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

Abstract

We present a spectroscopic analysis of main sequence (MS) stars in the young globular cluster NGC 1818 (age∼40 Myrs) in the Large Magellanic Cloud. Our photometric survey on Magellanic Clouds clusters has revealed that NGC 1818, 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 v sini 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 v sini =71±10 km s −1 (σ=37km s −1 ) and v sini =202±23 km s −1 (σ=91km s −1 ), respectively; (ii) eMSTO stars have different v sini , which are generally lower than those inferred for red-MS stars, and (iii) as expected, Be stars display the highest v sini 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.

How stellar rotation shapes the colour−magnitude diagram of the massive intermediate-age star cluster NGC 1846

“…It is also consistent with the observation of split main sequences in young stellar clusters that are characterised by different rotation rates (e.g. Marino et al 2018).…”

“…However, recent observations have directly linked the position of stars in the CMDs to their projected rotational velocity (v sin i) values (e.g. Dupree et al 2017;Kamann et al 2018b;Bastian et al 2018;Marino et al 2018).…”

Multiple populations in globular clusters and their parent galaxies

“…Similarly, the extended main-sequence turn offs and multiple main sequences (MSs) observed in clusters younger than ∼ 2 Gyr in both the Milky Way and MCs are interpreted as due to stellar rotation(e.g. D' Antona et al 2015;Li et al 2017;Cordoni et al 2018;Marino et al 2018c;Marino et al 2018a;Milone et al 2018a), rather than to chemical variations, and are possibly associated with age spreads (e.g. Goudfrooij et al 2011Goudfrooij et al , 2017.…”

MUSE Observations of NGC330 in the Small Magellanic Cloud: Helium Abundance of Bright Main-sequence Stars*

“…More recently, in young stellar clusters (age less than 600 Myr) have been observed not only the eMSTO, but also the split of the MS, similar to that detected in Milky Way GCs (Milone et al 2015(Milone et al , 2016. These photometric evidences could be interpreted with the presence of stellar populations with different ages, but also with coeval populations with different stellar rotation velocity (e.g., Bastian & de Mink 2009;Brandt & Huang 2015;D'Antona et al 2015;Marino et al 2018). Until now, spectroscopic studies of stars in MCs clusters younger than ∼ 2 Gyr do not show differences in light elements between stellar populations of young massive clusters (Mucciarelli et al 2014;Martocchia et al 2017), contrary to what is observed in the GGCs.…”