Temporal variability of the wind from the star τ Boötis

Nicholson, Belinda; Vidotto, A. A.; Mengel, M. W.; Brookshaw, Leigh; Carter, B.; Petit, P.; Marsden, S. C.; Jeffers, S. V.; Farès, R.

doi:10.1093/mnras/stw731

Cited by 63 publications

(55 citation statements)

References 40 publications

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“…From these studies, authors have found τ Boo A to have a magnetic cycle with polarity reversals in phase with its chromospheric activity cycle of 120 day, as observed for the Sun and 61 Cyg A. Its mass loss rate is not observationally constrained, but MHD simulations of the stellar wind surrounding τ Boo A have been produced by Nicholson et al (2016), using maps from some of the ZDI epochs considered here. We include these results in Figure 5 using blue squares to indicate their derived mass loss rates and angular momentum loss rates.…”

Section: τ Bootis Amentioning

confidence: 85%

“…To date, no single model (including M15) precisely reproduces the observed mass-rotation distributions, but M15 reproduces the broad dependences of rotation rates on mass and age. The torque in this model has two regimes, either unsaturated where the stellar Rossby 2 Average mass loss rate from the MHD simulations of Nicholson et al (2016).…”

Section: Angular Momentum Loss Prescriptionsmentioning

confidence: 99%

“…The predicted R A values for τ Boo A follow a shallower slope than the other dipolar dominated stars, due to the weaker dependence of the octupolar geometry, compared with the dipole or quadrupole geometries, on wind magnetisation in equa-tion (3). The MHD model results of Nicholson et al (2016) for the R A of τ Boo A, are also plotted with light blue squares in Figure 6. Their values for R A are shown to be in good agreement with results from the FM18 braking law.…”

Section: Predicted Alfvén Radiimentioning

confidence: 99%

See 2 more Smart Citations

The Effect of Magnetic Variability on Stellar Angular Momentum Loss. II. The Sun, 61 Cygni A, ϵ Eridani, ξ Bootis A, and τ Bootis A

Finley

See

2019

ApJ

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The magnetic fields of low-mass stars are observed to be variable on decadal timescales, ranging in behaviour from cyclic to stochastic. The changing strength and geometry of the magnetic field should modify the efficiency of angular momentum loss by stellar winds, but this has not been well quantified. In we investigated the variability of the Sun, and calculated the time-varying angular momentum loss rate in the solar wind. In this work, we focus on four low-mass stars that have all had their surface magnetic fields mapped for multiple epochs. Using mass loss rates determined from astrospheric Lyman-α absorption, in conjunction with scaling relations from the MHD simulations of Finley & Matt (2018), we calculate the torque applied to each star by their magnetised stellar winds. The variability of the braking torque can be significant. For example, the largest torque for Eri is twice its decadal averaged value. This variation is comparable to that observed in the solar wind, when sparsely sampled. On average, the torques in our sample range from 0.5 − 1.5 times their average value. We compare these results to the torques of Matt et al. (2015), which use observed stellar rotation rates to infer the long-time averaged torque on stars. We find that our stellar wind torques are systematically lower than the long-time average values, by a factor of ∼ 3 − 30. Stellar wind variability appears unable to resolve this discrepancy, implying that there remain some problems with observed wind parameters, stellar wind models, or the long-term evolution models, which have yet to be understood.

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Section: τ Bootis Amentioning

confidence: 85%

Section: Angular Momentum Loss Prescriptionsmentioning

confidence: 99%

Section: Predicted Alfvén Radiimentioning

confidence: 99%

See 1 more Smart Citation

The Effect of Magnetic Variability on Stellar Angular Momentum Loss. II. The Sun, 61 Cygni A, ϵ Eridani, ξ Bootis A, and τ Bootis A

Finley

See

2019

ApJ

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“…Typically used to determine an initial 3D field solution, then a magnetohydrodynamics code evolves this initial state in time until a steady state solution for the wind and magnetic field geometry is attained (e.g. Vidotto et al 2011;Cohen et al 2011;Garraffo et al 2016b;Réville et al 2016;Alvarado-Gómez et al 2016;Nicholson et al 2016;do Nascimento Jr et al 2016). These works are less conducive to the production of semi-analytical formulations, as the principle drivers of the spin-down process are hidden within complex field geometries, rotation and wind heating physics.…”

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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“…Of relevance to this work, in modelling efforts that attempt to explain stellar spin-down, the stellar orientation is usually assumed fixed, and only the spin-aligned component of angular momentum is typically investigated, preventing models from revealing tilting torques. In reality, stellar winds are highly variable and turbulent (Tu & Marsch 1995;Cranmer et al 2015;Kiyani et al 2015), particularly in the young stages (Nicholson et al 2016;Folsom et al 2016). Turbulence and random perturbations will lead to some degree of stochastic variability in the mean angular momentum axis of the wind (Scherer et al 2001).…”

Section: Introductionmentioning

confidence: 99%

Stellar Winds As a Mechanism to Tilt the Spin Axes of Sun-like Stars

Spalding

2019

ApJ

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The rotation axis of the Sun is misaligned from the mean angular momentum plane of the Solar system by about 6 degrees. This obliquity significantly exceeds the ∼ 1 − 2 degree distribution of inclinations among the planetary orbits and therefore requires a physical explanation. In concert, Sun-like stars are known to spin down by an order of magnitude throughout their lifetimes. This spin-down is driven by the stellar wind, which carries angular momentum from the star. If the mean angular momentum axis of the stellar wind deviates from that of the stellar spin axis, it will lead to a component of the spin-down torque that acts to tilt the star. Here, we show that Solar-like tilts of 6 degrees naturally arise during the first 10-100 Myr after planet formation as a result of stellar winds that deviate by about 10 degrees from the star's spin axis. These results apply to the idealized case of a dipole field, mildly inclined to the spin axis. Time-variability in the misalignment between the magnetic and spin poles is modeled as stochastic fluctuations, autocorrelated over timescales comparable to the primordial spin-down time of several million years. In addition to wind direction, time-variability in mass-loss rate and magnetic topology over the stellar lifetime may alternatively generate obliquity. We hypothesize that the gaseous environments of young, open clusters may provide forcing over sufficient timescales to tilt the astrospheres of young stars, exciting modest obliquities. The more extreme, retrograde stellar obliquities of extrasolar planetary systems likely arise through separate mechanisms.

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Temporal variability of the wind from the star τ Boötis

Cited by 63 publications

References 40 publications

The Effect of Magnetic Variability on Stellar Angular Momentum Loss. II. The Sun, 61 Cygni A, ϵ Eridani, ξ Bootis A, and τ Bootis A

The Effect of Magnetic Variability on Stellar Angular Momentum Loss. II. The Sun, 61 Cygni A, ϵ Eridani, ξ Bootis A, and τ Bootis A

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

Stellar Winds As a Mechanism to Tilt the Spin Axes of Sun-like Stars

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