On the internal pollution mechanisms in the globular cluster NGC 6121 (M4): heavy-element abundances and AGB models

TL;DR

This study investigates the internal pollution mechanisms in globular cluster NGC 6121 by analyzing heavy-element abundances and comparing them with AGB star models, aiming to identify the main polluters responsible for stellar population variations.

Contribution

It provides new Y and Rb abundance measurements for NGC 6121 and introduces updated AGB stellar yields including heavy elements, assessing their role in cluster pollution.

Findings

Y abundances are constant across the cluster.

Heavy element abundances show no variation, constraining AGB pollution models.

Uncertainties in stellar model parameters limit definitive conclusions.

Abstract

Globular clusters display significant variations in their light-element content, pointing to the existence of a second stellar generation formed from the ejecta of an earlier generation. The nature of these internal polluters is still a matter of debate: the two most popular scenarios indicate intermediate-mass AGB stars and fast rotating massive stars. Abundances determination for some key elements can help distinguish between these competitor candidates. We present in this paper Y abundances for a sample of 103 red giant branch stars in NGC 6121. Within measurement errors, we find that the [Y/Fe] is constant in this cluster contrary to a recent suggestion. For a subsample of six stars we also find [Rb/Fe] to be constant, consistent with previous studies showing no variation in other s-process elements. We also present a new set of stellar yields for intermediate-mass AGB stellar models of 5 and 6 solar masses, including heavy element s-process abundances. The uncertainties on the mass-loss rate, the mixing-length parameter, and the nuclear reaction rates have a major impact on the stellar abundances. Within the IM-AGB pollution scenario, the constant abundance of heavy elements inside the cluster requires a marginal s-process efficiency in IM-AGB stars. Such a constrain could still be satisfied by the present models assuming a stronger mass-loss rate. The uncertainties mentioned above are limiting the predictive power of intermediate-mass AGB models. For these reasons, at the moment we are not able to clearly rule out their role as main polluters of the second population stars in globular clusters.