NGC1818 unveils the origin of the extended main-sequence turn-off in young Magellanic Clouds clusters

TL;DR

This study uses the main-sequence turn-on point in NGC1818 to distinguish between stellar rotation and age effects, revealing rapid star formation within 8 Myr and challenging the age spread hypothesis for extended main-sequence turn-offs.

Contribution

It introduces a novel method analyzing the main-sequence turn-on to differentiate between rotation and age effects in young clusters, providing new insights into their star formation history.

Findings

Star formation in NGC1818 occurred within 8 Myr.

Age differences are unlikely to explain the extended main-sequence turn-offs.

Stellar rotation alone cannot account for observed features.

Abstract

The origin of young star clusters represents a major challenge for modern stellar astrophysics. While stellar rotation partially explains the colour spread observed along main-sequence turn-offs, i.e. where stars leave the main-sequence after the exhaustion of hydrogen in their core, and the multiple main sequences in the colour-magnitude diagrams of stellar systems younger than approximately 2 Gyr, it appears that an age difference may still be required to fulfill the observational constraints. Here we introduce an alternative approach that exploits the main-sequence turn-on, i.e. the point alongside the colour-magnitude diagram where pre-main-sequence stars join the main-sequence, to disentangle between the effects of stellar rotation and age to assess the presence, or lack thereof, of prolonged star formation in the approximately 40-Myr-old cluster NGC1818. Our results provide evidence for a fast star formation, confined within 8 Myr, thus excluding age differences as responsible for the extended main-sequence turn-offs, and leading the way to alternative observational perspectives in the exploration of stellar populations in young clusters.