Testing the Rossby Paradigm: Weakened Magnetic Braking in early K-type Stars

Abstract

There is an intricate relationship between the organization of large-scale magnetic fields by a stellar dynamo and the rate of angular momentum loss due to magnetized stellar winds. An essential ingredient for the operation of a large-scale dynamo is the Coriolis force, which imprints organizing flows on the global convective patterns and inhibits the complete cancellation of bipolar magnetic regions. Consequently, it is natural to expect a rotational threshold for large-scale dynamo action and for the efficient angular momentum loss that it mediates through magnetic braking. Here we present new observational constraints on magnetic braking for an evolutionary sequence of six early K-type stars. To determine the wind braking torque for each of our targets, we combine spectropolarimetric constraints on the large-scale magnetic field, Ly-alpha or X-ray constraints on the mass-loss rate, as well as uniform estimates of the stellar rotation period, mass, and radius. As identified previously from similar observations of hotter stars, we find that the wind braking torque decreases abruptly by more than an order of magnitude at a critical value of the stellar Rossby number. Given that all of the stars in our sample exhibit clear activity cycles, we suggest that weakened magnetic braking may coincide with the operation of a subcritical stellar dynamo.

ApJ accepted, 13 pages including 6 figures and 1 table. Python code is available at https://github.com/travismetcalfe/FinleyMatt2018

References in corpus (42)

• The surprising magnetic topology of tauSco: fossil remnant or dynamo output?
• Accurate, Empirical Radii and Masses of Planets and their Host Stars with Gaia Parallaxes
• Stellar magnetism: empirical trends with age and rotation
• Parallax Systematics and Photocenter Motions of Benchmark Eclipsing Binaries in Gaia EDR3
• A Guide to Realistic Uncertainties on Fundamental Properties of Solar-Type Exoplanet Host Stars
• Empirical, Accurate Masses and Radii of Single Stars with TESS and Gaia
• Magnetism, rotation, and nonthermal emission in cool stars -- Average magnetic field measurements in 292 M dwarfs
• PEPSI: The high-resolution echelle spectrograph and polarimeter for the Large Binocular Telescope
• When Do Stalled Stars Resume Spinning Down? Advancing Gyrochronology with Ruprecht 147
• The evolution of surface magnetic fields in young solar-type stars II: The early main sequence (250-650 Myr)
• A Temporary Epoch of Stalled Spin-Down for Low-Mass Stars: Insights from NGC 6811 with Gaia and Kepler
• The Mount Wilson Observatory S-index of the Sun
• New Observational Constraints on the Winds of M Dwarf Stars
• Magnetic Activity Cycles in the Exoplanet Host Star epsilon Eridani
• The connection between stellar activity cycles and magnetic field topology
• Weakened magnetic braking supported by asteroseismic rotation rates of Kepler dwarfs
• The Metastable Dynamo Model of Stellar Rotational Evolution
• Powering Stellar Magnetism: Energy Transfers in Cyclic Dynamos of Sun-like Stars
• The Missing Magnetic Morphology Term in Stellar Rotation Evolution
• Rotation Periods, Inclinations, and Obliquities of Cool Stars Hosting Directly Imaged Substellar Companions: Spin-Orbit Misalignments are Common
• Five Decades of Chromospheric Activity in 59 Sun-like Stars and New Maunder Minimum Candidate HD 166620
• The Origin of Weakened Magnetic Braking in Old Solar Analogs
• Evidence for Metallicity-Dependant Spin Evolution in the Kepler field
• Gyro-Kinematic Ages for around 30,000 Kepler Stars
• Starspots and Magnetism: Testing the Activity Paradigm in the Pleiades and M67
• Bridging the gap -- the disappearance of the intermediate period gap for fully convective stars, uncovered by new ZTF rotation periods
• A calibration of the Rossby number from asteroseismology
• Further Evidence of Modified Spin-down in Sun-like Stars: Pileups in the Temperature-Period Distribution
• Multi-instrumental view of magnetic fields and activity of $\epsilon$ Eridani with SPIRou, NARVAL, and TESS
• The Evolution of Rotation and Magnetic Activity in 94 Aqr Aa from Asteroseismology with TESS
• Tess asteroseismology of the known planet host star ${\lambda }^2$ Fornacis
• Magnetic and Rotational Evolution of $\rho$ CrB from Asteroseismology with TESS
• LBT/PEPSI Spectropolarimetry of a Magnetic Morphology Shift in Old Solar-type Stars
• Constraints on Magnetic Braking from the G8 Dwarf Stars 61 UMa and $\tau$ Cet
• Deciphering stellar chorus: apollinaire, a Python 3 module for Bayesian peakbagging in helio- and asteroseismology
• Weakened Magnetic Braking in the Exoplanet Host Star 51 Peg
• Asteroseismology and Spectropolarimetry of the Exoplanet Host Star $\lambda$ Serpentis
• Detectability of Rotational Modulation in Kepler Sun-like Stars as a Function of Age
• TESS asteroseismology of $\beta$ Hydri: a subgiant with a born-again dynamo
• A multi-wavelength view of the multiple activity cycles of $\epsilon$~Eridani
• Overview and Validation of the Asteroseismic Modeling Portal v2.0
• Extending the Asteroseismic Calibration of the Stellar Rossby Number

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