During the final evolutionary stages the nuclear fuel is consumed, and the star emits radiation owing to cooling. It is relatively cold and has a very high density, while the pressure arises mainly from the matter degeneracy. Chandrasekhar [322] in 1931 obtained the fundamental result that a star where the pressure is due to degenerate electrons has a maximum mass. At ρ > 1.15 × 109 g cm−3 (for 56Fe, for other nuclei see Table 10.1) the neutronization begins, and stars become unstable. Stable (neutron) stars reappear only at ρ c ≈ 1.5 × 1014 g cm−3 and exist to densities ρ c ≈ 1.15 × 1015 g cm−3, where instability arises from general relativity (GR) effects. Oppenheimer and Volkov established in 1939 the existence of a mass limit for neutron stars [517], but its value has been recalculated many times for various equations of state. Solving the equilibrium equations for cold stars in Newtonian theory (9.3.13a) and (10.1.1a) and in GR (11.2.3) and (11.2.4) at a given equation of state P(ρ) has allowed derivation of the curve M(ρ c ) demonstrating the existence of two maximum masses and an instability region (falling M(ρ c )). Figure 11.1 represents the curve M(ρ c ) from [267] for the equation of state corresponding to a minimum energy of matter in full thermodynamic equilibrium.

On the compatibility of thermal and hydrostatic equilibrium in thin radiative accretion disks

Final Stages of Stellar Evolution

First, we give a concise introduction to the formation, theory and stability of protoplanetary accretion disks. In a second part, we present an overview of the physical processes that are currently thought to be of importance for the evolution of a protoplanetary disk. Finally, we discuss some new ideas of how chemical reactions might influence the global evolution of such disks.

Physics and chemistry of protoplanetary accretion disks.

At the innermost part of the accretion disk where the disk grazes the surface of the central star frictional interaction decelerates the disk material to stellar rotational velocity and a transition zone (boundary layer, BL) is formed. In the case of a slowly rotating star, nearly half of the total accretion energy is liberated here. Therefore the inclusion of this boundary layer is very important and crucial for a complete and selfconsistent description of the accretion disk structure. The correct theoretical description of the BL structure is necessarily a two-dimensional problem and has to be solved selfconsistently. Therefore the author developed an algorithm which solves the coupled 2-D radiation-hydrodynamic equations for axisymmetric flows. It is a mixed explicit-implicit numerical method including viscosity, angular momentum and radiation transport treated in the flux-limited diffusion approximation. It was possible to study the detailed structure of the disk-star interaction for different parameters (stellar mass, viscosity, etc.).

On the compatibility of thermal and hydrostatic equilibrium in thin radiative accretion disks

Citations

Final Stages of Stellar Evolution

Physics and chemistry of protoplanetary accretion disks.

Radiation Hydrodynamics of the Boundary Layer of Accretion Disks in Cataclysmic Variables

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