How Efficient is Rotational Mixing in Massive Stars ?

Brott, I.; Hunter, I.; Anders, Peter; Langer, N.

doi:10.1063/1.2905562

Cited by 11 publications

(11 citation statements)

References 17 publications

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“…As it must, the effect vanishes for Sc → 0. Schatzman suggested such a relation D T = Re * ν with Re * =O(100) in order to explain diffusion processes in radiative zones of stars [166,167,168,169]. Obviously, the factor Re * , which in a wider sense can be considered as a microscopic Reynolds number (of a pattern cell) can be computed as due to magnetic instability with the presented models.…”

Section: Mixing Of a Passive Scalarmentioning

confidence: 98%

Stability and instability of hydromagnetic Taylor–Couette flows

et al. 2018

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Decades ago S. Lundquist, S. Chandrasekhar, P. H. Roberts and R. J. Tayler first posed questions about the stability of Taylor-Couette flows of conducting material under the influence of large-scale magnetic fields. These and many new questions can now be answered numerically where the nonlinear simulations even provide the instability-induced values of several transport coefficients. The cylindrical containers are axially unbounded and penetrated by magnetic background fields with axial and/or azimuthal components. The influence of the magnetic Prandtl number Pm on the onset of the instabilities is shown to be substantial. The potential flow subject to axial fields becomes unstable against axisymmetric perturbations for a certain supercritical value of the averaged Reynolds number Rm = √Re · Rm (with Re the Reynolds number of rotation, Rm its magnetic Reynolds number). Rotation profiles as flat as the quasi-Keplerian rotation law scale similarly but only for Pm 1 while for Pm 1 the instability instead sets in for supercritical Rm at an optimal value of the magnetic field. Among the considered instabilities of azimuthal fields, those of the Chandrasekhar-type, where the background field and the background flow have identical radial profiles, are particularly interesting. They are unstable against nonaxisymmetric perturbations if at least one of the diffusivities is non-zero. For Pm 1 the onset of the instability scales with Re while it scales with Rm for Pm 1. Even superrotation can be destabilized by azimuthal and current-free magnetic fields; this recently discovered nonaxisymmetric instability is of a double-diffusive character, thus excluding Pm = 1. It scales with Re for Pm → 0 and with Rm for Pm → ∞.The presented results allow the construction of several new experiments with liquid metals as the conducting fluid. Some of them are described here and their results will be discussed together with relevant diversifications of the magnetic instability theory including nonlinear numerical studies of the kinetic and magnetic energies, the azimuthal spectra and the influence of the Hall effect.

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Section: Mixing Of a Passive Scalarmentioning

confidence: 98%

Stability and instability of hydromagnetic Taylor–Couette flows

et al. 2018

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“…In the stellar interior, carbon is converted into nitrogen, which can reach the surface due to mixing and/or mass loss. Brott et al (2008) found that the surface nitrogen abundance can be enhanced by a factor of 11 with a realistic initial SMC mixture. A Solar-scaled mixture with the same iron abundance would lead to enhancement of only a factor of 4, for the same initial parameters.…”

Section: The Initial Chemical Compositionmentioning

confidence: 99%

Rotating massive main-sequence stars

Brott

Mink

Cantiello

et al. 2011

A&A

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899

1,758

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We present a dense grid of evolutionary tracks and isochrones of rotating massive main-sequence stars. We provide three grids with different initial compositions tailored to compare with early OB stars in the Small and Large Magellanic Clouds and in the Galaxy. Each grid covers masses ranging from 5 to 60 M and initial rotation rates between 0 and about 600 km s −1 . To calibrate our models we used the results of the VLT-FLAMES Survey of Massive Stars. We determine the amount of convective overshooting by using the observed drop in rotation rates for stars with surface gravities log g < 3.2 to determine the width of the main sequence. We calibrate the efficiency of rotationally induced mixing using the nitrogen abundance determinations for B stars in the Large Magellanic cloud. We describe and provide evolutionary tracks and the evolution of the central and surface abundances. In particular, we discuss the occurrence of quasi-chemically homogeneous evolution, i.e. the severe effects of efficient mixing of the stellar interior found for the most massive fast rotators. We provide a detailed set of isochrones for rotating stars. Rotation as an initial parameter leads to a degeneracy between the age and the mass of massive main sequence stars if determined from its observed location in the Hertzsprung-Russell diagram. We show that the consideration of surface abundances can resolve this degeneracy.

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“…The turbulent viscosity generated by the AMRI can be very effective at transporting angular momentum (Rüdiger et al 2015), with the magnetic contribution due to Maxwell stresses strongly dominating over the kinetic component. As a consequence, mixing is much less enhanced by the AMRI than angular momentum transport, and the Schmidt number (the ratio of turbulent viscosity and turbulent element diffusion coefficient) is large enough not to speed up stellar evolution significantly Schatzman (1977), Lebreton & Maeder (1987), Brott et al (2008). A distinctive property of the AMRI-induced viscosity is its dependence on the angular velocity shear present in the region where the instability develops.…”

Section: Introductionmentioning

confidence: 99%

Angular momentum transport efficiency in post-main sequence low-mass stars

Spada¹,

Gellert²,

Arlt³

et al. 2016

A&A

105

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Context. Using asteroseismic techniques, it has recently become possible to probe the internal rotation profile of low-mass (≈1.1−1.5 M ) subgiant and red giant stars. Under the assumption of local angular momentum conservation, the core contraction and envelope expansion occurring at the end of the main sequence would result in a much larger internal differential rotation than observed. This suggests that angular momentum redistribution must be taking place in the interior of these stars. Aims. We investigate the physical nature of the angular momentum redistribution mechanisms operating in stellar interiors by constraining the efficiency of post-main sequence rotational coupling. Methods. We model the rotational evolution of a 1.25 M star using the Yale Rotational stellar Evolution Code. Our models take into account the magnetic wind braking occurring at the surface of the star and the angular momentum transport in the interior, with an efficiency dependent on the degree of internal differential rotation. Results. We find that models including a dependence of the angular momentum transport efficiency on the radial rotational shear reproduce very well the observations. The best fit of the data is obtained with an angular momentum transport coefficient scaling with the ratio of the rotation rate of the radiative interior over that of the convective envelope of the star as a power law of exponent ≈3. This scaling is consistent with the predictions of recent numerical simulations of the Azimuthal Magneto-Rotational Instability. Conclusions. We show that an angular momentum transport process whose efficiency varies during the stellar evolution through a dependence on the level of internal differential rotation is required to explain the observed post-main sequence rotational evolution of low-mass stars.

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How Efficient is Rotational Mixing in Massive Stars ?

Cited by 11 publications

References 17 publications

Stability and instability of hydromagnetic Taylor–Couette flows

Stability and instability of hydromagnetic Taylor–Couette flows

Rotating massive main-sequence stars

Angular momentum transport efficiency in post-main sequence low-mass stars

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