2021ApJ...923..192A

A joint X-ray (0.2-2 keV) and ultraviolet (1150-3000 Å) time-domain study has been carried out on three nearby bright late-type stars, bracketing the Sun in properties. Alpha Cen A (HD 128620: G2 V) is a near twin to the Sun, although slightly more massive and luminous, slightly metal-rich, but older. Alpha Cen B (HD 128621: K1 V) is cooler than the Sun, somewhat less massive and lower in luminosity. Procyon (HD 61421: F5 IV-V) is hotter, more massive and more luminous than the Sun, half the age, but more evolved. Stellar observations were from Chandra X-ray Observatory and Hubble Space Telescope (HST). The Sun provided a benchmark through high-energy spectral scans from solar irradiance satellites and novel high-dispersion full-disk profiles of key UV species-Mg II, C II, and Si IV-from the Interface Region Imaging Spectrograph. Procyon's flux history was strikingly constant at all wavelengths, in contrast to the other three cycling-dynamo stars. Procyon also displays a strong subcoronal (T ∼ 1 x 10⁵ K) emission excess, relative to chromospheric Mg II (T <= 10⁴ K), although its X-rays (T ∼ 2 MK) appear to be more normal. At the same time, the odd sub-Gaussian shapes, and redshifts, of the subgiant's "hot lines" (such as Si IV and C IV) are remarkably similar to the solar counterparts (and α Cen AB). This suggests a Sun-like origin, namely a supergranulation network supplied by magnetic flux from a noncycling "local dynamo."