2010ApJ...720....1Y

We observed the Sh 2-233IR (S233IR) region with better sensitivity in the near-infrared than in previous studies of this region. By applying statistical subtraction of the background stars, we identified member sources and derived the age and mass of three distinguishable sub-groups in this region: Sh 2-233IR NE, Sh 2-233IR SW, and the "distributed stars" over the whole cloud. Star formation may occur sequentially with a relatively small age difference (∼0.2-0.3 Myr) between subclusters. We found that the slopes for the initial mass function (Γ ∼ -0.5) of two subclusters are flatter than those of Salpeter, which suggests that more massive stars were preferentially formed in those clusters compared to other Galactic star-forming regions. These subclusters may not result from the overall collapse of the whole cloud, but have formed by triggering before the previous star formation activities disturbed the natal molecular cloud. Additionally, high star formation efficiency (≳40%) of the subclusters may also suggest that stars form very efficiently in the center of the northeast.