Thermoconvective Instability in the Solar Core.
Abstract
Models of the present sun have a thin shell of He^3 at the edge of the core. The rates of nuclear burning of He^3 are very temperature sensitive and this can lead to thermal instability. Thermal instability in a stably stratified medium results in growing oscillations (overstability of gravity waves). The excitation of gravity modes by thermal instability is classically studied as a quasiadiabatic problem. However, the period of adiabatic oscillations increases with decreasing vertical to horizontal aspect ratio of these modes while the local thermal diffusion time decreases. It is shown through the study of a simplified example that such slow gravity modes can be trapped in a thin layer due to thermal effects, contrary to expectations based on quasiadiabatic arguments, which are not applicable to such slow modes. Using the smallness of the aspect ratio, amplitude equations can be derived for the wave amplitude under mildly unstable conditions. The amplitude equations admit nonlinear waves. Stationary wave solutions are also possible and these are described by a Hopf bifurcation that is subcritical in the thin layer limit, possibly leading to overturning motions. It is shown that if such monotonic or oscillatory convection of large horizontal scale exists in the solar core, the central temperature and therefore the high energy neutrino flux will be lower than that expected from the standard solar models. These "shellular" modes are expected to have periods of about two hours. If such modes are detected in the sun, they will be a powerful tool for seismic probing of the solar interior.
 Publication:

Ph.D. Thesis
 Pub Date:
 1992
 Bibcode:
 1992PhDT.........1G
 Keywords:

 Physics: Astronomy and Astrophysics, Physics: Elementary Particles and High Energy;
 Helium Isotopes;
 Solar Convection (Astronomy);
 Solar Interior;
 Thermal Diffusion;
 Thermal Instability;
 Gravitation;
 Gravity Waves;
 Neutrinos;
 Nonlinearity;
 Oscillations;
 Solar Physics