The true nature of Terzan 5: the most efficient "furnace" of MSPs in the Galaxy

TL;DR

Terzan 5 is a unique stellar system with multiple populations, high metallicity, and an exceptionally large population of millisecond pulsars, likely resulting from its massive past and supernova activity.

Contribution

This study reveals that Terzan 5 is not a typical globular cluster but a complex system with multiple populations and a high MSP count, linked to its supernova history.

Findings

Two distinct stellar populations with different metallicities.

High number of neutron stars explains MSP abundance.

Evidence of past supernova explosions shaping the system.

Abstract

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.