Stellar magnetic cycles in the solar-like stars Kepler-17 and Kepler-63

The stellar magnetic field plays a crucial role in the star internal mechanisms, as in the interactions with its environment. The study of starspots provides information about the stellar magnetic field, and can characterise 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 characterise the magnetic activity of stars. Here, we studied two solar-type stars Kepler-17 and Kepler-63 using two methods to estimate the magnetic cycle length. The first one characterises the spots (radius, intensity, and location) 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 in both time series. With the first method, we obtained $P_{\rm cycle}$ = 1.12 $\pm$ 0.16 yr (Kepler-17) and $P_{\rm cycle}$ = 1.27 $\pm$ 0.16 yr (Kepler-63), and for the second approach the values are 1.35 $\pm$ 0.27 yr and 1.27 $\pm$ 0.12 yr, respectively. The results of both methods agree with each other and confirm their robustness.
Comment: Accepted for publication in ApJ. 18 pages, 27 figures