The star formation history of the SMC star cluster NGC419

TL;DR

This study reconstructs the star formation history of the SMC cluster NGC419, revealing multiple star formation episodes over 700 Myr, and demonstrates the first such analysis for a cluster with multiple turn-offs.

Contribution

It applies the classical CMD reconstruction method to a star cluster with multiple turn-offs, providing the first detailed star formation history for such a cluster.

Findings

Star formation lasted at least 700 Myr.

A peak in star formation occurred around 1.5 Gyr ago.

Multiple star formation episodes likely caused the multiple turn-offs.

Abstract

The rich SMC star cluster NGC419 has recently been found to present both a broad main sequence turn-off and a dual red clump of giants, in the sharp colour-magnitude diagrams (CMD) derived from the High Resolution Channel of the Advanced Camera for Surveys on board the Hubble Space Telescope. In this work, we apply to the NGC419 data the classical method of star formation history (SFH) recovery via CMD reconstruction, deriving for the first time this function for a star cluster with multiple turn-offs. The values for the cluster metallicity, reddening, distance and binary fraction, were varied within the limits allowed by present observations. The global best-fitting solution is an excellent fit to the data, reproducing all the CMD features with striking accuracy. The corresponding star formation rate is provided together with estimates of its random and systematic errors. Star formation is found to last for at least 700 Myr, and to have a marked peak at the middle of this interval, for an age of 1.5 Gyr. Our findings argue in favour of multiple star formation episodes (or continued star formation) being at the origin of the multiple main sequence turn-offs in Magellanic Cloud clusters with ages around 1 Gyr. It remains to be tested whether alternative hypotheses, such as a main sequence spread caused by rotation, could produce similarly good fits to the data.