Can extended mixing in red giants be attributed to magnetic mechanisms (original) (raw)
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Magnetic Mixing in Red Giant and Asymptotic Giant Branch Stars
The Astrophysical Journal, 2008
The available information on isotopic abundances in the atmospheres of low-mass Red Giant Branch (RGB) and Asymptotic Giant Branch (AGB) stars requires that episodes of extensive mixing occur below the convective envelope, reaching down to layers close to the hydrogen burning shell (Cool Bottom Processing). Recently suggested that dynamo-produced buoyant magnetic flux tubes could provide the necessary physical mechanisms and also supply sufficient transport rates. Here, we present an α − Ω dynamo in the envelope of an RGB/AGB star in which shear and rotation drain via turbulent dissipation and Poynting flux. In this context, if the dynamo is to sustain throughout either phase, convection must resupply shear. Under this condition, volume-averaged, peak toroidal field strengths of B φ ≃ 3 × 10 3 G (RGB) and B φ ≃ 5 × 10 3 G (AGB) are possible at the base of the convection zone. If the magnetic fields are concentrated in flux tubes, the corresponding field strengths are comparable to those required by Cool Bottom Processing.
Can Extra Mixing in RGB and AGB Stars Be Attributed to Magnetic Mechanisms
Astrophysical Journal, 2007
It is known that there must be some weak form of transport (called cool bottom processing, or CBP) acting in low mass RGB and AGB stars, adding nuclei, newly produced near the hydrogen-burning shell, to the convective envelope. We assume that this extra-mixing originates in a stellar dynamo operated by the differential rotation below the envelope, maintaining toroidal magnetic fields near the hydrogen-burning shell. We use a phenomenological approach to the buoyancy of magnetic flux tubes, assuming that they induce matter circulation as needed by CBP models. This establishes requirements on the fields necessary to transport material from zones where some nuclear burning takes place, through the radiative layer, and into the convective envelope. Magnetic field strengths are determined by the transport rates needed by CBP for the model stellar structure of a star of initially 1.5 M ⊙ , in both the AGB and RGB phases. The field required for the AGB star in the processing zone is B 0 ∼ 5 × 10 6 G; at the base of the convective envelope this yields an intensity B E 10 4 G. For the RGB case, B 0 ∼ 5 × 10 4 − 4 × 10 5 G, and the corresponding B E are ∼ 450 − 3500 G. These results are consistent with existing observations on AGB stars. They also hint at the basis for high field sources in some planetary nebulae and the very large fields found in some white dwarfs. It is concluded that transport by magnetic buoyancy should be considered as a possible mechanism for extra mixing through the radiative zone, as is required by both stellar observations and the extensive isotopic data on circumstellar condensates found in meteorites.
Extra-mixing in AGB Stars from magnetic buoyancy
2007
We study the circulation of matter in AGB stars above the H-burning shell, which is known to yield the appearance of p-rich isotopes like 13C, 17O and the unstable 26Al at the stellar surface. These nuclei were observed in presolar grains of AGB origin and in some cases (e.g. C isotopes) in spectroscopic observations of evolved stars. For the physical mechanism driving the mixing we consider magnetic flux tube buoyancy. Magnetic tubes are formed below the convective envelope and one can express the parameters of the required mixing phenomena in terms of the magnetic field intensity |− →B |. We show that the required values of |− →B | can span a considerable range. If the mixing arrives only at the innermost layers of the convective envelope, fields smaller than the solar ones are sufficient, according to our previous analysis. If instead magnetic fields have to appear at the surface, as is suggested by recent observations, than field intensities in the Megagauss range are
The Astrophysical Journal, 2014
The advection of thermonuclear ashes by magnetized domains emerging from near the Hshell was suggested to explain AGB star abundances. Here we verify this idea quantitatively through exact MHD models. Starting with a simple 2D geometry and in an inertia frame, we study plasma equilibria avoiding the complications of numerical simulations. We show that, below the convective envelope of an AGB star, variable magnetic fields induce a natural expansion, permitted by the almost ideal MHD conditions, in which the radial velocity grows as the second power of the radius. We then study the convective envelope, where the complexity of macro-turbulence allows only for a schematic analytical treatment. Here the radial velocity depends on the square root of the radius. We then verify the robustness of our results with 3D calculations for the velocity, showing that, for both the studied regions, the solution previously found can be seen as a planar section of a more complex behavior, in which anyway the average radial velocity retains the same dependency on radius found in 2D. As a final check, we compare our results to approximate descriptions of buoyant magnetic structures. For realistic boundary conditions the envelope crossing times are sufficient to disperse in the huge convective zone any material transported, suggesting magnetic advection as a promising mechanism for deep mixing. The mixing velocities are smaller than for convection, but larger than for diffusion and adequate to extra-mixing in red giants.
3 He‐driven Mixing in Low‐Mass Red Giants: Convective Instability in Radiative and Adiabatic Limits
The Astrophysical Journal, 2008
We examine the stability and observational consequences of mixing induced by 3 He burning in the envelopes of first ascent red giants. We demonstrate that there are two unstable modes: a rapid, nearly adiabatic mode that we cannot identify with an underlying physical mechanism, and a slow, nearly radiative mode that can be identified with thermohaline convection. We present observational constraints that make the operation of the rapid mode unlikely to occur in real stars. Thermohaline convection turns out to be fast enough only if fluid elements have finger-like structures with a length to diameter ratio l/d 10. We identify some potentially serious obstacles for thermohaline convection as the predominant mixing mechanism for giants. We show that rotation-induced horizontal turbulent diffusion may suppress the 3 He-driven thermohaline convection. Another potentially serious problem for it is to explain observational evidence of enhanced extra mixing. The 3 He exhaustion in stars approaching the red giant branch (RGB) tip should make the 3 He mixing inefficient on the asymptotic giant branch (AGB). In spite of this, there are observational data indicating the presence of extra mixing in low-mass AGB stars similar to that operating on the RGB. Overmixing may also occur in carbon-enhanced metal-poor stars.
