Quantifying the Impact of Starspot-Crossing Events on Retrieved Parameters from Transit Lightcurves

Abstract

Starspot-crossing events (SCEs) in exoplanet transit lightcurves are becoming increasingly common as we focus on cooler host stars and observe higher precision photometric and spectroscopic lightcurves. In this work, we explore how these events affect our retrievals of transit depths and the accuracy with which we can derive spot properties. We inject and recover synthetic SCEs in photometric lightcurves using starry. We find that for high signal-to-noise ratio SCEs, we constrain the spot longitudes tightly (>80% within 1{\deg} of the true value) but degeneracies complicate retrieving spot contrasts, radii, and latitudes (within 17%, 19%, and 9{\deg} respectively). On average, the difference between injected and recovered transit depths is 0.78% or 78.3 ppm. In most (80%) injections, we recover the transit depth to within 0.6%. For transit depths inflated >1.3% by the transit light source effect (TLSE), fitting for a spot crossing improves the transit depth retrieval over masking the SCE in >95% of cases. However, we find that for spots with small contrasts (<5%) and/or covering fractions (<2%), we are likely to overcorrect for the TLSE, recovering a worse transit depth than simply masking. In addition, even when fitted, we find SCEs can inflate the uncertainties on recovered transit depths significantly, especially for JWST-like precisions. Finally, we present a new method, using SCE observables, of carving out the degenerate spot parameter space that provides informative priors for Markov Chain Monte Carlo sampling, demonstrating this technique on a real SCE observed in Kepler-51d's lightcurve.