Stability and Evolution of Supermassive Stars (SMS)

Abstract

Highly condensed gaseous objects with masses larger than 5 × 104 M⊙ are called Supermassive stars. They are thought to be possible precursors of Supermassive Black Holes in the centres of galaxies. In the quasi-stationary contraction phase the hydrostatic equilibrium is determined by radiation pressure and gravitation. The global structure is an n=3 polytrope which is at the stability limit. Small relativistic corrections for example can initiate a free fall collapse due to the "post Newtonian" instability. Since the outcome of the final collapse - Supermassive Black Hole or Hypernova - depends sensitively on the structure and the size of the object, when the instability sets in, it is important to investigate in more detail the contraction phase of the SMS. If the gaseous object is embedded in a dense stellar system, the central star cluster, the interaction and coupling of both components due to dynamical friction change the energy balance and evolution of the SMS dramatically. Dynamical friction between stars and gas, which can be estimated semi-analytically (see Just et al. 1986), has 3 different effects on the 2-component system. 1) The gas is heated by decelerating the stars. This may stall the contraction process for a while until the stars in the "loss cone", these which cross the SMS, lost their kinetic energy (for the total heating rate see Amaro-Seoane & Spurzem 2001). 2) This cooling of the loss cone stars lead to a mass segregation in the stellar component resulting in a much more condensed central stellar core. 3) The inhomogeneities due to the gravitational wakes in the gas changes the effective absorption coefficient of the gas. This affects the condition for hydrostatic equilibrium and may give essential deviations from the n=3 polytrope. We discuss, in which evolutionary stages and parameter range these interaction processes are relevant and how they can influence the stability and evolution of the SMS.