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
