The X‐Ray Sun in Time: A Study of the Long‐Term Evolution of Coronae of Solar‐Type Stars

Abstract

We have used the <em>ASCA</em> and <em>ROSAT</em> X-ray satellites to probe the coronae of a sample of nine solar-like G stars. These stars are all ostensibly single with ages ranging from 70 Myr to 9 Gyr and have X-ray luminosities ranging from 1 to 500 times that of the quiet Sun. Specifically, we investigate the dependence of the coronal temperature and emission measure structure of these stars on age and rotation period.<br />In the younger stars, a considerable portion of the volume emission measure resides at very high temperatures, reaching up to ∼20-30 MK in EK Dra. Such temperatures are comparable to temperatures that are achieved on the Sun during short flaring episodes. In two-temperature fits to <em>ROSAT</em> data, the higher temperature decays rapidly within the first few 100 Myr; the decay may be described by an inverse power law, <em>T</em>_hot ∝ age^-0.3. We also find a power-law dependence between the total X-ray luminosity and the higher temperature <em>L</em>_x ∝ <em>T</em>⁴_hot. We interpret this as evidence of a decrease in the efficiency of high-temperature coronal heating as a solar-like star ages and its rotation slows down. A reconstruction of the coronal differential emission measure (DEM) distribution in three of the stars using <em>ASCA</em> data indicates a bimodal distribution in temperature, with the hotter plasma at 12-30 MK and the cooler plasma below 10 MK. We infer, for the first time, a consistent evolution of the DEM structure in a solar-type star. The emission measure of the hotter component rapidly decreases with age and becomes unimportant at ages beyond ∼500 Myr. The emitted X-ray emission of the young Sun thus rapidly softened, which had important implications for the young planetary atmospheres. We suggest that the high-temperature component is the result of superimposed but temporally unresolved flaring events and support this picture by reconstructing the time-integrated (average) emission measure distribution of a typical solar X-ray flare. Radio observations of active stars fit well into this picture and suggest that the presence of nonthermal electrons in coronae is linked to the presence of hot (&gt;10 MK) plasma, very much the same situation as in solar flares. We find, however, that radio emission saturates, if at all, at smaller rotation periods than does X-ray emission.