Analysis of the Activity of 11 K-type Dwarfs with Planets in the Habitable Zone

Abstract

We report the results of a study of spot activity manifestations in 11 K-type dwarfs with established planets in the habitable zone according to the data from the Kepler space telescope archive acquired during 90-day long Q3-set observations. We found that the spot area $A$ on the surface of K-type dwarfs of our list is greater than the characteristic solar value, but is comparable to the maximum levels observed for the Sun, except for the active star KIC 9351316 with $A=7588$ MSH. We performed a detailed study of the K-type dwarf KIC 5938970 (Kepler-1540) with flare activity higher than that of the Sun and flare energy $\log E$ spanning the 32.95–34.48 interval. We estimated the rotation period of the star, $P=15\overset{\textrm{d}}{.}1447$, and its parameter $\Delta\Omega=0.024\pm 0.007$ rad/day. Based on the Kamogata Wide-field Survey data (99 $V$-band magnitude estimates in 3310 days) we conclude that photometric variations include cyclic changes and a weak monotonic brightness increase. The characteristic periods of possible cycles are equal to 2.8 and 4.4 years. Our study of the star’s activity in about half of the currently known K-type dwarfs with planets in the habitable zone (11 objects among 17 stars in the complete list) based on high-precision observations made with the Kepler telescope revealed certain activity manifestations (high flare activity in one star, high spot activity in another, etc.). We conclude that a detailed study of activity manifestations in each particular star is highly desirable when studying planetary systems of K-type stars including planets in the habitable zone.