Evidence for a fundamental stellar upper mass limit from clustered star formation

The observed masses of the most massive stars do not surpass about 150M⊙. This may either be a fundamental upper mass limit which is defined by the physics of massive stars and/or their formation, or it may simply reflect the increasing sparsity of such very massive stars so that observing even higher-mass stars becomes unlikely in the Galaxy and the Magellanic Clouds. It is shown here that if the stellar initial mass function (IMF) is a power-law with a Salpeter exponent (α = 2.35) for massive stars then the richest very young cluster R136 seen in the Large Magellanic Cloud (LMC) should contain stars with masses larger than 750M⊙. If, however, the IMF is formulated by consistently incorporating a fundamental upper mass limit then the observed upper mass limit is arrived at readily even if the IMF is invariant. An explicit turn-down or cutoff of the IMF near 150M⊙ is not required; our formulation of the problem contains this implicitly. We are therefore led to conclude that a fundamental maximum stellar mass near 150M⊙ exists, unless the true IMF has α > 2.8.