Stability and Evolution of Supermassive Stars (SMS)

Abstract

Highly condensed gaseous objects with masses larger than 5 × 10⁴ M_⊙ arecalled Supermassive stars. They are thought to be possible precursors ofSupermassive Black Holes in the centres of galaxies. In the quasi-stationary contraction phase the hydrostatic equilibrium is determined by radiationpressure and gravitation. The global structure is an n=3 polytrope which is at the stability limit. Small relativistic corrections for example can initiatea 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 theinstability sets in, it is important to investigate in more detail thecontraction phase of the SMS.If the gaseous object is embedded in a dense stellar system, the central starcluster, the interaction and coupling of both components due to dynamicalfriction change the energy balance and evolution of the SMS dramatically. Dynamicalfriction between stars and gas, which can be estimated semi-analytically (see Just et al. 1986), has 3 different effects on the2-component system. 1) The gas is heated by decelerating the stars. This maystall 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 seeAmaro-Seoane & Spurzem 2001). 2) This cooling of the losscone stars lead to a mass segregation in the stellar component resulting in amuch more condensed central stellar core. 3) The inhomogeneities due to thegravitational wakes in the gas changes the effective absorption coefficient ofthe gas. This affects the condition for hydrostatic equilibrium and may giveessential deviations from the n=3 polytrope. We discuss, in which evolutionarystages and parameter range these interaction processes are relevant and how theycan influence the stability and evolution of the SMS.