Clues to the star formation in NGC 346 across time and space

TL;DR

This study investigates the star formation history in NGC 346, revealing distinct age populations, spatial separation of star types, and implications for understanding star formation and mass estimation in galaxies.

Contribution

It introduces a novel method for identifying pre-main sequence stars and analyzes their properties to understand star formation across time and space in NGC 346.

Findings

Two main age groups of PMS stars at ~1 Myr and ~20 Myr

No spatial correlation between young and old PMS stars

Mass distribution varies significantly across the region

Abstract

We have studied the properties of the stellar populations in the field of the NGC 346 cluster in the Small Magellanic Cloud, using the results of a novel self-consistent method that provides a reliable identification of pre-main sequence (PMS) objects actively undergoing mass accretion, regardless of their age. The 680 identified bona-fide PMS stars show a bimodal age distribution, with two roughly equally numerous populations peaked respectively at ~1 Myr, and ~20 Myr. We use the age and other physical properties of these PMS stars to study how star formation has proceeded across time and space in NGC 346. We find no correlation between the locations of young and old PMS stars, nor do we find a correspondence between the positions of young PMS stars and those of massive OB stars of similar age. Furthermore, the mass distribution of stars with similar age shows large variations throughout the region. We conclude that, while on a global scale it makes sense to talk about an initial mass function, this concept is not meaningful for individual star-forming regions. An interesting implication of the separation between regions where massive stars and low-mass objects appear to form is that high-mass stars might not be "perfect" indicators of star formation and hence a large number of low-mass stars formed elsewhere might have so far remained unnoticed. For certain low surface density galaxies this way of preferential low-mass star formation may be the predominant mechanism, with the consequence that their total mass as derived from the luminosity may be severely underestimated and that their evolution is not correctly understood.