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Register a new userCorrecting for chromatic stellar activity effects in transits with multiband photometric monitoring: Application to WASP-52
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The properties of inhomogeneities on the surface of active stars (i.e. dark spots and bright faculae) significantly influence the determination of the parameters of an exoplanet. The chromatic effect they have on transmission spectroscopy could affect the analysis of data from future space missions such as JWST and Ariel. To quantify and mitigate the effects of those surface phenomena, we developed a modelling approach to derive the surface distribution and properties of active regions by modelling simultaneous multi-wavelength time-series observables. By using the StarSim code, now featuring the capability to solve the inverse problem, we analysed $sim$ 600 days of BVRI multiband photometry from TJO and STELLA telescopes exoplanet host star WASP-52. From the results, we simulated the chromatic contribution of surface phenomena on the observables of its transits. We are able to determine the relevant activity parameters of WASP-52 and reconstruct the time-evolving longitudinal map of active regions. The star shows a heterogeneous surface composed of dark spots with a mean temperature contrast of $575pm150$ K with filling factors ranging from 3 to 14 %. We studied the chromatic effects on the depths of transits obtained at different epochs with different stellar spot distributions. For WASP-52, with peak-to-peak photometric variations of $sim$7 % in the visible, the residual effects of dark spots on the measured transit depth, after applying the calculated corrections, are about $10^{-4}$ at 550 nm and $3times10^{-5}$ at 6$mu$m. We demonstrate that by using contemporaneous ground-based multiband photometry of an active star, it is possible to reconstruct the parameters and distribution of active regions over time, and thus, quantify the chromatic effects on the planetary radii measured with transit spectroscopy and mitigate them by about an order of magnitude.
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We present a novel, iterative method using an empirical Bayesian approach for modeling the limb darkened WASP-121b transit from the TESS light curve. Our method is motivated by the need to improve $R_{p}/R_{ast}$ estimates for exoplanet atmosphere modeling, and is particularly effective with the limb darkening (LD) quadratic law requiring no prior central value from stellar atmospheric models. With the non-linear LD law, the method has all the advantages of not needing atmospheric models but does not converge. The iterative method gives a different $R_{p}/R_{ast}$ for WASP-121b at a significance level of 1$sigma$ when compared with existing non-iterative methods. To assess the origins and implications of this difference, we generate and analyze light curves with known values of the limb darkening coefficients (LDCs). We find that non-iterative modeling with LDC priors from stellar atmospheric models results in an inconsistent $R_{p}/R_{ast}$ at 1.5$sigma$ level when the known LDC values are as those previously found when modeling real data by the iterative method. In contrast, the LDC values from the iterative modeling yields the correct value of $R_{p}/R_{ast}$ to within 0.25$sigma$. For more general cases with different known inputs, Monte Carlo simulations show that the iterative method obtains unbiased LDCs and correct $R_{p}/R_{ast}$ to within a significance level of 0.3$sigma$. Biased LDC priors can cause biased LDC posteriors and lead to bias in the $R_{p}/R_{ast}$ of up to 0.82$%$, 2.5$sigma$ for the quadratic law and 0.32$%$, 1.0$sigma$ for the non-linear law. Our improvement in $R_{p}/R_{ast}$ estimation is important when analyzing exoplanet atmospheres.
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