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$\omega$ Cen is a rare example of a globular cluster where the iron abundanceof the stars spans more than one order of magnitude. Many spectroscopicinvestigations of its red-giant- and sub-giant- branches have revealed multiplepeaks in the iron abundance distribution. The metallicity distribution ofmain-sequence (MS) stars is not well characterized yet, due to the faintness ofthe stars and lack of data. So far, almost all studies of MS stars are based onphotometric measurements. Our goal is to investigate the metallicitydistribution of a statistically significant sample of MS stars in $\omega$ Cen.In particular, we aim at revisiting the metallicity difference between the redand blue MS of the cluster. We used MUSE spectra obtained for the centralregion of $\omega$ Cen to derive metallicities for $\approx$4200 MS stars. Wefind that blue MS stars are on average $\approx$0.1 dex more metal-rich thantheir red counterparts. On the basis of this new estimate, we find that the twosequences can be fit on the Hubble Space Telescope color-magnitude diagram withtwo isochrones having the same global metallicity and age but a higher heliumabundance for the blue MS, that is $\Delta Y \lesssim$ 0.1. Furthermore, wedetermine the average metallicity of the five main populations along $\omega$Cen MS and these estimates are consistent with expectations from previousphotometric studies.

A stellar census in globular clusters with MUSE. A new perspective on the multiple main sequences of $\omega$ Centauri