Description
The magnetism and rotation of cool stars evolve in an intertwined manner over secular times. However, recent observations have questioned this link, in particular as solar-type stars older than the Sun possibly exhibit stronger magnetic proxies and rotate faster than predicted. To assess how the combination of rotation and convection via the Rossby number influences the type of rotation and magnetism established, we analyse a set of 15 global MHD models of solar-type stars dynamo, and confront it to current observations. These models harbor different rotation and dynamo regimes as a function of the Rossby number Ro and show an overall agreement with observational trends, making us able to propose a physics-based ‘Sun in Time’ scenario : While aging and reaching intermediate Ro values, the dynamo nature changes and cycle periods increase to decadal ranges. In addition, we find that the magnetic field amplitude reaches a minimum around Ro~1, suggesting that a stalling of cool stars’ spin-down could originate from a minimum of their magnetism. Then, they may undergo a transition towards anti-solar DR and lose cycles, while building back a stronger large-scale field, and possibly revive their spin-down. We first discuss the numerical evidence of such a scenario, and further present an observational study of the Kepler field, seeking to identify high-Rossby stellar targets in order to constrain the Ro~1 limit. To this end, we developed an new observational formula for Ro, that could also be applied to future missions, such as PLATO.
