Testing the Solar Activity Paradigm in the Context of Exoplanet Transits

Abstract

Transits of exoplanets across cool stars contain blended information about structures on the stellar surface and about the planetary body and atmosphere. To advance understanding of how this information is entangled, a surface-flux transport code, based on observed properties of the Sun's magnetic field, is used to simulate the appearance of hypothetical stellar photospheres from the visible near 4000 Angstrom to the near-IR at 1.6 micron, by mapping intensities characteristic of faculae and spots onto stellar disks. Stellar appearances are computed for a Sun-like star of solar activity up to a star with mean magnetic flux density ~30 times higher. Simulated transit signals for a Jupiter-class planet are compared with observations. This (1) suggests that the solar paradigm is consistent with observations for stars throughout the activity range explored provided that infrequent large active regions with fluxes up to $\sim 3\times 10^{23}$ Mx are included in the emergence spectrum, (2) quantitatively confirms that for such a model, faculae brighten relatively inactive stars while starspots dim more active stars, (3) suggests that large starspots inferred from transits of active stars are consistent with clusters of more compact spots seen in the model runs, (4) that wavelength-dependent transit-depth effects caused by stellar magnetic activity for the range of activity and the planetary diameter studied here can introduce apparent changes in the inferred exoplanetary radii across wavelengths from a few hundred to a few thousand kilometers, increasing with activity, and (5) that activity-modulated distortions of broadband stellar radiance across the visible to near-IR spectrum can reach several percent.