Activity of Five Young Dwarfs with Planetary Systems

Abstract

The activity of five young stars possessing planetary systems is studied: K2-231, EPIC 219 388 192, K2-136, Kepler-66, and Kepler-67. The ages of these objects were found from their cluster membership. K2-136 is the second planetary system discovered in the Hyades cluster. At the time of its discovery, it was the first multi-planet system in a young cluster. The system has three planets, the smallest of which with the shortest period is a terrestrial-type planet. The evolution of active areas on the surfaces of these stars is followed continuous over 70 days. The characters of the stars’ brightness variations are determined, and their rotation periods derived or improved. The rotation periods of these objects lie in the range from 9 to 15 days. The differential-rotation parameters of Kepler-66 and Kepler-67 are estimated to be ΔΩ = 0.04 − 0.05 and 0.04 rad/day. Maps of the temperature inhomogeneities on the surfaces of the five studied stars are constructed and conclusions drawn about the positional evolution of their active areas. The values of S, the fractional spotted areas of the stellar surfaces, are found to be 0.7–4.5% of their total visible surface areas. The positions of our objects in plots of S vs. age, S vs. rotation period, and S vs. Rossby number are considered, and found to agree with the general character of corresponding relations established earlier for 1570 M dwarfs.