Magnetic regions that brighten the Sun can appear dark on cool red dwarf stars. Researchers at the University of Graz and their international partners have investigated this effect using computer simulations. The study was selected by editors of journals published by the American Astronomical Society (AAS) for a Highlight on AAS Nova, the Society’s research highlights service.
Faculae, the small bright magnetic structures on the Sun, have never been observed directly on another star. Astronomers interpreting the light of other stars have therefore generally assumed that magnetic regions elsewhere behave as they do on the Sun. The Graz team put this assumption to the test.
Magnetic fields suppress the convection that carries heat to a star’s surface, so magnetic regions receive less heat from below and tend to become dark. At the same time, the magnetic field reduces the gas density inside them, allowing us to see deeper, hotter layers. On the Sun, this second effect dominates and faculae appear bright. On cool red dwarfs, however, the magnetic structures are shallower, and the temperature rises more gently with depth. The brightening is then too weak to compensate for the reduced heat supply, and the structures appear dark.
Cool red dwarfs are the most common stars in our Galaxy and are currently receiving particular attention in exoplanet research. Because these stars are small, a planet crossing in front of one blocks a comparatively large fraction of its light, bringing the atmospheres of rocky planets within reach of the James Webb Space Telescope. Astronomers analyse the starlight that passes through such an atmosphere to identify its gases. Magnetic regions on the star leave their own imprint on this light and, if not properly accounted for, can imitate features of the planet’s atmosphere. Whether these regions are bright or dark determines how the measured planetary signal is distorted.
Publication:
A. I. Shapiro, S. Seager, S. K. Solanki et al.: The Curious Case of Dark Faculae on M Dwarf Stars. The Astrophysical Journal 1008, 24 (2026). doi.org/10.3847/1538-4357/ae7105
The work was supported by the ERC Synergy Grant REVEAL, coordinated by the University of Graz.