Multiple stellar populations in Magellanic Cloud clusters. IV. The double main sequence of the young cluster NGC1755

TL;DR

This study investigates the young cluster NGC1755 in the Magellanic Clouds, revealing a split main sequence and extended MSTO that are best explained by populations with different stellar rotation rates, challenging age or composition variation hypotheses.

Contribution

The paper provides the first evidence of a split main sequence and eMSTO in a very young (~80 Myr) cluster, attributing these features to differences in stellar rotation rather than age or chemical composition.

Findings

NGC1755 exhibits a split main sequence with 25% blue and 75% red stars.

The extended MSTO is consistent with populations having different stellar rotation rates.

The split MS and eMSTO are not caused by observational errors or binary contamination.

Abstract

Nearly all the star clusters with ages of ~1-2 Gyr in both Magellanic Clouds exhibit an extended main-sequence turn off (eMSTO) whose origin is under debate. The main scenarios suggest that the eMSTO could be either due to multiple generations of stars with different ages or to coeval stellar populations with different rotation rates. In this paper we use Hubble-Space-Telescope images to investigate the ~80-Myr old cluster NGC1755 in the LMC. We find that the MS is split with the blue and the red MS hosting about the 25% and the 75% of the total number of MS stars, respectively. Moreover, the MSTO of NGC1755 is broadened in close analogy with what is observed in the ~300-Myr-old NGC1856 and in most intermediate-age Magellanic-Cloud clusters. We demonstrate that both the split MS and the eMSTO are not due to photometric errors, field-stars contamination, differential reddening, or non-interacting binaries. These findings make NGC1755 the youngest cluster with an eMSTO. We compare the observed CMD with isochrones and conclude that observations are not consistent with stellar populations with difference in age, helium, or metallicity only. On the contrary, the split MS is well reproduced by two stellar populations with different rotation, although the fit between the observed eMSTO and models with different rotation is not fully satisfactory. We speculate whether all stars in NGC1755 were born rapidly rotating, and a fraction has slowed down on a rapid timescale, or the dichotomy in rotation rate was present already at star formation. We discuss the implication of these findings on the interpretation of eMSTO in young and intermediate-age clusters.