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