# The HST Large Program on Omega Centauri. I. Multiple stellar populations at the bottom of the main sequence probed in NIR-Optical

## TL;DR

This study uses deep optical and near-infrared Hubble observations to identify and analyze multiple stellar populations along the main sequence of Omega Centauri, revealing complex chemical variations and subpopulations.

## Contribution

First detection of multiple main sequence populations in Omega Centauri across the entire main sequence using combined optical and NIR data.

## Key findings

- Identified two main stellar populations with different metallicity and helium content.
- Discovered subpopulations with varying oxygen abundances below the MS knee.
- Demonstrated the effectiveness of optical and NIR photometry in studying stellar populations.

## Abstract

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.