Theoretical and Observational Studies of Stellar Activity
Abstract
In the theoretical part of this thesis, doubly -diffusive MHD instabilities are studied as a means of breaking up a diffuse magnetic field at the bottom of the solar convection zone. The analysis is linear and local, and assumes short meridional wavelengths; the effects of rotation and diffusion of vorticity, magnetic fields and heat are included. Our results show that the instability depends sensitively on the temperature stratification, but rather insensitively on the assumed magnetic field configuration; instability time scales considerably less than the solar cycle period can be easily obtained. A new model for the region of convective overshoot underneath the solar convection zone is developed. It assumes highly asymmetric up- and down-flows, and utilizes semiempirical models, developed for the description of plume motions in the Earth's atmosphere. Using these plume equations together with the stellar structure equations, we derive a set of differential equations suitable for the study of convective overshoot phenomena in astrophysical contexts. Our results indicate the formation of an almost adiabatically stratified region underneath the "active" convection zone; in addition, we demonstrate the existence of a stabilising boundary layer between adiabatically and non-adiabatically stratified regions, and compute an upper limit of 500 km to its thickness. In the observational part of this thesis, we report the results of a survey of the X-ray emission of stars with shallow convection zones to study the onset of convection and dynamo activity along the main sequence. We extensively discuss the complications arising from stellar multiplicity; we demonstrate that binaries have statistically higher X-ray luminosities, and show that physical parameters can only be deduced from single stars. We further show that the X-ray luminosities of stars with spectral type in the color range 0.1 (LESSTHEQ) B-V (LESSTHEQ) 0.5 increase rapidly, whereas stars with B-V (TURN) 0.0 appear to have no intrinsic X-ray emission at presently detectable levels. We argue that the observed X-ray emission in our sample stars originates from coronae, produced by dynamo processes in the convection zones of these stars, and provide evidence supporting this point of view. A variety of statistical techniques was used to obtain the above results; these include two sample tests with censored data, adapted to astronomical usage from the field of survival analysis. A new method, based on maximum likelihood estimation in two dimensions, to determine correlation and linear regression coefficients in the presence of arbitrarily censored data, is developed. Further applications of the new method for likelihood ratio tests and principal component analysis in the presence of arbitrary censoring are also presented.
- Publication:
-
Ph.D. Thesis
- Pub Date:
- 1984
- Bibcode:
- 1984PhDT.........8S
- Keywords:
-
- MHD;
- STATISTICS;
- CONVECTIVE OVERSHOOT;
- Physics: Astronomy and Astrophysics;
- Astrophysics;
- Binary Stars;
- Convection Currents;
- Magnetohydrodynamic Stability;
- Stellar Models;
- Luminosity;
- Magnetic Field Configurations;
- Maximum Likelihood Estimates;
- Stellar Color;
- X Ray Spectra;
- Astrophysics