Speaker
Description
Globular Clusters (GC) are building blocks of any kind of galaxy. The Milky Way hosts hundreds of GCs, preferentially located in the galactic halo and bulge. A typical GC contains about 10^6 almost coeval stars, as old as \sim 13 Gyr. In the more advanced phase of the evolution, i.e., stars found in the RGB, HB, AGB and WD cooling sequence, stellar interiors are hot enough for the activation of thermal processes capable to produce hypothetical feebly interacting particles, such as Axions or Axion-Like-Particles (ALPs). Then, if the mass of these particles is smaller than \sim 100 KeV, they freely escape from the stellar core, thus acting as a net energy sink, whose effects are expected to modify the observed macroscopic stellar properties, such as the luminosity or the number of stars found in a given evolutionary phase. Therefore, precise measurements of these stellar properties my constrain the unknown properties of these particles, such as the strengths of the interactions with standard model particles (photons, electrons….) or their mass. In this talk, I will discuss the constraint of the axion-electron coupling as obtained from the observed luminosity of the RGB tip. I will show how the recent availability of more accurate astrometric data from GAIA (EDR3) and HST, coupled to the development of more reliable stellar models, provide the most stringent and robust upper bound to g_ae.
