We observed the Sh 2-233IR ( S233IR ) region with better sensitivity in near-infrared than previous studies for 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 be occurring sequentially with a relatively small age difference ( \sim 0.2 - 0.3 Myrs ) between subclusters .
We found that the slopes for initial mass function ( \Gamma \sim - 0.5 ) of two subclusters are flatter than that 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 ( \gtrsim 40 % ) of the subclusters may also suggest that stars form very efficiently in the center of NE .