One of the key questions in Astrophysics concerns the issue of whether there exists an upper-mass limit to stars , and if so , what physical mechanism sets this limit ?
The answer to this question might also determine if the upper-mass limit is metallicity ( Z ) dependent .
We argue that mass loss by radiation-driven winds mediated by line opacity is one of the prime candidates setting the upper-mass limit .
We present mass-loss predictions ( \mbox { $ \dot { M } $ } _ { wind } ) from Monte Carlo radiative transfer models for relatively cool ( T _ { eff } = 15kK ) inflated very massive stars ( VMS ) with large Eddington \Gamma factors in the mass range 10 ^ { 2 } -10 ^ { 3 } M _ { \odot } as a function of metallicity down to 1/100 Z / \mbox { $Z _ { \odot } $ } .
We employed a hydrodynamic version of our Monte Carlo method , allowing us to predict the rate of mass loss ( \mbox { $ \dot { M } $ } _ { wind } ) and the terminal wind velocity ( \varv _ { \infty } ) simultaneously .
Interestingly , we find wind terminal velocities ( \varv _ { \infty } ) that are low ( 100-500 km/s ) over a wide Z -range , and we propose that the slow winds from VMS are an important source of self-enrichment in globular clusters .
We also find mass-loss rates ( \mbox { $ \dot { M } $ } _ { wind } ) , exceeding the typical mass-accretion rate ( \mbox { $ \dot { M } $ } _ { accr } ) of 10 ^ { -3 } M _ { \odot } { yr } ^ { -1 } during massive-star formation .
We have expressed our mass-loss predictions as a function of mass and Z , finding \log \dot { M } = - 9.13 + 2.1 \log ( M / \mbox { $M _ { \odot } $ } ) + 0.74 \log ( Z / \mbox { $Z _ { \odot } $ } ) ~ { } ( M _ { \odot } / { yr } ) .
Even if stellar winds do not directly halt & reverse mass accretion during star formation , if the most massive stars form by stellar mergers , stellar wind mass loss may dominate over the rate at which stellar growth takes place .
We therefore argue that the upper-mass limit is effectively Z -dependent due to the nature of radiation-driven winds .
This has dramatic consequences for the most luminous supernovae , gamma-ray bursts , and other black hole formation scenarios at different Cosmic epochs .