Self-consistent atmospheric models of very massive stars and their early evolution in dense stellar systems

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The evolution of the dense cores of massive starburst clusters is affected by dynamical processes induced by N-body interactions. The resultant formation, via mergers, of very massive stars with masses up to several 1000 solar masses may be essential for the evolution of galaxies and decisive for the evolution of such clusters. For example, evidence has been collected that young dense clusters in galactic bulges harbor intermediate-mass black holes (IMBHs) with masses up to several 1000 solar masses; these objects could provide important clues for understanding the formation of supermassive black holes (SMBHs) in galactic centers.The evolution of such very massive stars (VMSs) is significantly influenced by their pronounced stellar winds, and the further evolution of merger products depends critically on their mass loss rates. The determination of the mass loss rates, however, requires a highly consistent treatment of the physical state of the outflowing gas and of the hydrodynamic and radiative processes in the expanding atmospheres.In preliminary prior work we have already combined consistent models of expanding atmospheres with stellar evolutionary calculations of VMSs for the purpose of modelling the evolution of dense stellar clusters. As suspected, the computed mass loss rates, terminal wind velocities, and spectral energy distributions revealed important implications for stellar evolutionary calculations. Solar-metallicity stars, for instance, were shown to have high mass-loss rates, but presumably not so high as to exclude VMSs, formed by dynamical processes in dense clusters, from ending their life massive enough to form IMBHs.The primary objective of the present proposal is to develop an advanced diagnostic method of O-type stellar atmospheres, including an assessment of the accuracy of the determinations of abundances, stellar and wind parameters by means of detailed interpretations of observational results; as a central task, mass loss rates of VMSs for different compositions will be computed with an improved level of consistency. The basic parameters deduced from these computations will be used for comparisons with present predictions from the evolution of very massive stars, and we will focus on the importance of the improved mass loss rates for the formation of IMBHs.