Mass loss in globular cluster red giants - an evolutionary investigation

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Abstract

The evolution of Population II red giants has been studied adopting Reimer's formalism for the efficiency of mass loss. Evolutionary computations for a model of 0.8 solar mass, Y = 0.23, Z = 0.0002 are presented for selected assumptions on the value of the parameter eta governing the mass-loss rate. Comparison with canonical models indicates that (1) the evolution of the internal He core is unaffected by mass loss, and (2) the luminosity of the red giant clump follows the actual stellar mass. Models with eta greater than 1.0 fail to ignite He, crossing the HR diagram toward the final cooling as He white dwarfs. However, one finds that the stellar core keeps ignoring the surface conditions well beyond this phase, unsuccessfully trying to ignite He along the cooling tracks, when log L/L(solar) is about 0. Exploring such an evidence, we found that for eta = 0.75 the cooling dwarf does succeed in igniting He at the quoted luminosity. As a result, one expects an extremely blue horizontal-branch star which evolves in about 200 x 10 exp 6 yr from log L/L(solar) = 1.2 to log L/L(solar) = 2.4 before starting the final cooling as a CO white dwarf. The resulting observational scenario is finally discussed, also in connection with the problematic of UV fluxes from old stellar populations.

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