Impact of Simultaneous Stellar Modeling Uncertainties on the Tip of the Red Giant Branch for Axion-Election Coupling

Abstract

We present a novel method for incorporating the effects of stellar modeling uncertainties into constraints on the axion-electron coupling constant found using the observed calibration of the tip of the red giant branch (TRGB) I band magnitude $M_I$.~We simulate grids of models with varying initial stellar mass, helium abundance, metallicity, and axion-electron coupling ${\alpha }_{26}=10^{26}{g}_{ae}^2/4\pi$ but different (fixed) mixing lengths and mass loss efficiencies.~We then train separate machine learning emulators to predict $M_I$ as a function of the varying parameters for each grid.~Our emulators enable the use of Markov Chain Monte Carlo simulations where ${\alpha }_{26}$ is varied simultaneously with the stellar parameters.~One of our grids yields a bound ${\alpha }_{26}\le 0.75$ at the 95\% confidence limit, a factor of $\sim 3.7$ weaker than previous bounds;~while the other grid yields ${\alpha }_{26}\le 1.58$ at the 95\% confidence limit, a factor $\sim 7.8$ weaker than previous bounds.~We demonstrate that the different values we find are due to covariances between stellar and axion physics that are not accounted for by single parameter variations.~Our results suggest that the bound on ${\alpha }_{26}$ derived using empirical calibrations of the TRGB I band magnitude need to be reevaluated using simultaneous parameter variation.~Alternative methods that use the bolometric luminosity instead of $M_I$ are more robust because they are not reliant upon theoretical predictions of the effective temperature.

Comments: 37 pages, 17 figures, 2 table, dataset at https://zenodo.org/record/7896061 Version accepted for publication in Physics of the Dark Universe. Substantial edits include: Expanded to two separate model grids with different input physics with additional machine learning and MCMC analysis. Added analysis of wind loss and mixing length effects on the TRGB. Expansion of discussion and conclusions