assuming the Ca II emission variation to follow that of Mg II, the corresponding peak-to-peak amplitude in the Mount Wilson K-line index would be on the order of 1%, making this the lowest amplitude stellar activity cycle so far detected.