STELLAR EVOLUTION AS A PROBE OF NEUTRINO PROPERTIES

We present the results of evolutionary computations devoted to a study of the effect of a nonvanishing neutrino magnetic moment (n.m.m.) on the evolution of stars. In agreement with previous investigations, we found that n.m.m affects the evolution of low-mass stars, increasing the cooling mechanisms in the core of red giants and thus increasing the mass M(c) of the He core at the onset of central He burning. One finds that in the range 0 < mu(nu) < 10(-11) mu(B) (Bohr magnetons), M(c) appears linearly dependent on the value of mu(nu). However, pushing this investigation toward larger stellar masses, giants around the so called transition phase shows a much larger nonlinear dependence of M(c) on mu(nu), because n.m.m. is governing whether or not the star experiences electron degeneracy in the core. The evolution of selected models through the whole phase of He burning has been followed. Contrary to previous suggestions, we found that the direct influence of n.m.m. on these structures appears negligible. As a whole, we calibrate the luminosity of He-burning stars in terms of n.m.m. Comparison with observational constraints concerning the pulsational properties of RR Lyrae variables in the well studied globular cluster M3 gives an upper limit to n.m.m. of mu(nu) < 10(-12) mu(B). The same upper limit is derived by considerations of the luminosity of He-burning giants in the galactic clusters Hyades and Praesepe.