To appear in ApJ Preprint typeset using LATEX style emulateapj v. 04/17/13 THE INSIDIOUS BOOSTING OF TP-AGB STARS IN INTERMEDIATE-AGE MAGELLANIC CLOUD CLUSTERS

In the recent controversy about the role of TP-AGB stars in evolutionary population synthesis (EPS) models of galaxies, one particular aspect is puzzling: TP-AGB models aimed at reproducing the lifetimes and integrated fluxes of the TP-AGB phase in Magellanic Cloud (MC) clusters, when incor-porated into EPS models, are found to overestimate, to various extents, the TP-AGB contribution in resolved star counts and integrated spectra of galaxies. In this paper, we call attention to a particular evolutionary aspect, linked to the physics of stellar interiors, that in all probability is the main cause of this conundrum. As soon as stellar populations intercept the ages at which RGB stars first appear, a sudden and abrupt change in the lifetime of the core He-burning phase causes a temporary “boost” in the production rate of subsequent evolutionary phases, including the TP-AGB. For a timespan of about 0.1 Gyr, triple TP-AGB branches develop at slightly different initial masses, causing their frequency and contribution to the integrated luminosity of the stellar population to increase by a factor of ∼2. The boost occurs for turn-off masses of ∼1.75M, just in the proximity of the expected peak in the TP-AGB lifetimes (for MC metallicities), and for ages of ∼ 1.6 Gyr. Coincidently, this relatively narrow age interval happens to contain the few very massive MC clusters that host mos