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

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 ω Centauri with a recent kinematical distance determination based on Gaia data release 2. 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 extragalactic case, we thus find an updated constraint on the axion-electron coupling of gae < 1.6 × 10−13 and μν < 1.5 × 10−12 μB (95% C.L.) on a possible neutrino dipole moment, whereas ω Centauri as the best galactic target provides instead gae < 1.3 × 10−13 and μν < 1.2 × 10−12 μB . The reduced observational errors imply that stellar evolution theory and bolometric corrections begin to dominate the overall uncertainties.