Differential rotation of solar α sunspots and implications for stellar light curves

¹ Leibniz-Institut für Astrophysik Potsdam (AIP), An der Sternwarte 16, 14482 Potsdam, Germany
² Universität Potsdam, Institut für Physik und Astronomie, Karl-Liebknecht-Straße 24/25, 14476 Potsdam, Germany
³ Geneva Observatory, University of Geneva, Chemin Pegasi 51, 1290 Versoix, Switzerland
⁴ Eidgenössische Technische Hochschule Zürich, Wolfgang-Pauli-Str. 27, 8093, Zürich, Switzerland
⁵ Istituto Ricerche Solari Aldo e Cele Daccò (IRSOL), Via Patocchi 57, 6605 Locarno, Switzerland

^★ Corresponding author: apietrow@aip.de

Received: 24 May 2025
Accepted: 11 August 2025

Abstract

Context. Differential rotation is a key driver of magnetic activity and dynamo processes in the Sun and other stars, especially as the rate differs across the solar layers, and also in active regions.

Aims. We aim to accurately quantify the velocity at which round α spots traverse the solar disk as a function of their latitude, and compare these rates to those of the quiet Sun and other sunspot types. We then extend this work to other stars and investigate how differential rotation affects the modulation of stellar light curves by introducing a generalized stellar differential rotation law.

Methods. We manually identified and tracked 105 α sunspots in the 6173 Å continuum using the Helioseismic and Magnetic Imager (HMI) aboard the Solar Dynamics Observatory (SDO). We measured the angular velocities of each spot through center-of-mass and geometric ellipse-fitting methods to derive a differential rotation law for round α sunspots.

Results. Using over a decade of HMI data, we derived a differential rotation law for α sunspots. Compared to previous measurements, we find that α sunspots rotate 1.56% faster than the surrounding quiet Sun, but 1.35% slower than the average sunspot population. This supports the hypothesis that the depth at which flux tubes are anchored influences sunspot motions across the solar disk. We extend this analysis to other stars by introducing a scaling law based on the rotation rates of these stars. This scaling law is implemented with the Stellar Activity Grid for Exoplanets (SAGE) code to illustrate how differential rotation alters the photometric modulation of active stars.

Conclusions. Our findings emphasize the necessity of considering differential rotation effects when modeling stellar activity and exoplanet transit signatures.