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

TL;DR

This study provides the first direct spectroscopic evidence linking stellar rotation velocities to the multiple main sequences observed in the young globular cluster NGC1818, revealing distinct rotation profiles for different stellar populations.

Contribution

It offers the first spectroscopic measurements of stellar rotation velocities in a young cluster's double main sequence, connecting rotation to multiple stellar populations.

Findings

Blue-MS stars have lower vsini (~71 km/s) than red-MS stars (~202 km/s).

eMSTO stars show varied vsini, generally lower than red-MS stars.

Be stars exhibit the highest vsini values, confirming rapid rotation.

Abstract

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.