Terzan 5: an alternative interpretation for the split horizontal branch

TL;DR

This paper proposes an alternative explanation for the split horizontal branch in Terzan 5, attributing it to different stellar populations with varying metallicity and helium content, rather than age differences.

Contribution

It introduces a model where the split HB results from second-generation stars enriched by supernova ejecta, challenging the previous age-difference interpretation.

Findings

The observed HB morphology aligns with a model of metal-enriched, helium-rich second-generation stars.

A small age difference (~100 Myr) suffices to explain the split, negating the need for prolonged star formation.

The iron enrichment can be explained by supernova ejecta contamination or environmental gas accretion.

Abstract

We consider the horizontal branch (HB) of the Globular Cluster Terzan 5, recently shown to be split into two parts, the fainter one (delta M_K ~ 0.3mag) having a lower metallicity than the more luminous. Both features show that it contains at least two stellar populations. The separation in magnitude has been ascribed to an age difference of ~6 Gyr and interpreted as the result of an atypical evolutionary history for this cluster. We show that the observed HB morphology is also consistent with a model in which the bright HB is composed of second generation stars that are metal enriched and with a helium mass fraction larger (by delta Y ~ 0.07) than that of first generation stars populating the fainter part of the HB. Terzan 5 would therefore be anomalous, compared to most "normal" clusters hosting multiple populations, only because its second generation is strongly contaminated by supernova ejecta; the previously proposed prolonged period of star formation, however, is not required. The iron enrichment of the bright HB can be ascribed either to contamination from Type Ia supernova ejecta of the low-iron, helium rich, ejecta of the massive asympotic giant branch stars of the cluster, or to its mixing with gas, accreting on the cluster from the environment, that has been subject to fast metal enrichment due to its proximity with the galactic bulge. The model here proposed requires only a small age difference, of ~100Myr.