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&

Jets from compact objects

Some topics in the theory of jets are reviewed. These include jet precession, unconfined jets, the origin of knots, the internal shock model as a unifying theme from protostellar jets to Gamma-ray bursts, relations between the Blandford-Znajek and MHD disk-wind models, and jet collimation in magnetic acceleration models.Comment: To appear in Highly Energetic Physical Processes .... (IAU Symp 195) P. C. H. Martens and S. Tsuruta, ed

Semiconvection: numerical simulations

A grid of numerical simulations of double-diffusive convection is presented for the astrophysical case where viscosity (Prandtl number Pr) and solute diffusivity (Lewis number Le) are much smaller than the thermal diffusivity. As in laboratory and geophysical cases convection takes place in a layered form. The proper translation between subsonic flows in a stellar interior and an incompressible (Boussinesq) fluid is given, and the validity of the Boussinesq approximation for the semiconvection problem is checked by comparison with fully compressible simulations. The predictions of a simplified theory of mixing in semiconvection given in a companion paper are tested against the numerical results, and used to extrapolate these to astrophysical conditions. The predicted effective He-diffusion coefficient is nearly independent of the double-diffusive layering thickness $d$. For a fiducial main sequence model (15 $M_\odot$) the inferred mixing time scale is of the order $10^{10}$ yr. An estimate for the secular increase of $d$ during the semiconvective phase is given. It can potentially reach a significant fraction of a pressure scale height.Comment: arXiv admin note: substantial text overlap with arXiv:1012.585

Convective settling in main sequence stars: Li and Be depletion

The process of convective settling is based on the assumption that a small fraction of the low-entropy downflows sink from the photosphere down to the bottom of the star's envelope convection zone retaining a substantial entropy contrast. We have previously shown that this process could explain the slow Li depletion observed in the Sun. We construct a parametric model of convective settling to investigate the dependence of Li and Be depletion on stellar mass and age. Our model is generally in good agreement with the Li abundances measured in open clusters and solar twins, although it seems to underestimate the Li depletion in the first ~1 Gyr. The model is also compatible with the Be abundances measured in a sample of field stars.Comment: 8 pages, 9 figures, accepted for publication in A&