Emergence of Anchored Flux Tubes Through the Convection Zone

We model the evolution of buoyant magnetic flux tubes in the Sun's convection zone. A flux tube is assumed to lie initially near the top of the stably stratified radiative core below the convection zone, but a segment of it is perturbed into the convection zone by gradual heating and convective overshoot motions. The ends ("footpoints") of the segment remain anchored at the base of the convection zone, and if the segment is sufficiently long, it may be buoyantly unstable, rising through the convection zone in a short time. The length of the flux tube determines the ratio of buoyancy to magnetic tension: short loops of flux are arrested before reaching the top of the convection zone, while longer loops emerge to erupt through the photosphere. Using Spruit's convection zone model, we compute the minimum footpoint separation $L_c$ required for erupting flux tubes. We explore the dependence of $L_c$ on the initial thermal state of the perturbed flux tube segment and on its initial magnetic field strength. Following an investigation of thermal diffusion time scales and the dynamic rise times of unstable flux tube segments, we conclude that the most likely origin for magnetic flux which erupts to the surface is from short length scale perturbations ($L < L_c$) which are initially stable, but which are subsequently destabilized either by diffusion of heat into the tube or by stretching of the anchor points until $L$ just exceeds $L_c$. In either case, the separation of the anchor points of the emergent tube should lie between the critical distance for a tube in mechanical equilibrium and one in thermal equilibrium. Finally, after comparing the dispersion of dynamic rise times with the much shorter observed active region formation time scales, we conclude that active regions form from the emergence of a single flux tube segment.Comment: 13 pages, 2 figures, 1 table. Publishing information: Solar System Plasma Physics: Geophysical Monograph 54. Edited by J. H., Jr. Waite, J. L. Burch and R. L. Moore. ISBN 0-87590-074-7; QC809.P5S65 1989. Published by the American Geophysical Union, Washington, DC USA, 1989, p.4

The growth of helium burning cores

Helium burning in the convective cores of horizontal branch and `red clump' stars appears to involve a process of `ingestion' of unburnt helium into the core, the physics of which has not been identified yet. I show here that a limiting factor controlling the growth is the buoyancy of helium entering the denser C+O core. It yields a growth rate which scales directly with the convective luminosity of the core, and agrees with constraints on core size from current asteroseismology.Comment: Accepted for publication in A&

Semiconvection: theory

A model is developed for the transport of heat and solute in a system of double-diffusive layers under astrophysical conditions (viscosity and solute diffusivity low compared with the thermal diffusivity). The process of formation of the layers is not part of the model but, as observed in geophysical and laboratory settings, is assumed to be fast compared to the life time of the semiconvective zone. The thickness of the layers is \tbf{a} free parameter of the model. When the energy flux of the star is specified, the effective semiconvective diffusivities are only weakly dependent on this parameter. An estimate is given of the evolution of layer thickness with time in a semiconvective zone. The model predicts that the density ratio has a maximum for which a stationary layered state can exist, R_\rho\la \mr{Le}^{-1/2}. Comparison of the model predictions with a grid of numerical simulations is presented in a companion paper.Comment: Accepted for publication in Astronomy & Astrophysic