Dissecting the Extended Main Sequence Turn-off of the Young Star Cluster NGC1850

TL;DR

This study uses Hubble Space Telescope data to analyze the young star cluster NGC1850, revealing an extended main sequence turn-off and double main sequence, which are explained by a combination of age spread and stellar rotation effects.

Contribution

It demonstrates that a combination of age spread and stellar rotation rates explains the extended MSTO and double MS in NGC1850, advancing understanding of stellar populations in young clusters.

Findings

Extended MSTO and double MS observed in NGC1850.

Stellar rotation and age spread together reproduce CMD features.

Rapidly rotating Be stars contribute to MSTO morphology.

Abstract

We use the Wide Field Camera 3 onboard the Hubble Space Telescope to obtain deep, high-resolution photometry of the young (~ 100 Myr) star cluster NGC1850 in the Large Magellanic Cloud. We analyze the cluster colour-magnitude diagram (CMD) and find that it hosts an extended main sequence turn-off (MSTO) and a double MS. We demonstrate that these features cannot be due to photometric errors, field star contamination, or differential reddening. From a comparison with theoretical models and Monte Carlo simulations, we show that a coeval stellar population featuring a distribution of stellar rotation rates can reproduce the MS split quite well. However, it cannot reproduce the observed MSTO region, which is significantly wider than the simulated ones. Exploiting narrow-band Halpha imaging, we find that the MSTO hosts a population of Halpha-emitting stars which are interpreted as rapidly rotating Be-type stars. We explore the possibility that the discrepancy between the observed MSTO morphology and that of the simulated simple stellar population (SSP) is caused by the fraction of these objects that are highly reddened, but we rule out this hypothesis. We demonstrate that the global CMD morphology is well-reproduced by a combination of SSPs that cover an age range of ~ 35 Myr as well as a wide variety of rotation rates. We derive the cluster mass and escape velocity and use dynamical evolution models to predict their evolution starting at an age of 10 Myr. We discuss these results and their implications in the context of the extended MSTO phenomenon.