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Magnetic Evolution and the Disappearance of Sun-like Activity Cycles  

Travis Metcalfe   Submitted: 2017-08-04 15:34

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 describe 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 TESS mission to determine precise masses and ages for bright stars with known cycles.

Authors: Travis S. Metcalfe and Jennifer van Saders
Projects: Mount Wilson Observatory,Other

Publication Status: Accepted for publication in Solar Physics
Last Modified: 2017-08-07 12:04
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Stellar Evidence that the Solar Dynamo may be in Transition  

Travis Metcalfe   Submitted: 2016-06-29 10:54

Precise photometry from the Kepler space telescope allows not only the measurement of rotation in solar-type field stars, but also the determination of reliable masses and ages from asteroseismology. These critical data have recently provided the first opportunity to calibrate rotation-age relations for stars older than the Sun. The evolutionary picture that emerges is surprising: beyond middle-age the efficiency of magnetic braking is dramatically reduced, implying a fundamental change in angular momentum loss beyond a critical Rossby number (Ro~2). We compile published chromospheric activity measurements for the sample of Kepler asteroseismic targets that were used to establish the new rotation-age relations. We use these data along with a sample of well characterized solar analogs from the Mount Wilson HK survey to develop a qualitative scenario connecting the evolution of chromospheric activity to a fundamental shift in the character of differential rotation. We conclude that the Sun may be in a transitional evolutionary phase, and that its magnetic cycle might represent a special case of stellar dynamo theory.

Authors: Travis S. Metcalfe, Ricky Egeland, Jennifer van Saders
Projects: Mount Wilson Observatory,Other

Publication Status: ApJL (accepted)
Last Modified: 2016-06-29 13:47
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Magnetic Activity Cycles in the Exoplanet Host Star epsilon Eridani  

Travis Metcalfe   Submitted: 2012-12-18 10:47

The active K2 dwarf epsilon Eri has been extensively characterized, both as a young solar analog and more recently as an exoplanet host star. As one of the nearest and brightest stars in the sky, it provides an unparalleled opportunity to constrain stellar dynamo theory beyond the Sun. We confirm and document the 3 year magnetic activity cycle in epsilon Eri originally reported by Hatzes and coworkers, and we examine the archival data from previous observations spanning 45 years. The data show coexisting 3 year and 13 year periods leading into a broad activity minimum that resembles a Maunder minimum-like state, followed by the resurgence of a coherent 3 year cycle. The nearly continuous activity record suggests the simultaneous operation of two stellar dynamos with cycle periods of 2.95±0.03 years and 12.7±0.3 years, which by analogy with the solar case suggests a revised identification of the dynamo mechanisms that are responsible for the so-called ''active'' and ''inactive'' sequences as proposed by Bohm-Vitense. Finally, based on the observed properties of epsilon Eri we argue that the rotational history of the Sun is what makes it an outlier in the context of magnetic cycles observed in other stars (as also suggested by its Li depletion), and that a Jovian-mass companion cannot be the universal explanation for the solar peculiarities.

Authors: T.S. Metcalfe, A.P. Buccino, B.P. Brown, S. Mathur, D.R. Soderblom, T.J. Henry, P.J.D. Mauas, R. Petrucci, J.C. Hall, S. Basu
Projects: None

Publication Status: ApJ Letters (accepted)
Last Modified: 2012-12-18 15:04
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Abstracts by Author
Magnetic Evolution and the Disappearance of Sun-like Activity Cycles
Stellar Evidence that the Solar Dynamo may be in Transition
Magnetic Activity Cycles in the Exoplanet Host Star epsilon Eridani

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