AAS 201st Meeting, January, 2003
Session 17. Stellar Evolution and Metal-Poor Stars
Poster, Monday, January 6, 2003, 9:20am-6:30pm, Exhibit Hall AB

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[17.01U] The Eddington Limit Upper Boudary for Cool Stars on the HR Diagram

P.J. Mendygral, B.L. Babler, J.P. Cassinelli (University of Wisconsin-Madison)

We have examined the properties of cool, low metallicity, super-massive forming stars, which may exist at high redshift. We seek to address the location of the Eddington limit and possible evolution scenarios for such stars. The location of the modified Eddington limit was determined across the Hertzsprung Russell Diagram by using Kurucz model atmospheres. Radiative acceleration and \Gammamax = grad/gmass were calculated for a range of gravities at specific temperatures from model atmospheres that accounted for presence of convection zones. Results for Teff and mass gravity were converted to a location on the HR Diagram using a mass luminosity relation that fit upper main sequence stars and extrapolated to \Gammaelectron\:scattering = 1 for very massive stars. Models were calculated for both Z/Z\sun = 1.0 and 0.03. On the hot side of the HR Diagram, our results agree well with the modified Eddington limit derived by Lamers and Noordhoek (1993) and Ulmer and Fitzpatrick (1998). Our results also agree well with the Humphreys-Davidson limit for the most luminous stars, reaching a minimal Lmax at about 12000 K. Our results extend to stars on the cool side of this minimum. We find that the opacity decreases well below the fully ionized electron scattering opacity allowing the upper limit to reach values 104 higher than the H-D limit at Teff = 5000 K for Z/Z\sun = 1 and 105 higher for Z/Z\sun = 0.03.

This apex region might be of interest for two classes of stars. a) Luminous Blue Variables could reach the cool apex region by crossing the divide from the Blue Hypergiant region to the yellow side during their outburst phases. b) The collapse and formation of the earliest generation of super-massive stars could also lead to objects that might be found observationally only as hyperluminous cool stars and not as Wolf-Rayet type hot stars as is commonly assumed.

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