The spin-down of cool stars on the main-sequence, like our Sun, is a critical area of modern astrophysics, with strong links to stellar structure, evolution, and exoplanet atmospheres. Cool stars (0.2 to 1.5 solar masses) host dynamo-driven magnetic fields that heat their atmospheres and drive stellar winds. Stellar winds also remove angular momentum from their host star, causing them to spin down over the main sequence. This, in turn, weakens the dynamo-generated magnetic field and modulates stellar activity. Both a star’s magnetic activity and the stellar wind affect the environments of orbiting exoplanets and can significantly alter the secular evolution of an exoplanet’s atmosphere. The interdependence of rotation, magnetic activity and angular momentum loss links a star’s rotation rate to its age during the main sequence. This is the basis of gyrochronology, a technique used to estimate ages from rotation periods. The upcoming PLATO mission aims to refine these age estimates, along with magneto-gyrochronology and asteroseismology. However, observations from the Kepler/K2 mission found that our understanding of spin-down still has gaps, particularly concerning the spin-down of older stars like our Sun. Despite recent advances, we still do not understand the physical mechanism(s) that cause stellar spin-down to stall temporarily across the intermediate rotation period gap or that cause the weakening of magnetic braking for older stars than the Sun. These gaps limit the accuracy of gyrochronology and magnetochronology, necessitating further investigation.

This research topic aims to explore potential mechanisms that could cause the observed gaps or epochs of stalling in the spin-down of cool stars. The focus is placed on better understanding the physics that produces the stalling, from the redistribution of angular momentum between the core and envelope, to evolving stellar magnetic field geometries and the weakening of the stellar wind outflow. By examining closely the physics of spin-down across these gaps, this research topic aims to improve the reliability of stellar age estimates derived from gyrochronology and magnetochronology ahead of the ESA PLATO mission.

To better understand the hurdles faced by modern gyrochronology, we welcome articles addressing, but not limited to, the following themes:

• Reviews and mini-reviews on spin-down and angular momentum-loss in cool stars.
• Original research from observations of cool star magnetism to models of stellar coronae, winds, and spin-down.
• Hypotheses and theories that address the observations of stalling (intermediate rotation periods) and weakening (late ages) of spin-down.
• Exploration of the potential limitations for gyrochronology and magnetochronology.
• Analysis of photometric and asteroseismic rotation periods.
• Contextual work for the upcoming ESA PLATO mission.

Are you interested in contributing a manuscript to this Research Topic?

Pushing the Limits of Gyrochronology: New Research on Cool Star Activity and Spin-Down