Buoyant magnetic flux tubes as a site for 26Al production in AGB stars
Memorie della Societa Astronomica Italiana, 2006
We address the diffusive circulation of matter occurring in AGB stars above the H-burning shell, which triggers the production of p-rich isotopes like 13 C and 17 O and of the unstable nucleus 26 Al, observed in presolar grains and in Early Solar System materials. As a physical mechanism for these phenomena we consider the buoyancy of magnetic flux tubes formed below the convective envelope and we arrive at expressions for the relevant mixing parameters in terms of the required strength of the magnetic field | − → B|. We show how values of | − → B| like those normally expected in the radiative layers above the H-burning shell of a red giant can indeed trigger chemical mixing at the required efficiency and should therefore be considered for explaining the abundance peculiarities induced in such stars by proton captures. The technique discussed here is suitable for chemically stratified regions. A different formalism should be used for studying the more internal, well-mixed He layers, where similar circulation phenomena induce the formation of the neutron source 13 C during dredge-up.
The magnetic fields at the surface of active single G-K giants
Astronomy & Astrophysics, 2015
Aims. We investigate the magnetic field at the surface of 48 red giants selected as promising for detection of Stokes V Zeeman signatures in their spectral lines. In our sample, 24 stars are identified from the literature as presenting moderate to strong signs of magnetic activity. An additional 7 stars are identified as those in which thermohaline mixing appears not to have occured, which could be due to hosting a strong magnetic field. Finally, we observed 17 additional very bright stars which enable a sensitive search to be performed with the spectropolarimetric technique. Methods. We use the spectropolarimeters Narval and ESPaDOnS to detect circular polarization within the photospheric absorption lines of our targets. We treat the spectropolarimetric data using the least-squares deconvolution (LSD) method to create high signal-to-noise ratio mean Stokes V profiles. We also measure the classical S-index activity indicator for the Ca ii H&K lines, and the stellar radial velocity. To infer the evolutionary status of our giants and to interpret our results, we use state-of-the-art stellar evolutionary models with predictions of convective turnover times. Results. We unambiguously detect magnetic fields via Zeeman signatures in 29 of the 48 red giants in our sample. Zeeman signatures are found in all but one of the 24 red giants exhibiting signs of activity, as well as 6 out of 17 bright giant stars. The majority of the magnetically detected giants are either in the first dredge up phase or at the beginning of core He burning, i.e. phases when the convective turnover time is at a maximum: this corresponds to a 'magnetic strip' for red giants in the Hertzsprung-Russell diagram. A close study of the 16 giants with known rotational periods shows that the measured magnetic field strength is tightly correlated with the rotational properties, namely to the rotational period and to the Rossby number Ro. Our results show that the magnetic fields of these giants are produced by a dynamo, possibly of α-ω origin since Ro is in general smaller than unity. Four stars for which the magnetic field is measured to be outstandingly strong with respect to that expected from the rotational period/magnetic field relation or their evolutionary status are interpreted as being probable descendants of magnetic Ap stars. In addition to the weak-field giant Pollux, 4 bright giants (Aldebaran, Alphard, Arcturus, η Psc) are detected with magnetic field strength at the sub-gauss level.
MHD and deep mixing in evolved stars. I. 2D and 3D analytical models for the AGB
2016
The advection of thermonuclear ashes by magnetized domains emerging from near the Hshell was suggested to explain AGB star abundances. Here we verify this idea quantitatively through exact MHD models. Starting with a simple 2D geometry and in an inertia frame, we study plasma equilibria avoiding the complications of numerical simulations. We show that, below the convective envelope of an AGB star, variable magnetic fields induce a natural expansion, permitted by the almost ideal MHD conditions, in which the radial velocity grows as the second power of the radius. We then study the convective envelope, where the complexity of macro-turbulence allows only for a schematic analytical treatment. Here the radial velocity depends on the square root of the radius. We then verify the robustness of our results with 3D calculations for the velocity, showing that, for both the studied regions, the solution previously found can be seen as a planar section of a more complex behavior, in which anyway the average radial velocity retains the same dependency on radius found in 2D. As a final check, we compare our results to approximate descriptions of buoyant magnetic structures. For realistic boundary conditions the envelope crossing times are sufficient to disperse in the huge convective zone any material transported, suggesting magnetic advection as a promising mechanism for deep mixing. The mixing velocities are smaller than for convection, but larger than for diffusion and adequate to extra-mixing in red giants.
Thermohaline mixing in low-mass giants
Proceedings of The International Astronomical Union, 2008
Thermohaline mixing has recently been proposed to occur in low mass red giants, with large consequences for the chemical yields of low mass stars. We investigate the role of thermohaline mixing during the evolution of stars between 1 M⊙ and 3 M⊙, in comparison to other mixing processes acting in these stars. We confirm that thermohaline mixing has the potential to destroy most of the 3 He which is produced earlier on the main sequence during the red giant stage. In our models we find that this process is working only in stars with initial mass M ∼ < 1.5 M⊙. Moreover, we report that thermohaline mixing can be present during core helium burning and beyond in stars which still have a 3 He reservoir. While rotational and magnetic mixing is negligible compared to the thermohaline mixing in the relevant layers, the interaction of thermohaline motions with differential rotation and magnetic fields may be essential to establish the time scale of thermohaline mixing in red giants.