We here develop an improved way of using a rotating star as a clock, set it using the Sun, and demonstrate that it keeps time well. This technique, called gyrochronology, permits the derivation of ages for solar-and late-type main sequence stars using only their rotation periods and colors. The technique is clarified and developed here, and used to derive ages for illustrative groups of nearby, late-type field stars with measured rotation periods. We first demonstrate the reality of the interface sequence, the unifying feature of the rotational observations of cluster and field stars that makes the technique possible, and extends it beyond the proposal of Skumanich by specifying the mass dependence of rotation for these stars. We delineate which stars it cannot currently be used on. We then calibrate the age dependence using the Sun. The errors are propagated to understand their dependence on color and period. Representative age errors associated with the technique are estimated at ∼15% (plus possible systematic errors) for late F, G, K, & early M stars. Ages derived via gyrochronology for the Mt. Wilson stars are shown to be in good agreement with chromospheric ages for all but the bluest stars, and probably superior. Gyro ages are then calculated for each of the active main sequence field stars studied by Strassmeier and collaborators where other ages are not available. These are shown to be mostly younger than 1 Gyr, with a median age of 365 Myr. The sample of single, late-type main sequence field stars assembled by Pizzolato and collaborators is then assessed, and shown to have gyro ages ranging from under 100 Myr to several Gyr, and a median age of 1.2 Gyr. Finally, we demonstrate that, in contrast to the other techniques, the individual components of the three wide binaries ξBooAB, 61CygAB, & αCenAB yield substantially the same gyro ages.8 He used the the function:5) but f can of course be written in terms of any convenient function of stellar mass. We will modify the expression for f below. 9 I have learned from Ed Guinan (2006, personal communication) that he has been using the Hyades rotational sequence and the Skumanich relation to derive stellar ages. That would make it substantially similar to the technique developed here. 10 We note here that chromospheric emission measurements also require repeated measurement to ensure that they are averages over the variability from rotation or from stellar cycles. 11 In fact, the satellite has been built and launched.

We have constructed a new set of isochrones, called the Y 2 Isochrones, that represent an update of the Revised Yale Isochrones (RYI), using improved opacities and equations of state. Helium diffusion and convective core overshoot have also been taken into consideration. This first set of isochrones is for the scaled solar mixture. A subsequent paper will consider the effects of α-element enhancement, believed to be relevant in many stellar systems. Two additionally significant features of these isochrones are that (1) the stellar models start their evolution from the pre-main sequence birthline instead of from the zero-age main sequence, and (2) the color transformation has been performed using both the latest table of Lejeune et al., and

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.