Published online by Cambridge University Press: 04 August 2017
Observational constraints on the form of the high-mass stellar IMF are reviewed. The evidence includes star counts in the solar neighborhood, individual and composite star clusters, and nearby galaxies, and arguments based on integrated light and chemical evolution modeling. There is no convincing evidence for any systematic variations of the shape of the high-mass IMF. However, the various determinations are very uncertain, and do not allow any firm estimate of the logarithmic slope of the upper IMF; the appropriate value is somewhere between −1.3 and −2.3, with region-to-region variations smaller than about ±0.5. A number of lines of evidence suggest that the lower mass limit or mode mass of the IMF increases with increasing star formation rate, reaching perhaps 10–15 m⊙ in some starburst galaxies. It is also possible that the upper mass limit depends on metallicity, based on variations in excitation conditions of HII regions.