Rotational mixing in low-mass stars

In the two previous papers of this series, we have discussed the importance of the l-gradients due to helium settling in rotation-induced mixing, first in an approximate analytical way, second in a two-dimensional numerical simulation. We have found that, for slowly rotating low-mass stars, a process of '' creeping paralysis,'' in which the circulation and the diffusion are nearly frozen, may take place below the convective zone. Here we apply this theory to the case of lithium and beryllium in Galactic clusters, especially the Hyades. We take into account the rotational braking with rotation velocities adjusted to the present observations. We find that two different cells of meridional circulation appear on the hot side of the '' lithium dip '' and that the creeping paralysis process occurs, not directly below the convective zone, but deeper inside the radiative zone, at the top of the second cell. As a consequence, the two cells are disconnected, which may be the basic reason for the lithium increase with effective temperature on this side of the dip. On the cool side, there is just one cell of circulation, and the paralysis has not yet set in at the age of the Hyades; the same modeling accounts nicely for the beryllium observations as well as for the lithium ones.

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“…When they are introduced, the two loops reappear (Palacios et al 2003); this is discussed again below.…”

Section: Previous Theoretical Studiesmentioning

confidence: 89%

On the Coupling between Helium Settling and Rotation‐induced Mixing in Stellar Radiative Zones. III. Applications to Light Elements in Population I Main‐Sequence Stars

Théado

Vauclair

2003

ApJ

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“…Additionally, the low lithium abundance for such an intermediate-mass star is a robust signature of rotationinduced mixing when the star was on the main sequence (see e.g. Palacios et al 2003). The 3.5 M rotating model was computed with the same input physics and assumptions as Lagarde et al (2012).…”

Section: Mass Evolutionary Status and Surface Abundances Of β Cetimentioning

confidence: 99%

Magnetic field structure in single late-type giants:βCeti in 2010–2012

Tsvetkova

Petit

Aurière

et al. 2013

A&A

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Aims. We study the behavior of the magnetic field and the line activity indicators of the single late-type giant β Ceti. Using spectropolarimetric data, we aim to reconstruct the magnetic field structure on the star's surface and to present the first magnetic maps for β Ceti. Methods. The data were obtained using two spectropolarimeters -Narval at the Bernard Lyot Télescope, Pic du Midi, France, and ESPaDOnS at CFHT, Hawaii. Thirty-eight circularly-polarized spectra have been collected in the period June 2010-January 2012. The least square deconvolution method was applied for extracting high signal-to-noise ratio line profiles, from which we measured the surface-averaged longitudinal magnetic field B l . Chromospheric activity indicators CaII K, Hα, CaII IR (854.2 nm), and radial velocity were simultaneously measured, and their variability was analyzed along with the behavior of B l . The Zeeman Doppler imaging (ZDI) inversion technique was employed for reconstructing the large-scale magnetic field and two magnetic maps of β Ceti are presented for two periods (June 2010-December 2010 and June 2011-January 2012). Results. The B l stays with a same positive polarity for the whole observational period and shows significant variations in the interval 0.1-8.2 G. The behavior of the line activity indicators is in good agreement with the B l variations. Searching for periodic signals in the Stokes V time series, we found a possible rotation period of 215 days. The two ZDI maps show a mainly axisymmetric and poloidal magnetic topology and a simple surface magnetic field configuration dominated by a dipole. Little evolution is observed between the two maps, in spite of a 1 yr interval between both subsets. We also use state-of-the-art stellar evolution models to constrain the evolutionary status of β Ceti. We derive a mass of 3.5 M and propose that this star is already in the central helium-burning phase. Conclusions. Considering all our results and the evolutionary status of the star, we suggest that dynamo action alone may not be efficient enough to account for the high magnetic activity of β Ceti. As an alternate option, we propose that it is a descendant of an Ap star presently undergoing central helium-burning and still exhibiting a remnant of the Ap star magnetic field.

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“…In this case, all the mixing models used thus far in stellar structure calculations (e.g., Mathis et al 2004;Palacios et al 2003Palacios et al , 2006Charbonnel & Talon 2005 assume the existence of a finite Ri(cr) since their starting point is the relation…”

Section: Resultsmentioning

confidence: 99%

Stellar mixing

Canuto

2011

A&A

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In this paper, salt−fingers (also called thermohaline convection) and semi-convection are treated under the name of double−diffusion (DD). We present and discuss the solutions of the RSM (Reynolds stress models) equations that provide the momentum, heat, μ fluxes, and their corresponding diffusivities denoted by K m,h,μ . Such fluxes are given by a set of linear, algebraic equations that depend on the following variables: mean velocity gradient (differential rotation), temperature gradients (for both stable and unstable regimes), and μ-gradients (DD). Some key results are as follows. Salt−fingers. When shear is strong and DD is inefficient, heat and μ diffusivities are identical. Second, when shear is weak K μ > K h and the difference can be sizeable O(10) meaning that heat and μ diffusivities must therefore be treated as different. Third, for strong-to-moderate shears and for R μ less than 0.8, both heat and μ diffusivities are practically independent of R μ . Fourth, the latter result favors parameterizations of the type K h,μ ∼ CR 0 μ suggested by some authors. Our results, however, show that C is not a constant but a linear function of the Reynolds number Re = ε(νN 2 ) −1 defined in terms of the kinematic viscosity ν, the Brunt-Väisälä frequency N, and the rate of energy input into the system, ε. Fifth, we suggest that ε is an essential ingredient that has been missing in all diffusivity models, but which ought to be present because without a source of energy, turbulence dies out and so does the turbulent mixing (for example, the turbulent kinetic energy is proportional to the power 2/3 of ε). Moreover, since different stellar environments have different ε, its presence is necessary for differentiating mixing regimes in different stars. Semi−convection. In this case the destabilizing effect is the T -gradient, and when shear is weak, K h > K μ . Since the model is symmetric under the change R μ to R −1 μ , most of the results obtained in the previous case can be translated to this case.

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Rotational mixing in low-mass stars

Cited by 172 publications

References 45 publications

On the Coupling between Helium Settling and Rotation‐induced Mixing in Stellar Radiative Zones. III. Applications to Light Elements in Population I Main‐Sequence Stars

On the Coupling between Helium Settling and Rotation‐induced Mixing in Stellar Radiative Zones. III. Applications to Light Elements in Population I Main‐Sequence Stars

Magnetic field structure in single late-type giants:βCeti in 2010–2012

Stellar mixing

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