Revisiting the modified Eddington limit for massive stars

We have determined the location of the line-opacity modified Eddington limi t for stars in the LMC using the most recent atmosphere models combined wit h a precise mapping to the H-R diagram through up-to-date stellar evolution calculations. While we find, in agreement with previous studies, that the shape of the modified Eddington limit qualitatively corresponds to the Hump hreys-Davidson (HD) limit defined by the most luminous supergiants, the mod ified limit is actually a full magnitude higher than the upper luminosity l imit observed for LMC stars. The observed limit is consistent with atmosphe re models in which the maximum value of the ratio of the radiation force ou tward to the gravitational force inward (Y-max) is similar to 0.9, i.e., th e photospheres of stars at the observed luminosity limit are bound. As mass ive stars evolve, they move to higher, and therefore less stable, values of Y-max, so mass loss, either sporadic or continuous, may halt their natural redward evolution as they approach the observed Y-max approximate to 0.9 l imit. We assess the metallicity dependence of this limit. If mass loss does limit the redward evolution of the most luminous stars, and if the value o f Y-max corresponding to the luminosity limit in the LMC is universal, then the brightest supergiants of the SMC should be only marginally brighter (0 .3 mag) than those of the LMC, in agreement with observations. Moreover, th e brightest supergiants in M31 should be 0.75 mag fainter than those in the LMC.