After decades of effort, the solar activity cycle is exceptionally well characterized, but it remains poorly understood. Pioneering work at the Mount Wilson Observatory demonstrated that other ...Sun-like stars also show regular activity cycles, and suggested two possible relationships between the rotation rate and the length of the cycle. Neither of these relationships correctly describes the properties of the Sun, a peculiarity that demands explanation. Recent discoveries have started to shed light on this issue, suggesting that the Sun’s rotation rate and magnetic field are currently in a transitional phase that occurs in all middle-aged stars. Motivated by these developments, we identify the manifestation of this magnetic transition in the best available data on stellar cycles. We propose a reinterpretation of previously published observations to suggest that the solar cycle may be growing longer on stellar evolutionary timescales, and that the cycle might disappear sometime in the next 0.8 – 2.4 Gyr. Future tests of this hypothesis will come from ground-based activity monitoring of
Kepler
targets that span the magnetic transition, and from asteroseismology with the
Transiting Exoplanet Survey Satellite
(TESS) mission to determine precise masses and ages for bright stars with known cycles.
Nearly half a century has passed since the initial indications that stellar rotation slows while chromospheric activity weakens with a power-law dependence on age, the so-called Skumanich relations. ...Subsequent characterization of the mass-dependence of this behavior up to the age of the Sun led to the advent of gyrochronology, which uses the rotation rate of a star to infer its age from an empirical calibration. The efficacy of the method relies on predictable angular momentum loss from a stellar wind entrained in the large-scale magnetic field produced by global dynamo action. Recent observational evidence suggests that the global dynamo begins to shut down near the middle of a star's main-sequence lifetime, leading to a disruption in the production of large-scale magnetic field, a dramatic reduction in angular momentum loss, and a breakdown of gyrochronology relations. For solar-type stars this transition appears to occur near the age of the Sun, when rotation becomes too slow to imprint Coriolis forces on the global convective patterns, reducing the shear induced by differential rotation, and disrupting the large-scale dynamo. We use data from Barnes to reveal the signature of this transition in the observations that were originally used to validate gyrochronology. We propose that chromospheric activity may ultimately provide a more reliable age indicator for older stars, and we suggest that asteroseismology can be used to help calibrate activity-age relations for field stars beyond the middle of their main-sequence lifetimes.
The magnetic activity of the Sun becomes stronger and weaker over roughly an 11 year cycle, modulating the radiation and charged particle environment experienced by the Earth as "space weather." ...Decades of observations from the Mount Wilson Observatory have revealed that other stars also show regular activity cycles in their Ca ii H+K line emission, and identified two different relationships between the length of the cycle and the rotation rate of the star. Recent observations at higher cadence have allowed the discovery of shorter cycles with periods between . Some of these shorter cycles coexist with longer cycle periods, suggesting that two underlying dynamos can operate simultaneously. We combine these new observations with previous data, and show that the longer and shorter cycle periods agree remarkably well with those expected from an earlier analysis based on the mean activity level and the rotation period. The relative turbulent length scales associated with the two branches of cyclic behavior suggest that a near-surface dynamo may be the dominant mechanism that drives cycles in more active stars, whereas a dynamo operating in deeper layers may dominate in less active stars. However, several examples of equally prominent long and short cycles have been found at all levels of activity of stars younger than 2.3 Gyr. Deviations from the expected cycle periods show no dependence on the depth of the convection zone or on the metallicity. For some stars that exhibit longer cycles, we compute the periods of shorter cycles that might be detected with future high-cadence observations.
The 11-year activity cycle of the Sun is a consequence of a dynamo process occurring beneath its surface. We analyzed photometric data obtained by the CoRoT space mission, showing solarlike ...oscillations in the star HD49933, for signatures of stellar magnetic activity. Asteroseismic measurements of global changes in the oscillation frequencies and mode amplitudes reveal a modulation of at least 120 days, with the minimum frequency shift corresponding to maximum amplitude as in the Sun. These observations are evidence of a stellar magnetic activity cycle taking place beneath the surface of HD49933 and provide constraints for stellar dynamo models under conditions different from those of the Sun.
