Semiconvection: numerical simulations
نویسندگان
چکیده
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 ) the inferred mixing time scale is of the order 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.
منابع مشابه
Does Rotation Influence Double-Diffusive Fluxes in Polar Oceans?
The diffusive (or semiconvection) regime of double-diffusive convection (DDC) is widespread in the polar oceans, generating ‘‘staircases’’ consisting of high-gradient interfaces of temperature and salinity separated by convectively mixed layers. Using two-dimensional direct numerical simulations, support is provided for a previous theory that rotation can influence DDC heat fluxes when the thic...
متن کاملLayer Formation in Semiconvection
Layer formation in a thermally destabilized fluid with stable density gradient has been observed in laboratory experiments and has been proposed as a mechanism for mixing molecular weight in late stages of stellar evolution in regions which are unstable to semiconvection. It is not yet known whether such layers can exist in a very low viscosity fluid: this work undertakes to address that questi...
متن کاملFirst Stars . I . Evolution without mass loss
The first generation of stars was formed from primordial gas. Numerical simulations suggest that the first stars were predominantly very massive, with typical masses M ≥ 100M⊙. These stars were responsible for the reionization of the universe, the initial enrichment of the intergalactic medium with heavy elements, and other cosmological consequences. In this work, we study the structure of Zero...
متن کاملSemiconvection: 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 ...
متن کاملDynamic and Diiusive Instabilities in Core Collapse Supernovae
The instabilities that are believed to play an important role in the supernova mechanism are reviewed. We then investigate the dynamics of two of these instabilities, prompt convection and neutron ngers, and its consequences for the supernova outcome. We nd that prompt convection occurs immediately after shock propagation and is primarily entropy driven. A number of detailed one-dimensional sph...
متن کامل