Theoretical Uncertainties in Red Giant Branch Evolution: The Red Giant Branch Bump

Abstract

A Monte Carlo simulation exploring uncertainties in standard stellar evolution theory on the red giant branch of metal-poor globular clusters has been conducted. Confidence limits are derived on the absolute V-band magnitude of the bump in the red giant branch luminosity function (M_v,b) and the excess number of stars in thebump, R_b. The analysis takes into account uncertainties in the primordial helium abundance, abundance of alpha-capture elements, radiative and conductive opacities, nuclear reaction rates, neutrino energy losses, the treatments of diffusion and convection, the surface boundary conditions, and color transformations. The uncertainty in theoretical values for the red giant bump magnitude varies with metallicity between +0.13/-0.12 mag at [Fe/H] = -2.4 and +0.23/-0.21 mag at [Fe/H] = -1.0$.Thedominantsourcesofuncertaintyaretheabundanceofthealpha-captureelements,themixinglength,andthelow-temperatureopacities.Thetheoreticalvaluesof{M}_v,bareingoodagreementwithobservations.Theuncertaintyinthetheoreticalvalueof{R}_{b}is+/-0.01atallmetallicitiesstudied.Thedominantsourcesofuncertaintyaretheabundanceofthealpha-captureelements,themixinglength,andthehigh-temperatureopacities.Themedianvalueof{R}_{b}variesfrom0.44at[Fe/H]=-2.4$ to 0.50 at [Fe/H] = -1.0. These theoretical values for R_b are in agreement with observations.

30 pages, 6 figures. To appear in ApJ

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