Spectroscopy Unveils the Complex Nature of Terzan 5

TL;DR

This study analyzes the chemical composition of stars in Terzan 5, revealing two distinct populations with different metallicities and enrichment histories, challenging its classification as a typical globular cluster.

Contribution

It provides the first detailed chemical abundance analysis of 33 stars in Terzan 5, uncovering its complex stellar populations and chemical evolution history.

Findings

Two stellar populations with distinct [Fe/H] and [alpha/Fe] ratios

Absence of typical globular cluster anti-correlation patterns

Evidence of prolonged star formation and chemical enrichment

Abstract

We present the chemical abundance analysis of 33 red giant stars belonging to the complex stellar system Terzan 5. We confirm the discovery of two stellar populations (Ferraro et al. 2009, Nature, 462,483) with distinct iron abundances: a relatively metal-poor component with [Fe/H]=-0.25 +/- 0.07 r.m.s., and another component with [Fe/H]=+0.27 +/- 0.04 r.m.s., exceeding in metallicity any known Galactic globular cluster. The two populations also show different [alpha/Fe] abundance ratios. The metal-poor component has an average [alpha/Fe]=+0.34 +/- 0.06 r.m.s., consistent with the canonical scenario for rapid enrichment by core collapse supernovae (SNe). The metal-rich component has [alpha/Fe]=+0.03 +/-i 0.04 r.m.s., suggesting that the gas from which it formed was polluted by both type II and type Ia SNe on a longer timescale. Neither of the two populations shows evidence of the [Al/Fe] over [O/Fe] anti-correlation, that is typically observed in Galactic globular clusters. Because these chemical abundance patterns are unique, we propose that Terzan 5 is not a true globular cluster, but a stellar system with a much more complex history of star formation and chemical enrichment.