  The method developed by Raffelt (1990a,b,c) to estimate a possible increase
in the standard values of the helium-core mass at the tip of the red giant
branch, $\Mc$, from properties of the color-magnitude diagrams of Galactic
globular clusters is employed. In the present study, we revise and update
Raffelt's database, including also constraints from RR Lyrae pulsation, and
find that a small increase, by $\Delta\Mc \approx 0.01\pm 0.015 \Msun$, cannot
be ruled out with the present data and evolutionary models. Our new upper
limits on $\Delta\Mc$ are less restrictive than those previously obtained by
Raffelt, as are the corresponding constraints on novel astroparticle phenomena
which may affect the evolution of low-mass red giants. Within the estimated
uncertainties, however, the standard values of $\Mc$ may also be acceptable.
Raffelt's method does not rule out a low envelope helium abundance in globular
cluster giants, though again the standard values are compatible with the
available constraints. The influence of a non-solar ratio for the
$\alpha$-capture elements upon these results is also investigated. In addition,
we review several aspects of the input physics employed in red giant stellar
evolutionary calculations, with the purpose of evaluating possible sources of
uncertainty in the value of the helium-core mass at the helium flash that is
obtained from evolutionary computations, such as: heat conduction by electrons
in the degenerate core; Coulomb effects upon the Equation of State;
triple-$\alpha$ reaction rates and screening factors; neutrino emission rates,
both standard and enhanced by a possible non-zero magnetic moment; stellar
rotation; microscopic element diffusion; and energy losses by axions and Weakly
Interacting Massive Particles.
