The HST large programme on 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 ? 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 colour 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 hydrogenburning limit. We have compared the observations with suitable synthetic spectra of MS stars and conclude that the two main sequences (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 launch 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.