We present a catalog of revised effective temperatures for stars observed in long-cadence mode in the Kepler Input Catalog (KIC). We use Sloan Digital Sky Survey (SDSS) griz filters tied to the ...fundamental temperature scale. Polynomials for griz color-temperature relations are presented, along with correction terms for surface gravity effects, metallicity, and statistical corrections for binary companions or blending. We compare our temperature scale to the published infrared flux method (IRFM) scale for V sub(T)JK sub(s) in both open clusters and the Kepler fields. We find good agreement overall, with some deviations between (J - K sub(s))-based temperatures from the IRFM and both SDSS filter and other diagnostic IRFM color-temperature relationships above 6000 K. For field dwarfs, we find a mean shift toward hotter temperatures relative to the KIC, of order 215 K, in the regime where the IRFM scale is well defined (4000 K to 6500 K). This change is of comparable magnitude in both color systems and in spectroscopy for stars with T sub(eff) below 6000 K. Systematic differences between temperature estimators appear for hotter stars, and we define corrections to put the SDSS temperatures on the IRFM scale for them. When the theoretical dependence on gravity is accounted for, we find a similar temperature scale offset between the fundamental and KIC scales for giants. We demonstrate that statistical corrections to color-based temperatures from binaries are significant. Typical errors, mostly from uncertainties in extinction, are of order 100 K. Implications for other applications of the KIC are discussed.
The magnetic field of the Sun is generated by internal dynamo process with a cyclic period of 11 years or a 22 year magnetic cycle. The signatures of the Sun’s magnetic cycle are observed in the ...different layers of its atmosphere and in its internal layers. In this review, we use the same diagnostics to understand the magnetic cycles of other stars with the same internal structure as the Sun. We review what is currently known about mapping the surface magnetic fields, chromospheric and coronal indicators, cycles in photometry and asteroseismology. We conclude our review with an outlook for the future.
Abstract
The rotation rates of main-sequence stars slow over time as they gradually lose angular momentum to their magnetized stellar winds. The rate of angular momentum loss depends on the strength ...and morphology of the magnetic field, the mass-loss rate, and the stellar rotation period, mass, and radius. Previous observations suggested a shift in magnetic morphology between two F-type stars with similar rotation rates but very different ages (88 Leo and
ρ
CrB). In this Letter, we identify a comparable transition in an evolutionary sequence of solar analogs with ages between 2–7 Gyr. We present new spectropolarimetry of 18 Sco and 16 Cyg A and B from the Large Binocular Telescope, and we reanalyze previously published Zeeman Doppler images of HD 76151 and 18 Sco, providing additional constraints on the nature and timing of this transition. We combine archival X-ray observations with updated distances from Gaia to estimate mass-loss rates, and we adopt precise stellar properties from asteroseismology and other sources. We then calculate the wind braking torque for each star in the evolutionary sequence, demonstrating that the rate of angular momentum loss drops by more than an order of magnitude between the ages of HD 76151 and 18 Sco (2.6–3.7 Gyr) and continues to decrease modestly to the age of 16 Cyg A and B (7 Gyr). We suggest that this magnetic transition may represent a disruption of the global dynamo arising from weaker differential rotation, and we outline plans to probe this phenomenon in additional stars spanning a wide range of spectral types.
Observations of Sun-like stars over the past half-century have improved our understanding of how magnetic dynamos, like that responsible for the 11 yr solar cycle, change with rotation, mass, and ...age. Here we show for the first time how metallicity can affect a stellar dynamo. Using the most complete set of observations of a stellar cycle ever obtained for a Sun-like star, we show how the solar analog HD 173701 exhibits solar-like differential rotation and a 7.4 yr activity cycle. While the duration of the cycle is comparable to that generated by the solar dynamo, the amplitude of the brightness variability is substantially stronger. The only significant difference between HD 173701 and the Sun is its metallicity, which is twice the solar value. Therefore, this provides a unique opportunity to study the effect of the higher metallicity on the dynamo acting in this star and to obtain a comprehensive understanding of the physical mechanisms responsible for the observed photometric variability. The observations can be explained by the higher metallicity of the star, which is predicted to foster a deeper outer convection zone and a higher facular contrast, resulting in stronger variability.
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