The true nature of Terzan 5: the most efficient "furnace" of MSPs in the
  Galaxy
  Terzan5 is the globular cluster (GC)-like stellar system harboring the
largest known population of MSPs. Using the Multi-Conjugate Adaptive Optics
demonstrator MAD at the ESO - VLT, we recently obtained a superb (K, J - K)
color-magnitude diagram, which has revealed the existence of two horizontal
branches (HBs) well separated in magnitude and colour (Ferraro et al. 2009,
Nature,462, 483). A prompt spectroscopic follow-up with NIRSPEC@Keck has shown
that the two populations have (1) significantly different iron content ([Fe/H]=
-0.2 and +0.3 for the faint and the bright HB, respectively), (2) distinct
[alpha/Fe] abundance patterns and (3) no evidence of the Al-O anti-correlation
commonly observed in GCs. All these properties suggest that Ter 5 is far from
being a genuine globular. Instead it has experienced the explosion of a huge
number of supernovae (SNe), thus accounting for its high metal content and it
should have been much more massive in the past than today, thus to retain the
SN ejecta within its potential well. The many type II SNe should have also
produced a large number of neutron stars (NSs), which could finally explain its
exceptionally large population of MSPs.
