Terzan 5 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 .