925 research outputs found
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
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