A stellar census in globular clusters with MUSE. A new perspective on
  the multiple main sequences of $\omega$ Centauri
  $\omega$ Cen is a rare example of a globular cluster where the iron abundance
of the stars spans more than one order of magnitude. Many spectroscopic
investigations of its red-giant- and sub-giant- branches have revealed multiple
peaks in the iron abundance distribution. The metallicity distribution of
main-sequence (MS) stars is not well characterized yet, due to the faintness of
the stars and lack of data. So far, almost all studies of MS stars are based on
photometric measurements. Our goal is to investigate the metallicity
distribution of a statistically significant sample of MS stars in $\omega$ Cen.
In particular, we aim at revisiting the metallicity difference between the red
and blue MS of the cluster. We used MUSE spectra obtained for the central
region of $\omega$ Cen to derive metallicities for $\approx$4200 MS stars. We
find that blue MS stars are on average $\approx$0.1 dex more metal-rich than
their red counterparts. On the basis of this new estimate, we find that the two
sequences can be fit on the Hubble Space Telescope color-magnitude diagram with
two isochrones having the same global metallicity and age but a higher helium
abundance for the blue MS, that is $\Delta Y \lesssim$ 0.1. Furthermore, we
determine the average metallicity of the five main populations along $\omega$
Cen MS and these estimates are consistent with expectations from previous
photometric studies.
