Magneto-Thermohaline Mixing in Red Giants

Abstract

We revise a magnetic buoyancy model that has recently been proposed as a mechanism for extra mixing in the radiative zones of low-mass red giants. The most important revision is our accounting of the heat exchange between rising magnetic flux rings and their surrounding medium. This increases the buoyant rising time by five orders of magnitude, therefore the number of magnetic flux rings participating in the mixing has to be increased correspondingly. On the other hand, our revised model takes advantage of the fact that the mean molecular weight of the rings formed in the vicinity of the hydrogen burning shell has been reduced by 3He burning. This increases their thermohaline buoyancy (hence, decreases the total ring number) considerably, making it equivalent to the pure magnetic buoyancy produced by a frozen-in toroidal field with B_phi ~ 10 MG. We emphasize that some toroidal field is still needed for the rings to remain cohesive while rising. Besides, this field prevents the horizontal turbulent diffusion from eroding the mu contrast between the rings and their surrounding medium. We propose that the necessary toroidal magnetic field is generated by differential rotation of the radiative zone, that stretches a pre-existing poloidal field around the rotation axis, and that magnetic flux rings are formed as a result of its buoyancy-related instability.

31 pages, 4 figures, 2 tables, accepted by ApJ after minor changes

References in corpus (17)

• Thermohaline mixing: A physical mechanism governing the photospheric composition of low-mass giants
• Short-lived Nuclei in the Early Solar System: Possible AGB Sources
• Deep Mixing of He-3: Reconciling Big Bang and Stellar Nucleosynthesis
• Rotational mixing in low-mass stars II. Self-consistent models of Pop II RGB stars
• Can Extra Mixing in RGB and AGB Stars Be Attributed to Magnetic Mechanisms?
• The Monitor project: Rotation of low-mass stars in the open cluster NGC 2516
• Carbon-Enhanced Metal-Poor Stars. III. Main-Sequence Turn-Off Stars from the SDSS/SEGUE Sample
• Isolated vs. Common Envelope Dynamos in Planetary Nebula Progenitors
• First stars X. The nature of three unevolved Carbon-Enhanced Metal-Poor stars
• CS 30322-023: an ultra metal-poor TP-AGB star?
• Inhibition of thermohaline mixing by a magnetic field in Ap star descendants: Implications for the Galactic evolution of 3He
• AGB stars of the intermediate-age LMC cluster NGC 1846 II. Dredge up along the AGB
• 3He-Driven Mixing in Low-Mass Red Giants: Convective Instability in Radiative and Adiabatic Limits
• The Impact of Carbon Enhancement on Extra Mixing in Metal-Poor Stars
• From Canonical to Enhanced Extra Mixing in Low-Mass Red Giants: Tidally Locked Binaries
• Observational consequences of the hypothesized helium rich stellar population in Omega Centauri
• Magneto-Rotational Transport in the Early Sun

Cited by in corpus (56)

