The insidious boosting of TP-AGB stars in intermediate-age Magellanic Cloud clusters

TL;DR

This paper identifies a temporary boost in TP-AGB star production in intermediate-age Magellanic Cloud clusters caused by evolutionary phase changes, which biases galaxy models and requires revised star population assessments.

Contribution

It reveals a specific evolutionary phase causing a transient increase in TP-AGB stars, impacting the accuracy of stellar population synthesis models.

Findings

TP-AGB contribution estimates are biased high due to the boost.

The boost occurs around 1.6 Gyr in stellar age.

Current models do not account for this transient effect.

Abstract

(Abridged) 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 incorporated into EPS models, are found to overestimate the TP-AGB contribution in resolved star counts and integrated spectra of galaxies. In this paper, we call attention to a particular evolutionary aspect 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 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 just in the proximity of the expected peak in the TP-AGB lifetimes, and for ages of 1.6 Gyr. Coincidently, this relatively narrow age interval happens to contain the few very massive MC clusters that host most of the TP-AGB stars used to constrain stellar evolution and EPS models. This concomitance makes the AGB-boosting particularly insidious in the context of present EPS models. The effect brings about three main consequences. (1) Present estimates of the TP-AGB contribution to the integrated light of galaxies derived from MC clusters, are biased towards too large values. (2) The relative TP-AGB contribution of single-burst populations falling in this critical age range cannot be accurately derived by the fuel consumption theorem. (3) A careful revision of AGB star populations in intermediate-age MC clusters is urgently demanded.