Identificação de ciclos magnéticos em estrelas do tipo solar observadas pelo satélite Kepler

Abstract

The stellar magnetic field plays a crucial role in the star internal mechanisms, as well as in the interactions with its environment. The study of starspots gives us information about the magnetic field of the star, and characterizes the cycle. Moreover, the analysis of solar-type stars is also useful to shed light onto the origin of the solar magnetic field. The objective of this work is to characterize the magnetic activity of stars. Initially, we studied two solar-type stars Kepler-17 and Kepler-63. Two methods were used to estimate the magnetic cycle length. The first one characterizes the spots (radius and intensity) by fitting the small variations in the light curve of a star caused by the occultation of a spot during a planetary transit. This approach yields the number of spots present in the stellar surface and the flux deficit subtracted from the star by their presence during each transit. The second method estimates the activity from the excess in the residuals of the transit lightcurves. This excess is obtained by subtracting a spotless model transit from the lightcurve, and then integrating all the residuals during the transit. The presence of long term periodicity is estimated from the analysis of a Lomb-Scargle periodogram of both time series. With the first method, we obtained Pcycle = 1.12 0.16 year (Kepler-17) and Pcycle = 1.27 0.16 year (Kepler-63), and for the second approach the values are 1.35 0.27 year and 1.27 0.12 year, respectively. Since the results of both methods agreed with each other, we used the second method to estimate short magnetic cycles of four more active stars with transiting planets observed by Kepler. These periods are consistent with short cycle periods of stars found in the literature.