The Shape of the Red Giant Branch Bump as a Diagnostic of Partial Mixing Processes in Low‐Mass Stars

We suggest to use the shape of the Red Giant Branch (RGB) Bump in metal-richglobular clusters as a diagnostic of partial mixing processes between the baseof the convective envelope and the H-burning shell. The Bump located along thedifferential luminosity function of cluster RGB stars is a key observable toconstrain the H-profile inside these structures. In fact, standard evolutionarymodels that account for complete mixing in the convective unstable layers andradiative equilibrium in the innermost regions do predict that the firstdredge-up lefts over a very sharp H-discontinuity at the bottom of theconvective region. Interestingly enough we found that both atomic diffusion anda moderate convective overshooting at the base of the convective regionmarginally affects the shape of the RGB Bump in the differential LuminosityFunction (LF). As a consequence, we performed several numerical experiments toestimate whether plausible assumptions concerning the smoothing of theH-discontinuity, due to the possible occurrence of extra-mixing below theconvective boundary, affects the shape of the RGB Bump. We found that thedifference between the shape of RGB Bump predicted by standard and by smoothedmodels can be detected if the H-discontinuity is smoothed over an enveloperegion whose thickness is equal or larger than 0.5 pressure scale heights.Finally, we briefly discuss the comparison between theoretical predictions andempirical data in metal-rich, reddening free Galactic Globular Clusters (GGCs)to constrain the sharpness of the H-profile inside RGB stars.Comment: 15 pages, 8 postscript figures, ApJ in pres