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We present a spectroscopic analysis of main sequence (MS) stars in the youngglobular cluster NGC1818 (age~40 Myrs) in the Large Magellanic Cloud. Ourphotometric survey on Magellanic Clouds clusters has revealed that NGC1818,similarly to the other young objects with age 600 Myrs, displays not only anextended MS Turn-Off (eMSTO), as observed in intermediate-age clusters (age~1-2Gyrs), but also a split MS. The most straightforward interpretation of thedouble MS is the presence of two stellar populations: a sequence ofslowly-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 thefirst direct spectroscopic measurements of projected rotational velocitiesvsini for the double MS, eMSTO and Be stars of a young cluster. The analysis ofline profiles includes non-LTE effects, required for correctly deriving v sinivalues. Our results suggest that: (i) the mean rotation for blue- and red-MSstars is vsini=71\pm10 km/s (sigma=37 km/s) and vsini=202\pm23 km/s (sigma=91km/s), respectively; (ii) eMSTO stars have different vsini, which are generallylower than those inferred for red-MS stars, and (iii) as expected, Be starsdisplay the highest vsini values. This analyis supports the idea that distinctrotational velocities play an important role in the appearence of multiplestellar populations in the color-magnitude diagrams of young clusters, andposes new constraints to the current scenarios.

Different stellar rotation in the two main sequences of the young globular cluster NGC1818: first direct spectroscopic evidence