The Effect of Magnetic Topology on Thermally Driven Wind: Toward a General Formulation of the Braking Law

Réville, Victor; Brun, Allan Sacha; Matt, Sean P.; Strugarek, Antoine; Pinto, Rui

doi:10.1088/0004-637x/798/2/116

Cited by 221 publications

(354 citation statements)

References 55 publications

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“…van Saders et al (2016) suggested that magnetic braking might become less efficient in older stars from a concentration of the field into smaller spatial scales. Réville et al (2015) demonstrated that the dipole component of the global field is responsible for most of the angular momentum loss due to the magnetized stellar wind (see also Garraffo, Drake, and Cohen, 2016). The Alfvén radius is greater for the larger scale components of the field, and because both the open flux and the effective lever-arm increase with increasing Alfven radius, low-order fields consequently shed more angular momentum.…”

Section: Breakdown Of Magnetic Brakingmentioning

confidence: 96%

Magnetic Evolution and the Disappearance of Sun-Like Activity Cycles

Metcalfe

Saders

2017

Sol Phys

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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 middleaged 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.

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Section: Breakdown Of Magnetic Brakingmentioning

confidence: 96%

Magnetic Evolution and the Disappearance of Sun-Like Activity Cycles

Metcalfe

Saders

2017

Sol Phys

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“…More than half of the parameter space that we explore lies in the range where the energy density of the quadrupole mode is greater than that of the dipole (B 2 quad /B 2 dip > 1.0). For this study both the pure dipolar and quadrupolar fields are used as controls (both of which were studied in detail within Réville et al (2015a)), and 5 mixed cases parametrised by R dip values (R dip = 0.8, 0.5, 0.3, 0.2, 0.1). We include R dip = 0.8 to demonstrate the dominance of the dipole at higher values.…”

Section: Numerical Setupmentioning

confidence: 99%

“…Weber & Davis (1967) showed for a one-dimensional magnetised wind that the Alfvén radius represented a lever arm for the spin-down torque. Since the introduction of this result, many researchers have produced scaling laws for the Alfvén radius (Mestel 1984;Kawaler 1988;Matt & Pudritz 2008;Matt et al 2012;Ud-Doula et al 2009;Pinto et al 2011;Réville et al 2015a;Pantolmos. in prep) all of which highlight the importance of the magnetic field strength and mass loss rate in correctly parametrising a power law dependence.…”

Section: Introductionmentioning

confidence: 99%

The Effect of Combined Magnetic Geometries on Thermally Driven Winds. I. Interaction of Dipolar and Quadrupolar Fields

Finley

2017

ApJ

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Cool stars with outer convective envelopes are observed to have magnetic fields with a variety of geometries, which on large scales are dominated by a combination of the lowest order fields such as the dipole, quadrupole and octupole modes. Magnetised stellar wind outflows are primarily responsible for the loss of angular momentum from these objects during the main sequence. Previous works have shown the reduced effectiveness of the stellar wind braking mechanism with increasingly complex, but singular, magnetic field geometries. In this paper, we quantify the impact of mixed dipolar and quadrupolar fields on the spin-down torque using 50 MHD simulations with mixed field, along with 10 of each pure geometries. The simulated winds include a wide range of magnetic field strength and reside in the slow-rotator regime. We find that the stellar wind braking torque from our combined geometry cases are well described by a broken power law behaviour, where the torque scaling with field strength can be predicted by the dipole component alone or the quadrupolar scaling utilising the total field strength. The simulation results can be scaled and apply to all main-sequence cool stars. For Solar parameters, the lowest order component of the field (dipole in this paper) is the most significant in determining the angular momentum loss.

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“…The process begins at Ro ∼ 1, where many global convection simulations exhibit a transition from solar-like to anti-solar differential rotation ( [10]). This change in the character of differential rotation triggers a phase of rapid magnetic evolution leading to a reduction in the efficiency of magnetic braking, probably due to a shift in magnetic topology ( [11]). …”

Section: Motivationmentioning

confidence: 99%

The impact of Gaia DR1 on asteroseismic inferences from Kepler

Metcalfe

Creevey

Saders³

2017

EPJ Web Conf.

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Abstract. The Kepler mission has been fantastic for asteroseismology of solar-type stars, but the targets are typically quite distant. As a consequence, the reliability of asteroseismic modeling has been limited by the precision of additional constraints from highresolution spectroscopy and parallax measurements. A precise luminosity is particularly useful to minimize potential biases due to the intrinsic correlation between stellar mass and initial helium abundance. We have applied the latest version of the Asteroseismic Modeling Portal (AMP) to the complete Kepler data sets for 30 stars with known rotation rates and chromospheric activity levels. We compare the stellar properties derived with and without the measured parallaxes from the first data release of Gaia. We find that in most cases the masses and ages inferred from asteroseismology shift within their uncertainties. For a few targets that show larger shifts, the updated stellar properties only strengthen previous conclusions about anomalous rotation in middle-aged stars. MotivationThe correlation between stellar mass and initial helium abundance is a long-standing problem in modeling solar-type stars. Increasing either the mass or the initial helium yields a model with a higher luminosity, so we can trade off one parameter for the other while still satisfying the observational constraints ([1]). Asteroseismic observations can reduce the severity of this problem by providing a strong constraint on the stellar radius, but the issue cannot be avoided entirely without a more direct constraint on the stellar luminosity. Without such a constraint, the correlation can lead to systematic biases in the stellar properties inferred from asteroseismology. In this paper, we examine the impact of including luminosity constraints derived from Gaia DR1 parallaxes ([2]) for a sample of 30 Kepler targets with known rotation rates ([3]) and chromospheric activity levels ([4]). Our goal is to evaluate the possibility of systematic biases in the asteroseismic masses and ages for stars with anomalous rotation compared to empirical gyrochronology relations ([5]), with implications for a new theory of magnetic evolution beyond middle age ([6]).Until recently, it was presumed that rotation and magnetism decay together throughout the lives of solar-type stars ([7, 8]). Although stars are formed with a range of initial rotation rates, the stellar winds entrained in their magnetic fields lead to angular momentum loss from magnetic braking. This forces convergence to a single rotation rate at a given mass after roughly 500 Myr. The Kepler mission allowed the measurement of rotation periods in old field stars whose masses and ages could be determined from asteroseismology. These new data revealed a population of field stars rotating

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The Effect of Magnetic Topology on Thermally Driven Wind: Toward a General Formulation of the Braking Law

Cited by 221 publications

References 55 publications

Magnetic Evolution and the Disappearance of Sun-Like Activity Cycles

Magnetic Evolution and the Disappearance of Sun-Like Activity Cycles

The Effect of Combined Magnetic Geometries on Thermally Driven Winds. I. Interaction of Dipolar and Quadrupolar Fields

The impact of Gaia DR1 on asteroseismic inferences from Kepler

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