Non-grey hydrogen burning evolution of subsolar mass Population III stars

Abstract

The primordial elements, H, He and Li, are included in a low-temperature equation of state and monochromatic opacity function. The equation of state and opacity function are incorporated into the stellar evolution code ng-elms, which makes use of a non-grey model atmosphere computed at runtime. ng-elms is used to compute stellar evolution models for primordial and lithium-free element mixtures and for stars in the subsolar mass range 0.8–0.15 M⊙. We find that lithium has little or no effect upon the structure and observable properties of stars in this mass range. Furthermore, lithium is completely destroyed by fusion before the main sequence in stars of mass less than ∼0.7 M⊙. We find that on the red giant branch and Hayashi track, the use of a non-grey model atmosphere to provide the upper boundary conditions for the stellar evolution calculation results in significantly cooler less-luminous stars, across the mass range.