A chromosome map to unveil stellar populations with different magnesium
  abundances. The case of Omega Centauri
  Historically, photometry has been largely used to identify stellar
populations (MPs) in Globular Clusters (GCs) by using diagrams that are based
on colours and magnitudes that are mostly sensitive to stars with different
metallicities or different abundances of helium, carbon, nitrogen and oxygen.
In particular, the pseudo two-colour diagram called chromosome map (ChM),
allowed the identification and the characterization of MPs in about 70 GCs by
using appropriate filters of the Hubble Space Telescope (HST) that are
sensitive to the stellar content of He, C, N, O and Fe. We use here
high-precision HST photometry from F275W, F280N, F343N, F373N, and F814W images
of Omega Centauri to investigate its MPs. We introduce a new ChM whose abscissa
and ordinate are mostly sensitive to stellar populations with different
magnesium and nitrogen, respectively, in monometallic GCs. This ChM is
effective in disentangling the MPs based on their Mg chemical abundances,
allowing us to explore, for the first time, possible relations between the
production of these elemental species for large samples of stars in GCs. By
comparing the colours of the distinct stellar populations with the colours
obtained from appropriate synthetic spectra we provide 'photometric-like'
estimates of the chemical composition of each population. Our results show
that, in addition to first generation (1G) stars, the metal-poor population of
Omega Centauri hosts four groups of second-generation stars with different
[N/Fe], namely, 2GA--D. 2GA stars share nearly the same [Mg/Fe] as the 1G,
whereas 2GB, 2GC and 2GD stars are Mg depleted by ~0.15, ~0.25 and ~0.45 dex,
respectively. We provide evidence that the metal-intermediate populations host
stars with depleted [Mg/Fe].
