The Eddington Limit Upper Boundary for Cool Stars on the HR Diagram
Mendygral, P. J. and Cassinelli, J. P.
Abstract
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 Γ
max = g
rad/g
mass were calculated for a range of gravities at specific
temperatures from model atmospheres that accounted for presence of convection zones. Results for T
eff 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
Γ
electron 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.
Presented, AAS 201st
American Astronomical Society, 201st AAS Meeting, #17.01; Bulletin of the American Astronomical Society, Vol. 34, p.1126
Other Presentations
I gave a public presentation on this topic to the Wehr Astronomical Society in spring of 2003. The society is a group of amateur
astronomers who meet at the Wehr Nature Center in Greenfield, WI. It was a very fun event for me, and my family was kind enough to
take video of the event. The quality is obviously reduced for the web streaming and audience questions are tough to hear, but enjoy
anyway.
Wehr Astronomical Presentation
