Isochrone fitting of Galactic globular clusters - IX. Tip of the red-giant branch for 27 clusters and constraints on new particle physics

Abstract

Red-giant evolution is sensitive to non-standard channels of energy loss in stellar cores and therefore provides a probe of new physics. Such losses increase the luminosity of the tip of the red-giant branch (TRGB), the brightest point reached by a red giant in the color-magnitude diagram. Previous analyses of TRGB luminosities in globular clusters placed stringent bounds on the axion-electron coupling g_ae, but were limited by observational and theoretical uncertainties. To improve these constraints, we use a new homogeneous set of parameters for 27 globular clusters, derived from our fitting of the Stetson ground-based, Hubble Space Telescope, and Gaia data by isochrones from the Dartmouth Stellar Evolution Database and a Bag of Stellar Tracks and Isochrones together with state-of-the-art Modules for Experiments in Stellar Astrophysics models. For each cluster, we identify the most luminous non-variable red giant, infer its luminosity from the best-fit isochrones, and apply Monte-Carlo based corrections for discrete sampling of the red-giant branch and for the exclusion of variable stars. We model the TRGB luminosity as a function of g_ae, as well as its uncertainties, for each cluster individually, and combine the results in a likelihood analysis. We find no indication of anomalous energy losses and obtain g_ae<3.8*10^(-14) at 95% confidence level. The median TRGB bolometric absolute magnitude is -3.53+-0.05 mag. We briefly discuss the implications for the neutrino magnetic dipole moment, millicharged particles, and other new physics.