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.Comment: 17 pages. Accepted for publication in A
