Spectroscopy Unveils the Complex Nature of Terzan 5

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 with distinct iron abundances: a relatively metal-poor component with [Fe/H] = -0.25 ± 0.07 rms and another component with [Fe/H] = +0.27 ± 0.04 rms, exceeding in metallicity any known Galactic globular cluster (GC). The two populations also show different [α/Fe] abundance ratios. The metal-poor component has an average [α/Fe] = +0.34 ± 0.06 rms, consistent with the canonical scenario for rapid enrichment by core collapse supernovae (SNe). The metal-rich component has [α/Fe] = +0.03 ± 0.04 rms, 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 GCs. Because these chemical abundance patterns are unique, we propose that Terzan 5 is not a true GC, but a stellar system with a much more complex history of star formation and chemical enrichment.