On the Rotational Evolution of Solar‐ and Late‐Type Stars, Its Magnetic Origins, and the Possibility of Stellar Gyrochronology

Abstract

We propose a simple interpretation of the rotation period data for solar-and late-type stars. The open cluster and Mt. Wilson star observations suggest that rotating stars lie primarily on two sequences, initially called I and C. Some stars lie in the intervening gap. These sequences, and the fractional numbers of stars on each sequence evolve systematically with cluster age, enabling us to construct crude rotational isochrones allowing 'stellar gyrochronology', a procedure, upon improvement, likely to yield ages for individual field stars.The age and color dependences of the sequences allow the identification of the underlying mechanism, which appears to be primarily magnetic. The majority of solar-and late-type stars possess a dominant Sun-like, or Interface magnetic field, which connects the convective envelope both to the radiative interior of the star and to the exterior where winds can drain off angular momentum. These stars spin down Skumanich-style. An age-decreasing fraction of young G, K, and M stars, which are rapid rotators, possess only a Convective field which is not only inefficient in depleting angular momentum, but also incapable of coupling the surface convection zone to the inner radiative zone, so that only the outer zone is spun down, and on an exponential timescale. These stars do not yet possess large-scale dynamos.The large-scale magnetic field associated with the dynamo, apparently created by the shear between the decoupled radiative and convective zones, (re)couples the convective and radiative zones and drives a star from the Convective to the Interface sequence through the gap on a timescale that increases as stellar mass decreases. Fully convective stars do not possess such an interface, cannot generate an Interface dynamo, and hence can never make such a transition. Helioseismic results for the present day Sun agree with this scheme, which also explains the rotational bi-modality observed by Herbst and collaborators among pre-main sequence stars, and the termination of this bi-modality when stars become fully convective. This paradigm also provides a new basis for understanding stellar X-ray and chromospheric activity, light element abundances, and perhaps other stellar phenomena that depend on rotation.

X-ray emission regimes and rotation sequences in M 35

“…In agreement with the paradigm advanced by Barnes (2003a), M09 found that M 35 stars group into two main subpopulations that lie on narrow sequences in a colour-period diagram (see Fig. 6).…”

“…A strong gradient in angular velocity between the radiative core and the convective envelope is an essential ingredient for the generation of magnetic field at the base of the convection zone and magnetic fields are quite effective at transporting angular momentum (Spruit 2002). In line with the above discussion, Barnes (2003a) proposed that the C sequence is due to the coupling of the stellar wind to just the convective zone that is decoupled from the radiative zone, while the I sequence is due to the coupling of the wind with the entire star. The transition across the gap between these two rotation sequences would therefore be associated with a coupling between the radiative and the convective zone.…”

“…6) displays functional forms of the M 35 rotation isochrones of the I and C sequences. These were first introduced by Barnes (2003a). For the I sequence, I used the form subsequently modified by Barnes (2007) in line with the gyrochronology analysis of M 35 performed by M09.…”

“…Prosser et al 1995;Krishnamurthi et al 1998;Irwin et al 2007;Collier Cameron et al 2009;Hartman et al 2009Hartman et al , 2010. Based on these data, Barnes (2003a) found that young stars tend to group into two main sub-populations that lie on narrow sequences in diagrams where the measured rotation periods of the members of a stellar cluster are plotted against their B − V colour indices. One sequence, called the I sequence, consists of stars that form a diagonal band of increasing period with increasing B − V colour.…”

Differential Rotation in Sun-like Stars from Surface Variability and Asteroseismology

“…The M48 cluster is much older and predominantly consists of stars that have converged onto the main rotation sequence. These same features are seen in other clusters of various ages (Barnes 2003). The cluster members move toward a single well-defined main rotation sequence as the cluster ages.…”