Clues to the star formation in NGC 346 across time and space
  We have studied the properties of the stellar populations in the field of the
NGC 346 cluster in the Small Magellanic Cloud, using the results of a novel
self-consistent method that provides a reliable identification of pre-main
sequence (PMS) objects actively undergoing mass accretion, regardless of their
age. The 680 identified bona-fide PMS stars show a bimodal age distribution,
with two roughly equally numerous populations peaked respectively at ~1 Myr,
and ~20 Myr. We use the age and other physical properties of these PMS stars to
study how star formation has proceeded across time and space in NGC 346. We
find no correlation between the locations of young and old PMS stars, nor do we
find a correspondence between the positions of young PMS stars and those of
massive OB stars of similar age. Furthermore, the mass distribution of stars
with similar age shows large variations throughout the region. We conclude
that, while on a global scale it makes sense to talk about an initial mass
function, this concept is not meaningful for individual star-forming regions.
An interesting implication of the separation between regions where massive
stars and low-mass objects appear to form is that high-mass stars might not be
"perfect" indicators of star formation and hence a large number of low-mass
stars formed elsewhere might have so far remained unnoticed. For certain low
surface density galaxies this way of preferential low-mass star formation may
be the predominant mechanism, with the consequence that their total mass as
derived from the luminosity may be severely underestimated and that their
evolution is not correctly understood.
