ИСТИНА

The TESS space mission provides a unique opportunity to investigate the photometric variability of stars (including those with planetary systems) in young moving groups (YMGs). YMGs are dynamically unbound associations of stars that are identified based on their common motion. The ages of YMGs reach 300 Myr and they probe a more continuous range of ages than do young star clusters. The stellar environments in YMGs also differ from those found in high-density star clusters, such as Praesepe or Pleiades. The YMG clusters are less compact and, therefore, stellar dynamical interactions in them are less frequent. As a result, they may be more characteristic of the precursors of exoplanetary systems that orbit typical field stars. Dynamical studies indicate that stellar interactions in open clusters are unlikely to disrupt planetary systems, but milder impacts, such as changes in the eccentricity of planetary orbits, are quite possible. Finally, most known YMGs are substantially less distant than star clusters. This provides additional advantages for detailed characterization of the exoplanetary systems in YMGs through such techniques as transmission spectroscopy and radial velocity monitoring. We report the results of our analysis of the activity of two stars with planetary systems in 45 Myr Tuc – Hor group. DS TUC: A close-in exoplanet with a radius between those of Neptune and Saturn was discovered around star DS Tuc (DS Tuc A, HD 222259A) which is a member of the Tucana – Horologium (Tuc – Hor) group with an age of 45 Myr. DS Tuc is a visual binary consisting of a G6V primary and a K3V secondary. The G6V-type component may be considered as the prototype of a young Sun, while the planetary system itself may be considered as the prototype of a young Solar system. We analyzed the activity of the hot G6V component. The data processing is analogous to that performed by us previously when analyzing the data from the Kepler Space Telescope archive and the archive of TESS observations. According to our estimate, the stellar rotation period is P = 2.85±0.18 days, which coincides with its published estimates. If the appearance of the second peak corresponding to a period of 3.4 days in the power spectrum is interpreted as evidence for the presence of differential stellar rotation, then the differential rotation parameter of DS Tuc is ΔΩ = 0.357±0.010 rad/day. We performed a further analysis of the star’s photometric variability with a mean photometric period of 2.85 days. The light curve was analyzed in detail by the method of solving the inverse problem of reconstructing the maps of surface temperature inhomogeneities. We showed that, as a rule, there are concentrations of spots at two longitudes on the surface maps. The spot positions and sizes undergo changes and are sometimes unstable. The fraction of the spotted stellar surface S is about 3.3 %. We studied the object’s position on the S — age, S — rotation period, and S — Rossby number diagrams and concluded that it corresponds to the general pattern of the dependences established by us previously for young stars with planetary systems. For the first time we have estimated the possible activity cycle of DS Tuc, 1610 days (4.4 years), based on All Sky Automated Survey observations (astrouw.edu.pl/asas). AB PIC: On the base of high-precision data from the archive of the TESS mission the photometric variability of K1 dwarf K1 AB Pic (HD 44627), a member of the Tuc – Hor Association and the host of the distant giant planet with a mass of 13.5 Jupiter masses, was studied. During the observation interval with the TESS mission 48 superflares with energy 6.1•1032–3.4•1035 erg were registered. Estimates of the period of rotation of the star P = 3.860±0.020 days and of the amplitude of the brightness variability were made. The area of spots on the surface of AB Pic significantly exceeds that ones on the Sun, spottedness A varies between 54,000 and 95,000 MSH. It is suggested that during the observation interval the area of spots on the surface of the star had cyclical changes lasting 233 days which can be regarded as the analog of the Rieger cycle on the Sun. Estimates of longer AB Pic activity cycles were based on data from the All Sky Automated Survey observation archive and indicated possible activity cycles of 258 days, 1120 days (3.1 year) and 2640 days (7.2 year).