Axion and neutrino bounds improved with new calibrations of the tip of the red-giant branch using geometric distance determinations

TL;DR

This study refines the brightness calibration of the tip of the red-giant branch using geometric distances, leading to improved constraints on axion-electron coupling and neutrino dipole moments, with implications for stellar physics and particle physics.

Contribution

The paper provides new TRGB calibrations based on geometric distances, reducing uncertainties and tightening bounds on axion and neutrino properties.

Findings

Updated bounds on axion-electron coupling: g_{ae}<1.6×10^{-13} (NGC 4258)

Updated bounds on neutrino dipole moment: μ_ν<1.5×10^{-12} μ_B (NGC 4258)

Consistent calibrations improve constraints and highlight the role of observational errors.

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 expected by standard stellar models. The required absolute TRGB calibrations strongly improve with reliable geometric distances that have become available for the galaxy NGC 4258 that hosts a water megamaser and to the Large Magellanic Cloud based on 20 detached eclipsing binaries. Moreover, we revise a previous TRGB calibration in the globular cluster $\omega$ Centauri with a recent kinematical distance determination based on Gaia DR2 data. All of these calibrations have similar uncertainties and they agree with each other and with recent dedicated stellar models. Using NGC 4258 as the cleanest extra-galactic case, we thus find an updated constraint on the axion-electron coupling of $g_{ae}<1.6\times10^{-13}$ and $\mu_\nu<1.5\times10^{-12}\mu_{\rm B}$ (95\% CL) on a possible neutrino dipole moment, whereas $\omega$ Centauri as the best galactic target provides instead $g_{ae}<1.3\times10^{-13}$ and $\mu_\nu<1.2\times10^{-12}\mu_{\rm B}$. The reduced observational errors imply that stellar evolution theory and bolometric corrections begin to dominate the overall uncertainties.