Starspot occultations in infrared transit spectroscopy? The case of WASP-52b

Stellar activity in transit photometry affects the transit shape and the derived transit parameters. In transmission spectroscopy, starspots can mimic Rayleigh scattering in the visible part of the spectrum, which can erroneously be interpreted as the product of hazes in otherwise clear atmospheres. In the near-infrared (NIR), the signal of a starspot is less prominent, but very cool (<3000 K) activity features can potentially mimic water absorption in the transmission spectrum.

We discuss HST/WFC3 NIR observations of the active star WASP-52, obtained as part of a large program for the analysis of exoplanet atmospheres with HST. WASP-52 hosts a transiting hot Jupiter and, during transit, the spectroscopic light curves show a possible starspot occultation signal. We investigate whether the masking of the transit profile distortion or modeling it with a starspot model – which provides an estimate of the starspot temperature – or with a Gaussian process affects the shape of the transmission spectrum. Different methods produce spectra with the same shape and a robust detection of water vapor, and with <1σ different reference radii for the planet. Our WFC3 light curves of WASP-52b hint that starspot occultations might become more problematic with the James Webb Space Telescope’s higher sensitivity and complete coverage of the transit profile.

We combine this data set with HST/STIS and Spitzer WASP-52b observations obtained from other programs, and carry out retrieval exercises in order to place constraints on the atmospheric abundances.


 

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