Extracting starspot structures from exoplanet transit photometry

Context. Future space-based transit photometry can provide empirical comparisons of solar and stellar spot structures with high precision. Spot transit mapping provides a way to directly observe and characterize the location, size, intensity, and evolution of starspots to infer stellar rotation rates and differential rotation. Aims. We present a novel analysis technique to extend the scientific value of exoplanet transit mapping by extracting the umbral and penumbral structure of starspots from flux amplitude variations in transit light curves. We estimate the constraints on penumbral detectability according to transit depth, stellar brightness, and time-correlated and uncorrelated noise. Methods. Our approach used simulated transits of a solar active region to determine the resulting flux ratios of occulted umbral and penumbral regions. The detection threshold of starspot penumbrae could be expressed in terms of flux variations in transit light curves, commensurate with penumbral intensities. We then examined the residual differences between noiseless and noise-added light curves for simulated transits of super-Earth and sub-Neptune sized exoplanets across spotted Sun-like stars. We used the PLATO Solar-like Light-curve Simulator to synthesize realistic photometric noise. Results. We find that, under the right conditions, it is feasible to detect stellar umbrae and penumbrae with flux ratios matching solar values (1.4-4.2) in spotted transit light curves from the PLATO mission. The detection threshold was found to be a function of apparent stellar magnitude, as noise dominates for all but the brightest stars. In particular, penumbral flux variations can be distinguishable in the light curves of exoplanets as small as 3 R⊕ transiting the brightest Sun-like stars. Nevertheless, only the darkest penumbrae (0.66-0.70 Ic) are observed in the transit light curve of even the brightest 1 R⊙ star considered (mv = 8). However, for a 0.85 R⊙ star, the range of penumbral intensities is broader, at mv = 8 (0.66-0.85 Ic). A faint star limit of mv = 10 is found with only the darkest penumbrae (0.66-0.75 Ic) distinguishable, with penumbrae masked by the noise background for mv > 10. Conclusions. High-precision transit photometry such as that from the PLATO mission can provide empirical comparisons of solar and stellar spot structures for an improved understanding of magnetic stellar activity and dynamo mechanisms.