2011ApJ...726L..20O

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.