Evolution Of Galaxies .4. Highly Flattened Disks

NSF GP-18335, GP-32051Astronom

Advanced Evolution Of Massive Stars .4. Secondary Nucleosynthesis During Helium Burning

NSF GP-32051, GP-23282Astronom

Evolution Of Galaxies .2. Chemical Evolution Coefficients

NSF GP-18355, GP-32051Astronom

Turbulent Mixing in Stars: Theoretical Hurdles

A program is outlined, and first results described, in which fully three-dimensional, time dependent simulations of hydrodynamic turbulence are used as a basis for theoretical investigation of the physics of turbulence in stars. The inadequacy of the treatment of turbulent convection as a diffusive process is discussed. A generalization to rotation and magnetohydrodynamics is indicated, as are connection to simulations of 3D stellar atmospheres.Comment: 5 pages, 1 figure, IAU Symposium 265, 200

Chaos and Turbulent Nucleosynthesis Prior to a Supernova Explosion

Three-dimensional (3D), time dependent numerical simulations, of flow of matter in stars, now have sufficient resolution to be fully turbulent. The late stages of the evolution of massive stars, leading up to core collapse to a neutron star (or black hole), and often to supernova explosion and nucleosynthesis, are strongly convective because of vigorous neutrino cooling and nuclear heating. Unlike models based on current stellar evolutionary practice, these simulations show a chaotic dynamics characteristic of highly turbulent flow. Theoretical analysis of this flow, both in the Reynolds-averaged Navier-Stokes (RANS) framework and by simple dynamic models, show an encouraging consistency with the numerical results. It may now be possible to develop physically realistic and robust procedures for convection and mixing which (unlike 3D numerical simulation) may be applied throughout the long life times of stars. In addition, a new picture of the presupernova stages is emerging which is more dynamic and interesting (i.e., predictive of new and newly observed phenomena) than our previous one.Comment: 11 pages, 2 figures, Submitted to AIP Advances: Stardust, added figures and modest rewritin

Toward a consistent use of overshooting parametrizations in 1D stellar evolution codes

Several parametrizations for overshooting in 1D stellar evolution calculations coexist in the literature. These parametrizations are used somewhat arbitrarily in stellar evolution codes, based on what works best for a given problem, or even for historical reasons related to the development of each code. We bring attention to the fact that these different parametrizations correspond to different physical regimes of overshooting, depending whether the effects of radiation are dominant, marginal, or negligible. Our analysis is based on previously published theoretical results, as well as multidimensional hydrodynamical simulations of stellar convection where the interaction between the convective region and a stably-stratified region is observed. Although the underlying hydrodynamical processes are the same, the outcome of the overshooting process is profoundly affected by radiative effects. Using a simple picture of the scales involved in the overshooting process, we show how three regimes are obtained, depending on the importance of radiative effects. These three regimes correspond to the different behaviors observed in hydrodynamical simulations so far, and to the three types of parametrizations used in 1D codes. We suggest that the existing parametrizations for overshooting should coexist in 1D stellar evolution codes, and should be applied consistently at convective boundaries depending on the local physical conditions.Comment: 5 pages, 2 figures, to appear in A&A as a regular paper. Last version: language editing + typos in Eq. (6) & (9) correcte