Analysis of stellar magnetic activity through non-parametric modeling of transits in chromatic light curves: mapping of starspots with the ECLIPSE algorithm

Context. Much like the Sun, measurements of radius and temperature of starspots in young solar-like stars are key parameters to quantify the level of magnetic activity in such stars. The magnetic activity can be particularly impactful in stars hosting transiting exoplanets, because it induces brightness variations that negatively impact the accuracy of exoplanet transit measurements. In particular, these variations can affect the transit depth estimates, having the potential to alter planetary transmission spectra, that could undermine the interpretation of results in exoplanet atmospheric characterizations. 

Aims. Our objective is to study the magnetic activity of solar-like stars to be inferred by studying the starspots transited by hot-Jupiter exoplanets. For that, we used the planetary transit mapping method to probe for occulted spots by the exoplanets, in order to map the physical characteristics of the individual spots. Specifically, our main objective is to break the degeneracy between the radius and intensity (contrast) of stellar spots inferred from planetary transit mapping, using chromatic photometric observations in multiple bands.

Methods. Continuing the previous study of the stellar activity done by our research group, we propose a new non-parametric variation of the transit model incorporating the contamination from occulted starspots, using the ECLIPSE tool. For this purpose, differently from previous studies of targets from the CoRoT and Kepler missions, chromatic data provided by instruments in space and ground-based telescopes will be used, among the combined monochromatic signal and usually denomined as ‘white light’, by the sum of the individual channels. Based on the results from the model fitting of the exoplanet transits with starspots contamination, physical characteristics will be used to estimate the temperature and the intensity of the magnetic field of the spots. Moreover, a possible differential rotation may also be estimated, assuming the persistence of spots among subsequent transits of the exoplanet. 

Results. We employed the transit mapping method to probe starspot-crossing events of the solar-like star CoRoT-2, in transit observations of the exoplanet CoRoT-2 b, using archival observations from the CoRoT space mission, using its three colors (blue, green, and red) from the exoplanet channel. From the intensities of these spots, we estimated the temperatures assuming a blackbody emission and employing a PHOENIX stellar atmospheric model, as well as the relative sizes for the spots. Our results show spots up to about 200 K cooler than the stellar photosphere (5529 K) and having radii of 0.31 stellar radii, or 196 Mm, and both larger than values from previous studies for CoRoT-2 in white light, corresponding to solar penumbra temperatures for spots much larger than typical sunspots. So, we can conclude that the degeneracy between the radius and the intensity of the spots, present in the analysis in "white light", can underestimate the size and intensity of the starspots. We also compare the results from CoRoT to the results from ground-based observations using the multi-channel instruments SPARC4 and MuSCAT3.