Galactic-Field Initial Mass Functions of Massive Stars

Over the past years observations of young and populous star clusters have shown that the stellar initial mass function (IMF) appears to be an invariant featureless Salpeter power law with an exponent α=2.35 for stars more massive than a few M_solar. A consensus has also emerged that most, if not all, stars form in stellar groups and star clusters and that the mass function of young star clusters in the solar neighborhood and in interacting galaxies can be described, over the mass range of a few 10 to 10⁷M_solar, as a power law with an exponent β~2. These two results imply that galactic-field IMFs for early-type stars cannot, under any circumstances, be a Salpeter power law, but that they must have a steeper exponent, α_field>~2.8. This has important consequences for the distribution of stellar remnants and for the chemodynamical and photometric evolution of galaxies.