Different Stellar Rotations in the Two Main Sequences of the Young Globular Cluster NGC 1818: The First Direct Spectroscopic Evidence

We present a spectroscopic analysis of main sequence (MS) stars in the young globular cluster NGC 1818 (age ∼40 Myr) in the Large Magellanic Cloud. Our photometric survey of Magellanic Clouds clusters has revealed that NGC 1818, similar to other young objects with ages ∼600 Myr, displays not only an extended MS turnoff (eMSTO), as observed in intermediate-age clusters (age ∼1-2 Gyr), 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 (bMS) and a sequence of fast rotators, with rotation close to the breaking speed, defining a red-MS (rMS). We report the first direct spectroscopic measurements of projected rotational velocities v sin i for the double MS, eMSTO, and Be stars of a young cluster. The analysis of line profiles includes non-local thermodynamic equilibrium effects, required for correctly deriving v sin i values. Our results suggest that: (i) the mean rotation for bMS and rMS stars is v sin i =71 ± 10 km s-1 (σ=37 km s-1) and v sin i=202±23 km s-1 (σ= 91 km s-1), respectively; (ii) eMSTO stars have different v sin i, which are generally lower than those inferred for rMS stars, and (iii) as expected, Be stars display the highest v sin i values. This analysis supports the idea that distinct rotational velocities play an important role in the appearance of multiple stellar populations in the color-magnitude diagrams of young clusters, and poses new constraints on the current scenarios.