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 $\alpha_{26}= 10^{26} g^2_{ae}/4\pi$ 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 $\alpha_{26}\leq 0.75$ at the 95\% confidence limit, a factor of $\sim3.7$ weaker than previous bounds;~while the other grid yields $\alpha_{26}\leq1.58$ at the 95\% confidence limit, a factor $\sim7.8$ 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.