Revisiting the Modified Eddington Limit for Massive Stars

We have determined the location of the line-opacity modified Eddington limitfor stars in the LMC using the most recent atmosphere models combined with aprecise mapping to the HR Diagram through up-to-date stellar evolutioncalculations. While we find, in agreement with previous studies, that the shapeof the modified Eddington limit qualitatively corresponds to theHumphreys-Davidson (HD) limit defined by the most luminous supergiants, themodified limit is actually a full magnitude higher than the upper luminositylimit observed for LMC stars. The observed limit is consistent with atmospheremodels in which the maximum value of the ratio of the radiation force outwardsto the gravitational force inwards, Y_max, is 0.9, i.e., the photospheres ofstars at the observed luminosity limit are bound. As massive stars evolve, theymove to higher, and therefore less stable values of Y_max, so mass loss, eithersporadic or continuous, may halt their natural redward evolution as theyapproach the Y_max = 0.9 limit. We assess the metallicity dependence of thislimit. If the limit does determine the most luminous stars, and the value ofY_max corresponding to the luminosity limit in the LMC is universal, then thebrightest supergiants the SMC should be only marginally brighter (0.3 mag) thanthose of the LMC, in agreement with observations. Moreover, the brightestsupergiants in M31 should be 0.75~mag fainter than those in the LMC.Comment: 13 pages with 4 figures, AAS Latex, ApJ Submitted (August