The stellar and sub-stellar IMF of simple and composite populations

Details The stellar and sub-stellar IMF of simple and composite populations The stellar and sub-stellar IMF of simple and composite populations

2011-12-14

arxiv.org/abs/1112.3340v2

Astronomy and Astrophysics

Astrophysics

Cosmology and Extragalactic Astrophysics

165 pages, 37 figures, 3 tables, to appear in Stellar Systems and Galactic Structure, Vol.V. This revised version corrects equ

Scientific paper

The current knowledge on the stellar IMF is documented. It is usually described as being invariant, but evidence to the contrary has emerged: it appears to become top-heavy when the star-formation rate density surpasses about 0.1 Msun/(yr pc^3) on a pc scale and it may become increasingly bottom-heavy with increasing metallicity. It ends quite abruptly below about 0.1 Msun with brown dwarfs (BDs) and very low mass stars having their own IMFs. The most massive star of mass mmax formed in an embedded cluster with stellar mass Mecl correlates strongly with Mecl being a result of gravitation-driven but resource limited growth and fragmentation induced starvation. There is no convincing evidence whatsoever that massive stars do form in isolation. Massive stars form above a density threshold in embedded clusters which become saturated when mmax = mmax* about 150Msun which appears to be the canonical physical upper mass limit of stars. Super-canonical massive stars arise naturally due to stellar mergers induced by stellar-dynamical encounters in very young dense clusters. Various methods of discretising a stellar population are introduced: optimal sampling leads to a mass distribution that perfectly represents the exact form of the desired IMF and the mmax-Mecl relation, while random sampling results in statistical variations of the shape of the IMF. The observed mmax-Mecl correlation and the small spread of IMF power-law indices together suggest that optimally sampling the IMF may be the more realistic description of star formation than random sampling. Composite populations on galaxy scales, which are formed from many pc scale star formatiom events, need to be described by the integrated galactic IMF. This IGIMF varies systematically in dependence of galaxy type and star formation rate, with dramatic implications for theories of galaxy formation and evolution.

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