M. K. Das

On multimode pulsations of degenerate stellar models

Nonlinear dissipative radial oscillations of partially and completely degenerate stellar models have been studied. The general mathematical framework for the study of interaction between various radial modes of oscillation has been presented. The dynamical tools, e.g., Poincare surface of section and the spectrum of Liapunov exponents, have been used to investigate the nature of pulsation in the phase space. For simplicity, numerical computations have been made to study the interaction of only the first three modes, including the fundamental mode for both partially and completely degenerate stellar masses. For these models, with varying degrees of central degeneracy, the possible occurrence of quasi-periodic motions with as many as three irrationally related frequencies has been established in the dissipative regime. 56 refs.

The convection zone in stars with radiation pressure in the presence of magnetic field

The effect of a uniform magnetic field on the envelope convection zone of an 8.8M⊙ star has been studied. The adiabatic exponents Γi (i=1, 2, 3), adiabatic temperature gradient and specific heat of stellar matter has been computed. It is shown that the magnetic field tends to increase the values of adiabatic temperature gradient and specific heats of stellar matter in the envelope convection zone.

Radial oscillations of finite-temperature white dwarfs

Models of finite temperature completely degenerate stellar configurations are considered. The frequencies of fundamental radial modes of oscillations for these white dwarf models have been computed for different values of the central degeneracy parameter 1/y02 and for uniform temperatures of 20×106 K and 108 K. A variational formulation as well as a direct approach is employed to calculate the temperature induced modifications in the frequencies of oscillation of these white-dwarf models.

The structure and oscillations of low-mass stars in the presence of a magnetic field

The equilibrium structure and oscillations of a partially degenerate standard model in the presence of a poloidal magnetic field have been studied. The magnetic field in the interior has been matched with an outside dipole field. The effect of magnetic field on the various structural parameters, e.g., mass, central condensation, moment of inertia, and oblateness has been computed for different values of the central degeneracy of the model. We have also studied the effect of magnetic field on radial oscillations of the configuration. A variational formulation is used to compute the changes in the frequency of radial mode of oscillation. It has been shown that the changes in frequency computed for various models using a two-parameter eigenfunction are in fair agreement with the values obtained by using the exact eigenfunction.

Nonradial oscillations of low-mass stars in the presence of a magnetic field

Nonradial oscillations of a partially degenerate standard model, approximating a class of low-mass stars, have been studied in the presence of a weak poloidal magnetic field. The magnetic field in the interior of the configuration is taken to be continuous across the equilibrium surface and is matched with an external dipole field. Using a variational formulation, corrections to the oscillation frequencies of the Kelvin mode have been found for different values of the central degeneracy. It has been noted that the effect of the magnetic field is to increase the frequency of nonradial (l=2) mode of pulsation.

The effect of rotation on the luminosity of magnetic stars

The effect of rotation and a general magnetic field on the luminosity, radius, and effective temperature of the upper Main-Sequence stars has been investigated using a perturbation analysis. The magnetic field profile prevailing inside the star is assumed to have both poloidal and toroidal components. The case of constant as well as differential rotation is admitted. Model calculations indicate that these stellar parameters modify considerably as a result of coupling between rotation and the magnetic field.