| Abstract |
We discuss the star formation history of the Small Magellanic Cloud
(SMC) region NGC 346 based on Hubble Space Telescope images. The region
contains both field stars and cluster members. Using a classical
synthetic color-magnitude diagram (CMD) procedure applied to the field
around NGC 346 we find that the star formation pace there has been
rising, from a quite low rate 13 Gyr ago to ≈1.4 ×
10–8 M sun yr-1
pc–2 in the last 100 Myr. This value is significantly
higher than that in other star-forming regions of the SMC. For NGC 346
itself, we compare theoretical and observed 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 a high rate (up to 2
× 10–5 M sun yr-1
pc–2) for about 3 Myr and is now progressing at a lower
rate. Interestingly, sub-clusters mainly composed of low-mass PMS stars
now seem to experience 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 underthreshold conditions of
the parent gas and the second speculates that the initial mass function
is a function of time, with the youngest sub-clusters not having had
sufficient time to form more massive stars.
Based on observations with the NASA/ESA Hubble Space Telescope, obtained
at the Space Telescope Science Institute, which is operated by the
Association of Universities for Research in Astronomy (AURA), Inc.,
under NASA contract NAS5-26555. These observations are associated with
program GO10248. |