Spot Crossing & TLSE

Stellar surface inhomogeneities—spots and faculae—are a significant source of contamination in transmission spectroscopy. Their effects are wavelength-dependent, which means they can introduce spurious spectral features that mimic or mask real atmospheric signatures. ExoIris provides built-in support for modelling both spot-crossing events during transit and the transit light source effect (TLSE) from unocculted active regions.

Spot Crossing Events

When a transiting planet crosses a spot or bright region on the stellar surface, it produces a characteristic bump or dip in the light curve. The amplitude and shape of this event depend on the contrast between the spot and the surrounding photosphere—a contrast that varies with wavelength. A spot that is prominent in the optical may be nearly invisible in the infrared. Ignoring these events does not just leave residuals in the light curve; it biases the measured transit depth as a function of wavelength, directly contaminating the transmission spectrum.

ExoIris models spot crossings using a generalised Gaussian profile whose chromatic contrast is computed from the BT-Settl stellar atmosphere model grid. This ensures that the wavelength dependence of the crossing event is physically motivated rather than treated as an arbitrary free function. The model captures the essential physics—center time, amplitude, width, profile shape, and spot temperature—while remaining compact enough to fit alongside the transit and baseline parameters.

Transit Light Source Effect

Even when no spots are directly occulted, the presence of active regions on the visible stellar disk alters the effective spectrum of the light blocked by the planet. The planet occults a local patch of the photosphere, but the out-of-transit baseline is set by the disk-integrated flux, which includes contributions from spots and faculae. This mismatch—the transit light source effect—introduces a wavelength-dependent bias in the inferred transmission spectrum.

ExoIris models the TLSE through effective temperatures and covering fractions for both spots and faculae, using the same BT-Settl model grid to ensure physical consistency with the spot crossing model. Because the covering fractions can vary between epochs, the model naturally accommodates stellar variability across multi-epoch datasets.