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.
Highlights
The evolution of a low-mass star as it ascends the red-giant branch (RGB) is driven by the growing mass and shrinking size of its degenerate core until helium ignites and the core quickly expands [1]
Our main interest is to use the the red-giant branch (TRGB) as a particlephysics laboratory in the sense that the emission of new low-mass particles, notably axions or neutrinos with anomalous magnetic dipole moments, would provide additional cooling of the helium core, increase the core mass before helium ignites, and, lead to a brighter TRGB
A second motivation is the availability of new theoretical reference models explicitly for the purpose of TRGB
Summary
The evolution of a low-mass star as it ascends the red-giant branch (RGB) is driven by the growing mass and shrinking size of its degenerate core until helium ignites and the core quickly expands [1]. A second motivation is the availability of new theoretical reference models explicitly for the purpose of TRGB calibration, including detailed error estimates [15] These authors find that, for stellar parameters appropriate for the globular cluster M5, their calibration agrees perfectly with earlier models dedicated to M5 [8] after one corrects for the treatment of screening of nuclear reaction rates relevant for conditions on the RGB. Another empirical TRGB calibration uses red giants in the halos of galaxies, as these represent an old population of stars.
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