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 4000Å to the near-IR at 1.6 µm, 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× higher. Simulated transit signals for a Jupiter-class planet are compared with observations. This (1) indicates that the solar paradigm is consistent with transit observations for stars throughout the activity range explored, provided that infrequent large active regions with fluxes up to ∼ 3 × 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, and suggests (3) 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.

Forward modelling of Kepler-band variability due to faculae and spots

“…Consequently, mean intensity contrasts were derived for the quiet photosphere and facular regions with a range of mean magnetic field strengths in the wavelength range 149.5 nm ≤ λ ≤ 160000.0 nm, finding good agreement with solar observations at dif-ferent heliocentric angles. Facular contrast values based on MURaM calculations were used in Schrijver (2020) to model photospheric facular appearances on Sun-like stars in the ultraviolet (UV), visible and infrared (IR).…”

Torus-Stable Zone Above Starspots

“…Episodes of emergence, decay, and coalescence frequently occur. Forward modelling suggests the same (clustering and nesting) for starspots (Schrijver 2020;Işık et al 2020). If the spots are no longer monolithic, the presence of multiple shorter magnetic flux ropes becomes more likely than the presence a single, long rope.…”

Measurements of the magnetic field strengths at the bases of stellar coronae using the magnetic-field-induced transition theory

“…In this situation a photospheric magnetogram at the model inner boundary cannot be easily constructed from actual observations. Nevertheless, in the near future we plan to construct stellar magnetograms through other approaches, e.g., using stellar flux transport models (Schrijver 2020), and explore the suitability of coronal magnetic field diagnostics with the MIT method.…”