Activity of G Dwarf HD 284521 (K2-370) and Atmospheric Losses of Its Exoplanet

An analysis of the activity evidence of the chromospherically active solar-type dwarf HD 284521, which has a sub-Neptune-type planet, is presented. Based on the data from the Kepler and TESS missions, the stellar rotation period was estimated (\(P_{\textrm{rot}}=13{-}14\) days) and the evolution of cold spots on its surface was examined. Estimates of the areas of these spots were obtained, which, according to the data from the Kepler mission, equaled about 3.2\(\%\) and, according to TESS observations, about 2.3, 1.2, 2.3, and 2.5\(\%\) of its surface area. Based on the data from the long-term Kamogata Wide-field Survey (KWS) and data from the All Sky Automated Survey (ASAS) observation archive, the value of the star’s long-term activity cycle was found to be about 3800 days (10.4 yrs). The cycle length is comparable to that of the Sun, but HD 284521 rotates about twice as fast as the Sun. An analysis of measurements of the \(S\) index characterizing the chromospheric activity of the star also indicates the presence of a long-period variability. The results obtained from studying the stellar activity were used to estimate the loss of atmospheric matter from the planet HD 284521 b. An approximation formula corresponding to the energy-constrained atmospheric loss model was applied. Based on 135 estimates of the \(S\) parameter, the matter loss from the atmosphere of HD 284521 b over an interval of 1886 days (approximately five years) was calculated. The matter losses (\(\log M_{\textrm{loss}}\)) of the atmosphere of the planet HD 284521 b are in the range of 9.8 to 10.3 with a median value of 10.2. The high amount of matter loss from the atmosphere of this planet is due to a significant flux of XUV photons caused by the high activity of the solar-type star. The study of the planetary system HD 284521 continues our series of studies of planetary systems, in which the central star is an analog of the Sun.