Different stellar rotation in the two main sequences of the young
  globular cluster NGC1818: first direct spectroscopic evidence
  We present a spectroscopic analysis of main sequence (MS) stars in the young
globular cluster NGC1818 (age~40 Myrs) in the Large Magellanic Cloud. Our
photometric survey on Magellanic Clouds clusters has revealed that NGC1818,
similarly to the other young objects with age 600 Myrs, displays not only an
extended MS Turn-Off (eMSTO), as observed in intermediate-age clusters (age~1-2
Gyrs), but also a split MS. The most straightforward interpretation of the
double MS is the presence of two stellar populations: a sequence of
slowly-rotating stars lying on the blue-MS and a sequence of fast rotators,
with rotation close to the breaking speed, defining a red-MS. We report the
first direct spectroscopic measurements of projected rotational velocities
vsini for the double MS, eMSTO and Be stars of a young cluster. The analysis of
line profiles includes non-LTE effects, required for correctly deriving v sini
values. Our results suggest that: (i) the mean rotation for blue- and red-MS
stars is vsini=71\pm10 km/s (sigma=37 km/s) and vsini=202\pm23 km/s (sigma=91
km/s), respectively; (ii) eMSTO stars have different vsini, which are generally
lower than those inferred for red-MS stars, and (iii) as expected, Be stars
display the highest vsini values. This analyis supports the idea that distinct
rotational velocities play an important role in the appearence of multiple
stellar populations in the color-magnitude diagrams of young clusters, and
poses new constraints to the current scenarios.
