Revisiting the modified Eddington limit for massive stars

We have determined the location of the line-opacity modified Eddington limit for stars in the LMC using the most recent atmosphere models combined with 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 Humphreys-Davidson (HD) limit defined by the most luminous supergiants, the modified limit is actually a full magnitude higher than the upper luminosity limit observed for LMC stars. The observed limit is consistent with atmosphere models in which the maximum value of the ratio of the radiation force outward to the gravitational force inward (Y-max) is similar to 0.9, i.e., the photospheres of stars at the observed luminosity limit are bound. As massive 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 limit. 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 of 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, the brightest supergiants in M31 should be 0.75 mag fainter than those in the LMC.