X-Ray Emission from the Sun in Its Youth and Old Age

We have obtained ROSAT PSPC pointed observations of two nearby G stars of ages 70 Myr and 9½ Gyr that are of unique importance as proxies for the Sun at the two extremes of its main-sequence evolutionary lifetime. The younger star, HD 129333 (EK Dra; GO V), a rapid rotator with a 2.7 day period, is a strong source with an X-ray luminosity Lx(0.2-2.4 keV) = (7.5-11.5) × 1029 erg s-1. Modeling suggests a two- temperature corona with T1 = (2.0±0.3) × 106 K and T2 = (9.7±0.3) × 106 K (formal uncertainties). A continuous emission measure distribution, increasing to higher temperatures and with a cutoff at (20-30) × 106 K, yields even better fits to the data. The old star, β Hyi (HR 98; G2 IV), represents the Sun in the future, near the end of its hydrogen-core burning stage, when it should be rotating more slowly (present Prot = 25.4 day) and should have lower levels of activity. The ROSAT measurements yield LX = (0.9-3.0) × 1027 ergs s-1 and a rather cool, single coronal temperature of T = (1.7±0.4) × 106 K. For comparison, the Sun has LX 2 × 1027 ergs s-1 and a coronal temperature of about T = 2 × 106 K. These stars provide information on the decline of the stellar (and specifically solar) magnetic activity from extreme youth to old age. HD 129333 is also important in that it yields an estimate of the solar soft X-ray flux in the early solar system at the epoch of the terminal stages of planetary accretion.