Pre-Main sequence Turn-On as a chronometer for young clusters: NGC346 as
  a benchmark
  We present a novel approach to derive the age of very young star clusters, by
using the Turn-On (TOn). The TOn is the point in the color-magnitude diagram
(CMD) where the pre-main sequence (PMS) joins the main sequence (MS). In the MS
luminosity function (LF) of the cluster, the TOn is identified as a peak
followed by a dip. We propose that by combining the CMD analysis with the
monitoring of the spatial distribution of MS stars it is possible to reliably
identify the TOn in extragalactic star forming regions. Compared to alternative
methods, this technique is complementary to the turn-off dating and avoids the
systematic biases affecting the PMS phase. We describe the method and its
uncertainties, and apply it to the star forming region NGC346, which has been
extensively imaged with the Hubble Space Telescope (HST). This study extends
the LF approach in crowded extragalactic regions and opens the way for future
studies with HST/WFC3, JWST and from the ground with adaptive optics.
