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Abstract

The brightness of the tip of the red-giant branch (TRGB) allows one to constrain novel energy losses that would lead to a larger core mass at helium ignition and thus to a brighter TRGB than predicted by standard stellar models. The required absolute TRGB calibration depends on reliable distances to the observed ensembles of stars. Motivated by its role as a rung in the cosmic distance ladder, the TRGB was recently recalibrated with geometric distance determinations of the Magellanic Clouds based on detached eclipsing binaries (DEBs). Moreover, we revise previous TRGB calibrations of the galactic globular clusters M5 and $\omega$ Centauri with recent kinematical distance determinations based on Gaia DR2 data. All of these TRGB calibrations have similar uncertainties and they agree with each other and with recent dedicated stellar models. We thus find an updated constraint on the axion-electron coupling of $g_{ae}<1.6\times10^{-13}$ (95% CL) and $\mu_\nu<1.5\times10^{-12}\mu_{\rm B}$ (95% CL) on a possible neutrino dipole moment. The reduced observational errors imply that stellar evolution theory and bolometric corrections begin to dominate the overall uncertainties.