• The Dawes Review 2: Nucleosynthesis and stellar yields of low and intermediate-mass single stars
• Evolution, nucleosynthesis and yields of low mass AGB stars at different metallicities (II): the FRUITY database
• The s-Process in Low Metallicity Stars. II. Interpretation of High-Resolution Spectroscopic Observations with AGB models
• NuGrid Stellar Data Set. II. Stellar Yields from H to Bi for Stellar Models with Mzams = 1 to 25Msun and Z = 0.0001 to 0.02
• s-Process in Low Metallicity Stars. I. Theoretical Predictions
• Deep Mixing in Evolved Stars: I. The Effect of Reaction Rate Revisions from C to Al
• Helium enrichment and Carbon-star Production in Metal-rich Populations
• Numerical Simulations of Thermohaline Convection: Implications for Extra-Mixing in Low-Mass RGB Stars
• On Lithium-Rich Red Giants. I. Engulfment of Sub-Stellar Companions
• Thermohaline mixing in evolved low-mass stars
• Metal-Poor Lithium-Rich Giants in the Radial Velocity Experiment Survey
• Thermohaline Mixing: Does It Really Govern the Atmospheric Chemical Composition of Low-Mass Red Giants?
• Constraining Metallicity-dependent Mixing and Extra Mixing using [C/N] in Alpha-Rich Field Giants
• Application of a Theory and Simulation based Convective Boundary Mixing model for AGB Star Evolution and Nucleosynthesis
• Old puzzle, new insights: a lithium rich giant quietly burning helium in its core
• On the Need for Deep Mixing in AGB Stars of Low Mass
• Magnetic-buoyancy Induced Mixing in AGB Stars: Presolar SiC Grains
• Lithium-rich giants in the Galactic thick disk
• MHD and deep mixing in evolved stars. 1. 2D and 3D analytical models for the AGB
• The Gaia-ESO survey: impact of extra-mixing on C- and N-abundances of giant stars
• Deep Mixing in Evolved Stars. II. Interpreting Li Abundances in RGB and AGB Stars
• The Role of Thermohaline Mixing in Intermediate- and Low-Metallicity Globular Clusters
• BD+48 740 - Li overabundant giant star with a planet. A case of recent engulfment?
• The Gaia-ESO Survey: CNO abundances in the open clusters Trumpler 20, NGC 4815, and NGC 6705
• Lithium in red giant stars: Constraining non-standard mixing with large surveys in the Gaia era
• s-Processing in AGB Stars Revisited. I. Does the Main Component Constrain the Neutron Source in the 13C-Pocket?
• Li-rich RGB stars in the Galactic Bulge
• Carbon and oxygen isotopic ratios in Arcturus and Aldebaran: Constraining the parameters for non convective mixing on the RGB
• Survey of Li-rich giants among Kepler and LAMOST fields: Determination of Li-rich giants Evolutionary Phase
• The Penn State - Torun Centre for Astronomy Planet Search stars. II. Lithium abundance analysis of the Red Giant Clump sample
• Lithium Enrichment Signatures of Planetary Engulfment Events in Evolved Stars
• The evolution of CNO elements in galaxies
• Light Elements in the Universe
• C, N, O abundances and carbon isotope ratios in evolved stars of the open clusters Collinder 261 and NGC 6253
• Chemical Abundances for Evolved Stars in M5: Lithium through Thorium
• Theoretical Estimates of Stellar e-Captures. I. The half-life of 7Be in Evolved Stars
• Models of red giants in the CoRoT asteroseismology fields combining asteroseismic and spectroscopic constraints
• Fingering Convection in Red Giants Revisited
• A phenomenological modification of thermohaline mixing in globular cluster red giants
• The effects of surface fossil magnetic fields on massive star evolution: IV. Grids of models at Solar, LMC, and SMC metallicities
• The effect of rotation on fingering convection in stellar interiors
• Extra-Mixing in Luminous Cool Red Giants. Hints from Evolved Stars with and without Li
• A Deep Mixing Solution to the Aluminum and Oxygen Isotope Puzzles in Presolar Grains
• On the sodium overabundance of giants in open clusters: The case of the Hyades
• Spectroscopic study of two new super Li rich red clump K giants
• 3D hydrodynamics simulations of internal gravity waves in red giant branch stars
• CNO abundances and carbon isotope ratios in evolved stars of the open clusters NGC 2324, NGC 2477, and NGC 3960
• Lithium abundances in giants as a function of stellar mass: An evidence for He-flash as the source of Li enhancement in low mass giants
• An Investigation of Non-Canonical Mixing in Red Giant Stars Using APOGEE 12C/13C Ratios Observed in Open Cluster Stars
• Gaia-ESO Survey: Detailed elemental abundances in red giants of the peculiar globular cluster NGC 1851
• HD 16771: A lithium-rich giant in the red-clump stage
• Lithium in Kepler Red Giants: Defining Normal and Anomalous
• Lithium Inventory of 2 $M_{\odot }$ Red Clump Stars in Open Clusters: A Test of the Helium Flash Mechanism
• Thermohaline mixing in extremely metal-poor stars
• The true nature of HE0057-5959, the most metal-poor Li-rich star
• The Early Solar System - Chapter 6