Talk

The rotational evolution of solar-like stars: wind braking vs. rotational coupling

Federico Spada, Max-Planck-Institut für Sonnensystemforschung

Solar-like stars (M < 1.3 Msun) lose angular momentum through their magnetized winds. Measurements of surface rotation periods of the approximately coeval stars in Galactic open clusters provide important clues on the rotational evolution of solar-like stars. In the color-period diagram of young main sequence clusters (e.g., the 100 Myr-old Pleiades, or older) a sequence of relatively slower-rotating stars gradually emerges; so far, this sequence has been observed in clusters up to 4 Gyr of age. In general, stars belonging to the slow rotators sequence evolve coherently towards longer periods in progressively older clusters. Recent observations of the ~700-Myr old Praesepe and the 1-Gyr old NGC 6811 clusters, however, seem to contradict this general pattern. While the 1 Msun stars on the slow rotators sequence of the older NGC 6811 have longer periods than their counterparts in the younger Praesepe, as expected, the two sequences overlap at lower masses (~0.8 Msun). In other words, low-mass stars seem to have not been spinning down at all in the intervening 300 Myr. I will present a model for the rotational evolution of solar-like stars that is consistent with the recent observations of the slow rotators sequence. In particular, the apparent halt in the surface braking of low-mass stars between 700 Myr and 1 Gyr arises naturally in the model from simple assumptions on the wind braking law and the internal rotational coupling. More specifically, the angular momentum transported from the interior temporarily compensates for the loss due to wind braking at the surface. The constraints placed by this simple model on the processes that redistribute angular momentum in stellar interiors, as well as its predictions for stars of different masses and ages will be discussed.