Radiation pressure and velocity gradients in the Great Dimming context

Red supergiants are cool and massive stars that are known to lose a part of their mass during this evolutionary phase. However, the mechanisms responsible for triggering stellar winds remain poorly understood. The discovery of linear polarization in the atomic lines of the red supergiant Betelgeuse has unveiled a wealth of information about the dynamics of the photosphere, offering the possibility of reconstructing three-dimensional images of the surface of the star and revealing the presence of large convective cells evolving on different timescales. These images showed that some cells exhibited nearly constant velocities throughout the atmosphere, indicating the presence of a force counterbalancing gravity, already present at photospheric levels. In 2020, Betelgeuse underwent a historical dimming, known as the Great Dimming, associated with a strong mass-loss event. Linear polarization revealed a strong increase in opacity at photospheric levels months before the Great Dimming. This increase in opacity led to an increase in radiation pressure, thus counterbalancing gravity and allowing the plasma to escape Betelgeuse’s gravity. At the same time, the spectral line profiles showed the presence of low velocity gradients propagating through the atmosphere, consistent with a force acting against gravity. The magnetic field of Betelgeuse remained nearly constant during this period, at a level of a few gauss, suggesting that the magnetic field plays only a marginal role in the mass-loss process of red supergiants. These results show that radiation pressure at photospheric levels may be able to trigger mass-loss events in red supergiants.