Long-Term Variations of Stellar Magnetic Activity in Lower Main Sequence Stars

Abstract

Observed chromospheric emission fluxes and their time variation are undoubtedly universal phenomena in lower main sequence stars. Knowledge gained from the Sun indicates that stellar chromospheric activity is caused by magnetic variations. In lower main sequence stars, magnetic activity variations can be investigated by proxy with the chromospheric emission present in the Ca II H and K lines. Cross-sectional and time-serial studies reveal the dependence of average chromospheric activity levels and their variations on stellar macroscopic parameters such as mass, age and rotation. Twenty years of chromospheric Ca II H and K fluxes in cool stars monitored from Mt. Wilson Observatory have revealed long-term variations that are similar to the 11-year sunspot cycle. The behavior of the long-term activity variations is summarized. Cycle periodicities inferred for the majority of stars in the Mt. Wilson sample show no obvious dependence on rotation, mass or chromospheric activity level. Subgroups of the sample do, however, contain insight on magnetic cycle behavior: First, some F- and early G-type dwarf stars display short (≤4 yr), distinctly non-solar, cycle periods. Second, for stars with extremely obvious periodicities, the cycle period may depend upon the chromospheric radiative loss expressed as a fraction of stellar bolometric luminosity. Contributing to the tentativeness of the latter result is the inaccurate determination of cycle periods longer than ten years from the short, twenty-year baseline of the time series.