Abstract
The aim of our paper has been to study the activity of the FK Com type star rapidly rotating G5 III–IV giant HD 199178 (V1794 Cyg) based on available photometric data from the TESS mission in five sectors. The light curves of the star characterized the variability caused by rotational modulation due to the presence of cool spots on the surface, power spectra, and phase diagrams have been studied. In the power spectrum, based on the combined data of all observation sectors, the dominant peak has corresponded to the photometric period of \(P{{ = 3.28}^{d}} \pm {{0.12}^{d}}\) . It has been suggested that a decrease in the \(P\) value over time (over an interval of ~40 years) from 3.337d up to 3.28d may be due to the long-term evolution of the positions of the active areas (spots) on the surface of a differentially rotating star. The changes in the amplitude of the brightness variability and the shape of the phase curves of HD 199178 have been analyzed and it has been found that the fraction of the star’s surface occupied by spots varied from 7 to 12% of its surface area. The flare activity of the star has been studied. The parameters of the amplitude, duration, and energy of the flares have been determined for two flares. One of the considered flares is the strongest during the observations in five sectors (5.1 × 1035 erg) and in magnitude can characterize the upper limit of the flare energies in a given state of star activity. Maps of the surface temperature inhomogeneities of the star have been constructed.
