The HST Large Program on Omega Centauri. I. Multiple stellar populations
  at the bottom of the main sequence probed in NIR-Optical
  As part of a large investigation with Hubble Space Telescope to study the
faintest stars within the globular cluster Omega Centauri, in this work we
present early results on the multiplicity of its main sequence (MS) stars,
based on deep optical and near-infrared observations. By using appropriate
color-magnitude diagrams we have identified, for the first time, the two main
stellar populations I, and II along the entire MS, from the turn-off towards
the hydrogen-burning limit. We have compared the observations with suitable
synthetic spectra of MS stars and conclude that the two MSs are consistent with
stellar populations with different metallicity, helium, and light-element
abundance. Specifically, MS-I corresponds to a metal-poor stellar population
([Fe/H]~-1.7) with Y~ 0.25 and [O/Fe]~0.30. The MS-II hosts helium-rich
(Y~0.37-0.40) stars with metallicity ranging from [Fe/H]~-1.7 to -1.4. Below
the MS knee (mF160W~19.5, our photometry reveals that each of the two main MSs
hosts stellar subpopulations with different oxygen abundances, with very O-poor
stars ([O/Fe]~-0.5) populating the MS-II. Such a complexity has never been
observed in previous studies of M-dwarfs in globular clusters. A few months
before the lunch of the James Webb Space Telescope, these results demonstrate
the power of optical and near-infrared photometry in the study of multiple
stellar populations in globular clusters.
