History and modes of star formation in the most active region of the Small Magellanic Cloud, NGC 346

TL;DR

This study investigates the detailed star formation history and modes in the active NGC 346 region of the Small Magellanic Cloud, revealing multiple star formation regimes and proposing explanations for observed stellar populations.

Contribution

It provides a comprehensive analysis of star formation modes and history in NGC 346, including the identification of different regimes and the proposal of two scenarios for the lack of upper main sequence stars.

Findings

Star formation rate has increased over 13 Gyr, reaching high levels in the last 100 Myr.

NGC 346 experienced a dominant high-density star formation mode and a diffuse low-density mode.

Star formation in sub-clusters started about 6 Myr ago, with recent episodes involving mainly low-mass PMS stars.

Abstract

We discuss the star formation history of the SMC region NGC 346 based on Hubble Space Telescope images. The region contains both field stars and cluster members. Using a classical synthetic CMD procedure applied to the field around NGC 346 we find that there the star formation pace has been rising from a quite low rate 13 Gyr ago to \approx 1.4 \times 10^{-8} Mo yr^{-1}pc^{-2} in the last 100 Myr. This value is significantly higher than in other star forming regions of the SMC. For NGC 346 itself, we compare theoretical and observed Color-Magnitude Diagrams (CMDs) of several stellar sub-clusters identified in the region, and we derive their basic evolution parameters. We find that NGC 346 experienced different star formation regimes, including a dominant and focused "high density mode", with the sub-clusters hosting both pre-main sequence (PMS) and upper main sequence (UMS) stars, and a diffuse "low density mode", as indicated by the presence of low-mass PMS sub-clusters. Quantitatively, the star formation in the oldest sub-clusters started about 6 Myr ago with remarkable synchronization, it continued at high rate (up to 2 \times 10^{-5} Mo yr^{-1} pc^{-2}) for about 3 Myr and is now progressing at a lower rate. Interestingly, sub-clusters mainly composed by low mass PMS stars seem to experience now the first episode of star formation, following multi-seeded spatial patterns instead of resulting from a coherent trigger. Two speculative scenarios are put forth to explain the deficiency of UMS stars: the first invokes under-threshold conditions of the parent gas; the second speculates that the initial mass function (IMF) is a function of time, with the youngest sub-clusters not having had sufficient time to form more massive stars.