Constraints on Magnetic Braking from the G8 Dwarf Stars 61 UMa and $\tau$ Cet

Abstract

During the first half of their main-sequence lifetimes, stars rapidly lose angular momentum to their magnetized winds, a process known as magnetic braking. Recent observations suggest a substantial decrease in the magnetic braking efficiency when stars reach a critical value of the Rossby number, the stellar rotation period normalized by the convective overturn timescale. Cooler stars have deeper convection zones with longer overturn times, reaching this critical Rossby number at slower rotation rates. The nature and timing of the transition to weakened magnetic braking has previously been constrained by several solar analogs and two slightly hotter stars. In this Letter, we derive the first direct constraints from stars cooler than the Sun. We present new spectropolarimetry of the old G8 dwarf $\tau$ Cet from the Large Binocular Telescope, and we reanalyze a published Zeeman Doppler image of the younger G8 star 61 UMa, yielding the large-scale magnetic field strengths and morphologies. We estimate mass-loss rates using archival X-ray observations and inferences from Ly$\alpha$ measurements, and we adopt other stellar properties from asteroseismology and spectral energy distribution fitting. The resulting calculations of the wind braking torque demonstrate that the rate of angular momentum loss drops by a factor of 300 between the ages of these two stars (1.4-9 Gyr), well above theoretical expectations. We summarize the available data to help constrain the value of the critical Rossby number, and we identify a new signature of the long-period detection edge in recent measurements from the Kepler mission.

ApJ Letters (accepted), 6 pages including 3 figures and 1 table. Python code is available at https://github.com/travismetcalfe/FinleyMatt2018

References in corpus (21)

• Second ROSAT all-sky survey (2RXS) source catalogue
• Accurate, Empirical Radii and Masses of Planets and their Host Stars with Gaia Parallaxes
• Toroidal versus poloidal magnetic fields in Sun-like stars: a rotation threshold
• The effect of magnetic topology on thermally-driven winds: towards a general formulation of the braking law
• Empirical, Accurate Masses and Radii of Single Stars with TESS and Gaia
• PEPSI: The high-resolution echelle spectrograph and polarimeter for the Large Binocular Telescope
• Surface rotation and photometric activity for Kepler targets. II. G and F main-sequence stars, and cool subgiant stars
• The evolution of surface magnetic fields in young solar-type stars II: The early main sequence (250-650 Myr)
• The Mount Wilson Observatory S-index of the Sun
• New Observational Constraints on the Winds of M Dwarf Stars
• Weakened magnetic braking supported by asteroseismic rotation rates of Kepler dwarfs
• Estimating magnetic filling factors from Zeeman-Doppler magnetograms
• Solar-like oscillations in the G8 V star tau Ceti
• The Origin of Weakened Magnetic Braking in Old Solar Analogs
• The Debris Disk of Solar Analogue $\tau$ Ceti: Herschel Observations and Dynamical Simulations of the Proposed Multiplanet System
• A calibration of the Rossby number from asteroseismology
• Further Evidence of Modified Spin-down in Sun-like Stars: Pileups in the Temperature-Period Distribution
• Magnetic and Rotational Evolution of $\rho$ CrB from Asteroseismology with TESS
• LBT/PEPSI Spectropolarimetry of a Magnetic Morphology Shift in Old Solar-type Stars
• Detectability of Rotational Modulation in Kepler Sun-like Stars as a Function of Age
• Extrasolar Planets and Their Host Stars

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• TESS asteroseismology of $\beta$ Hydri: a subgiant with a born-again dynamo
• Towards a holistic magnetic braking model from the evolution of cataclysmic variables to stellar spin-down -- I: the spin-down of fully convective M-dwarfs
• Testing the Rossby Paradigm: Weakened Magnetic Braking in early K-type Stars
• Towards a holistic magnetic braking model -- II: explaining several long-term internal- and surface-spin properties of solar-like stars and the Sun
• Breaking gyrochronology through the collapse of coronal winds
• Zeeman Doppler Imaging of $\tau$Ceti: The Weakest Magnetic Field Detected in a Sun-like Star