MUSE spectroscopic observations of the Young Massive Cluster NGC1850
  NGC1850 is the nearest Young Massive Cluster of the Local Group with a mass
similar to those of Galactic globular clusters. Recent studies have revealed an
extended morphology of its MSTO, which can be interpreted as a spread in either
age or internal rotation. An accurate spectroscopic determination of its
chemical properties is still missing. We analyse spectra obtained with MUSE in
adaptive optics mode of 1167 stars in both components of this cluster (NGC1850A
and NGC1850B). Thanks to this dataset, we measure an average metallicity of
<[M/H]>=-0.31 +/- 0.01, a mean Ba abundance of <[Ba/Fe]>=+0.40 +/- 0.02 and a
systemic radial velocity of <v_{LOS}>=251.1 +/- 0.3 km/s. The dispersion of the
radial velocities suggests a dynamical mass of log(M/Ms)=4.84 +/- 0.1, while no
significant systemic rotation is detected. We detect a significant bimodality
in OI line strength among the TO stars of NGC1850A with ~66% of stars with
[O/Fe]~-0.16 and the rest with no detectable line. The majority of O-weak stars
populate preferentially the red side of the MSTO and show H lines in emission,
suggesting that they are Be stars rotating close to their critical velocity.
Among normal MSTO stars, red stars have on average broader line profiles than
blue ones, suggesting a correlation between colour and rotational velocity. The
mean metallicity of this cluster lies at the metal-rich side of the metallicity
distribution of the LMC following its age-metallicity relation. The Ba and O
abundances agree with those measured in the bar of this galaxy. The observed
spread in OI line width among its MS stars can be interpreted as an effect of
rotational mixing occurring in the envelopes of O-weak stars. The correlation
between line broadening and colour suggests that the observed colour spread
among turn-off stars can be due to a wide range in rotational velocity covered
by these stars.
