New Constraints on the Star Formation History of the Star Cluster NGC 1856

TL;DR

This study uses Hubble Space Telescope data and simulations to investigate the star formation history of NGC 1856, suggesting an age spread and potential multiple stellar populations, challenging previous claims of a single-age population.

Contribution

It provides new constraints on the age distribution and formation history of NGC 1856, incorporating dynamical models and addressing the role of stellar rotation.

Findings

The CMD is best explained by two stellar populations separated by 80 Myr.

The cluster's escape velocity was sufficient to retain stellar ejecta for ~100 Myr.

Results support an age spread in NGC 1856, contradicting earlier single-age assumptions.

Abstract

We use the Wide Field Camera 3 onboard the Hubble Space Telescope to obtain deep, high-resolution photometry of the young (age ~ 300 Myr) star cluster NGC1856 in the Large Magellanic Cloud. We compare the observed colour-magnitude diagram (CMD), after having applied a correction for differential reddening, with Monte Carlo simulations of simple stellar populations (SSPs) of various ages. We find that the main sequence turn-off (MSTO) region is wider than that derived from the simulation of a single SSP. Using constraints based on the distribution of stars in the MSTO region and the red clump, we find that the CMD is best reproduced using a combination of two different SSPs with ages separated by 80 Myr (0.30 and 0.38 Gyr, respectively). However, we can not formally exclude that the width of the MSTO could be due to a range of stellar rotation velocities if the efficiency of rotational mixing is higher than typically assumed. Using a King-model fit to the surface number density profile in conjunction with dynamical evolution models, we determine the evolution of cluster mass and escape velocity from an age of 10 Myr to the present age, taking into account the possible effects of primordial mass segregation. We find that the cluster has an escape velocity Vesc ~ 17 km/s at an age of 10 Myr, and it remains high enough during a period of ~ 100 Myr to retain material ejected by slow winds of first-generation stars. Our results are consistent with the presence of an age spread in NGC1856, in contradiction to the results of Bastian & Silva-Villa (2013).