2010ApJ...722..343D

Over the last couple of decades we have been carrying out a study of stellar magnetic activity, dynamos, atmospheric physics, and spectral irradiances from a sample of solar-type G0-5 V stars with different ages. One of the major goals of this program is to study the evolution of the Sun's X-ray through NUV spectral irradiances with age. Of particular interest is the determination of the young Sun's elevated levels of high-energy fluxes because of the critical roles that X-ray (coronal) through FUV (transition region (TR), chromospheric) emissions play on the photochemical and photoionization evolution (and possible erosion) of early, young planetary atmospheres and ionospheres. Motivated by the current exoplanetary search missions (such as Kepler and CoRoT, along with the planned Space Interferometry Mission and Darwin/Terrestrial Planet Finder missions) that are hunting for Earth-size planets in the habitable zones (liquid water) of nearby main-sequence G-M stars, we are expanding our program to cooler, less luminous, but very importantly, much more numerous main-sequence K-type stars, such as α Centauri B. The long life (2-3x longer than the Sun) and slow evolution of K stars provide nearly constant energy sources for possible hosted planets. This program parallels our "Sun in Time" program, but extends the study to stars with deeper convective zone depths. Presented here are X-ray (coronal; ROSAT, Chandra, XMM-Newton), UV (TR; International Ultraviolet Explorer (IUE)), NUV (chromospheric; IUE), and recently acquired FUV (TR/chromospheric; FUSE Cycles 7/8) observations of the K1 V star α Cen B (HD 128621; V = 1.33; (B - V) = +0.88; τ = 5.6±0.6 Gyr). These combined high-energy measures provide a more complete look into the nature of α Cen B's magnetic activity and X-ray-UV radiances. We find that α Cen B has exhibited significant long-term variability in X-ray through NUV emission fluxes, indicating a solar-like long-term activity cycle of P_cycle= 8.84±0.4 years. In addition, analysis of the short-term rotational modulation of mean light due to the effects of magnetically active regions has yielded a well-determined rotation period of P_rotation= 36.2±1.4 days. α Cen B is the only old main-sequence K star with a reliably determined age and rotation period, and for early K stars, as in the case of the Sun for G2 V stars, is an important calibrator for stellar age/rotation/activity relations.