THE HELIUM-CORE MASS AT THE HELIUM FLASH IN LOW-MASS RED GIANT STARS - OBSERVATIONS AND THEORY

The method developed by Raffelt to estimate a possible increase in the standard values of the helium-core mass at the tip of the red giant b ranch, M(c), from properties of the color-magnitude diagrams of Galact ic globular clusters is employed. In the present study, we revise and update Raffelts database, including also constraints from RR Lyrae pul sation, and find that a small increase, of Delta M(c) approximate to 0 .01 +/- 0.015 M(.), cannot be ruled out with the present data and evol utionary models. Our new upper limits on Delta M(c) are less restricti ve than those previously obtained by Raffelt, as are the corresponding constraints on novel astroparticle phenomena that may affect the evol ution of low-mass red giants. Within the estimated uncertainties, howe ver, the standard values of M(c) may also be acceptable. Raffelts meth od does not rule out a low envelope helium abundance in globular clust er giants, though again the standard values are compatible with the av ailable constraints. The influence of a nonsolar ratio for the alpha-c apture elements upon these results is also investigated. In addition, we review several aspects of the input physics employed in red giant s tellar evolutionary calculations, with the purpose of evaluating possi ble sources of uncertainty in the value of the helium-core mass at the helium hash that is obtained from evolutionary computations, such as heat conduction by electrons in the degenerate core; Coulomb effects u pon the equation of state; triple-alpha reaction rates and screening f actors; neutrino emission rates, both standard and enhanced by a possi ble nonzero magnetic moment; stellar rotation; microscopic element dif fusion; and energy losses by axions and weakly interacting massive par ticles (WIMPs).