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We discuss the star formation history of the SMC region NGC 346 based onHubble Space Telescope images. The region contains both field stars and clustermembers. Using a classical synthetic CMD procedure applied to the field aroundNGC 346 we find that there the star formation pace has been rising from a quitelow rate 13 Gyr ago to \approx 1.4 \times 10^{-8} Mo yr^{-1}pc^{-2} in the last100 Myr. This value is significantly higher than in other star forming regionsof the SMC. For NGC 346 itself, we compare theoretical and observedColor-Magnitude Diagrams (CMDs) of several stellar sub-clusters identified inthe region, and we derive their basic evolution parameters. We find that NGC346 experienced different star formation regimes, including a dominant andfocused "high density mode", with the sub-clusters hosting both pre-mainsequence (PMS) and upper main sequence (UMS) stars, and a diffuse "low densitymode", as indicated by the presence of low-mass PMS sub-clusters.Quantitatively, the star formation in the oldest sub-clusters started about 6Myr 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 alower rate. Interestingly, sub-clusters mainly composed by low mass PMS starsseem to experience now the first episode of star formation, followingmulti-seeded spatial patterns instead of resulting from a coherent trigger. Twospeculative scenarios are put forth to explain the deficiency of UMS stars: thefirst invokes under-threshold conditions of the parent gas; the secondspeculates that the initial mass function (IMF) is a function of time, with theyoungest sub-clusters not having had sufficient time to form more massivestars.

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