Dynamical model for spindown of solar-type stars

Abstract

Since their formation, stars slow down their rotation rates by the removal of angular momentum from their surfaces, e.g. via stellar winds. Despite the complexity of the processes involved, a traditional model, where the removal of angular momentum loss by magnetic fields is prescribed, has provided a useful framework to understand observational relations between stellar rotation and age and magnetic field strength. Here, a spindown model is proposed where loss of angular momentum by magnetic fields is evolved dynamically, instead of being kinematically prescribed. To this end, we evolve the stellar rotation and magnetic field simultaneously over stellar evolution time by extending our previous work on a dynamo model which incorporates the nonlinear feedback mechanisms on rotation and magnetic fields. Our extended model reproduces key observations and explains the presence of the two branches of (fast and slow rotating) stars which have different relations between rotation rate $Ω$ vs. time (age), magnetic field strength $|B|$ vs. rotation rate, and frequency of magnetic field $ω_{cyc}$ vs. rotation rate. For fast rotating stars we find: (i) an exponential spindown $Ω\propto e^{-1.35t}$, with $t$ measured in Gyrs, (ii) $|B|$ saturates for higher rotation rate, (iii) $ω_{cyc} \propto Ω^{0.85}$. For slow rotating stars we obtain: (i) a power law spindown $Ω\propto t^{-0.52}$, (ii) $|B|$ scales almost linearly with rotation rate, (iii) $ω_{cyc} \propto Ω^{1.16}$. The results obtained are in good agreement with observations. The Vaughan-Preston gap is consistently explained in our model by the shortest spindown timescale in this transition from fast to slow rotators. Our results highlight the importance of self-regulation of magnetic fields and rotation by direct and indirect interactions involving nonlinear feedback in stellar evolution.