Activity of the Spectral Class K2V Dwarf HD 189733 in a Hot-Jupiter Type Exoplanet

Abstract

Results are presented from an analysis of manifestations of the activity of the spectral class K2V dwarf HD189733, which has one of the best studied exoplanets of the hot-Jupiter type. Including the activity of the star makes analysis of the characteristics of the planet considerably more complicated (in particular, cool spots on the star’s surface introduce an additional signal in the relation between the depth of a transit and the size of the planet and prevent an exact determination of the radius of the planet). Photometric data for HD189733 in the archive of the TESS mission are analyzed and conclusions are reached on the values of the rotation period P_rot of the star, for which we estimated values of 12.5 and 11.3 days for sectors 41 and 54, respectively. A comparison of these with previous estimates based on earthbound photometric observations suggests that the differences in P_rot at different times are related to the fact of different observation times spots located at different latitudes on the surface of the differentially rotating star predominated. According to the TESS data the amplitude of the brightness variation of HD189733 is 1.6 and 0.9% of the mean brightness level of the star (for sectors 41 and 54, respectively). The standard method was used to estimate the spottedness parameter of the star (2.0 and 1.2% of the star’s surface area) and values of the area of the spotted surface A of the star in absolute terms (9200 MSH and 5400 MSH for sectors 41 and 54, respectively) were obtained. Based on 771 estimates of the star’s brightness in the V band presented in the Kamogata Wide-field Survey, an analysis was made of the long-term activity of HD189733 which yielded indications of the possible existence of activity cycles of duration 1140 and 4220 days (3.1 and 11.6 years, respectively). Based on an analysis of published data it has been concluded that the level of chromospheric activity of HD189733 exceeds the average values typical of other cool dwarfs with analogous values of the color index (B-V) and is higher than the solar value. It is concluded that HD189733 has a moderate (or somewhat higher than average) activity typical of analogous dwarfs in spectral class K but, nevertheless, requires that the properties and evolution of the planet formed around this star be taken into detailed account during modeling, regardless of whether a simplified method or an exact magnetohydrodynamic model is used to estimate them. It should also be noted that the activity of the star is a variable over time (on a time scale comparable to P_rot) and requires estimates that are made simultaneously with the main studies of the exoplanet’s atmosphere.