Differential rotation of Kepler-71 via transit photometry mapping of faculae and starspots

Abstract

Knowledge of dynamo evolution in solar-type stars is limited by the difficulty of using active region monitoring to measure stellar differential rotation, a key probe of stellar dynamo physics. This paper addresses the problem by presenting the first ever measurement of stellar differential rotation for a main-sequence solar-type star using starspots and faculae to provide complementary information. Our analysis uses modelling of light curves of multiple exoplanet transits for the young solar-type star Kepler-71, utilizing archival data from the Kepler mission. We estimate the physical characteristics of starspots and faculae on Kepler-71 from the characteristic amplitude variations they produce in the transit light curves and measure differential rotation from derived longitudes. Despite the higher contrast of faculae than those in the Sun, the bright features on Kepler-71 have similar properties such as increasing contrast towards the limb and larger sizes than sunspots. Adopting a solar-type differential rotation profile (faster rotation at the equator than the poles), the results from both starspot and facula analysis indicate a rotational shear less than about 0.005 rad d −1 , or a relative differential rotation less than 2 per cent, and hence almost rigid rotation. This rotational shear contrasts with the strong rotational shear of zero-age main-sequence stars and the modest but significant shear of the modern-day Sun. Various explanations for the likely rigid rotation are considered.

Superflares and Starspots: when size does not matter

“…Basically the height of this small flux variation is a measure of the spot's intensity whereas the duration of this bump reflects the spot's size. This method has been successfully applied to a few solar-type stars [1,20,32,36,44,45]. This technique of spot transit mapping yields the physical parameters of the spots such as: size, intensity, temperature, and location (longitude and latitude) on the surface of the star.…”

Activity and differential rotation of the early M dwarf Kepler-45 from transit mapping

“…The rotation period is then varied to find the one which best aligns the magnetic features with respect to longitude. The method is the same used previously for CoRoT-2 (Silva-Valio & Lanza 2011), Kepler-17 (Valio et al 2017), and Kepler-71 (Zaleski et al 2019).…”

“…Via transit photometry mapping, the characteristics of individual star-spots and faculae cumulatively build magnetic activity maps of the stellar surface. This technique (see Section 3) has been used to evaluate stellar activity and rotation at transit latitudes for solar-type stars CoRoT-2 (G7V) (Silva-Valio et al 2010;Silva-Valio & Lanza 2011), Kepler-17 (G2V) (Valio et al 2017), Kepler-63 (G5V) (Estrela & Valio 2016), and Kepler-71 (G7V) (Zaleski et al 2019).…”

Testing the Solar Activity Paradigm in the Context of Exoplanet Transits

“…All transit residuals reveal signatures of the facular brightening toward stellar limbs, recognized as dips near the beginnings and ends of the transit residuals (to be compared, for example, to a similarly shaped transit light curve for Kepler-71 shown in Figure 4 of Zaleski et al 2019). This is in effect a wavelength-and activitydependent distortion of the limb-darkening curve (which should be anticipated to be asymmetric because the stellar activity along the transit path is expected to be generally asymmetric).…”