Stellar contamination in transmission spectroscopy: Modelling spot-crossing events in transit light curves

Marylou Fournier Tondreau ( Université de Montréal )

The inhomogeneities of active stars, like starspots and faculae, complicate transmission spectroscopy and can significantly impact spectral features. The transits of HAT-P-18 b’s and WASP-52 b’s observed with the JWST/NIRISS show spot-crossing events. A possible spot-crossing was also suspected on the second transit observed of Trappist-1b. It is recommended to perform joint inference of the starspots and planet properties with the James Webb Space Telescope (JWST) instead of masking the active regions to avoid bias in the transit depth measurements (Rackham et al., 2022). This research aims to establish what properties of occulted starspots can be inferred from transits of exoplanets observed with the NIRISS (Near Infrared Imager and Slitless Spectrometer) instrument of the JWST. Thereby, I adapted and implemented SPOTROD (Béky et al., 2014), a model for transits of spotted stars, into the Juliet tool (Espinoza et al., 2019). I can efficiently fit transit light curves with spot crossings and retrieve the spot’s coordinates, radius and the spot contrast spectrum. I can then constrain the spot temperature and its surface gravity by fitting the spot contrast spectrum with PHOENIX stellar spectra (Husser et al., 2013; Bruno et al., 2022). However, a certain degree of degeneracy between those parameters is unavoidable (Béky et al., 2014). I will present my results for the different transits and discuss the challenge encountered due to the strong correlation between the fraction of occultation of the starspot and its temperature.