Cl* NGC 6231 SBL 380 , the SIMBAD biblio

UBVRI and Hα CCD photometry has been obtained for the young open cluster NGC 6231. From the R - Hα color, a measure of the Hα emission, we find 12 pre-main-sequence (PMS) members and seven possible PMS members in the cluster. Applying recent stellar evolutionary tracks and PMS evolutionary tracks, we derive the age, age spread, and initial mass function of the cluster. The initial mass function of NGC 6231 is a Gaussian with a turnover at log m ∼ 0.4 (m ∼ 2.5 M_☉) and shows an abrupt decrease in the number of low-mass stars. Possible reasons for the suppression of low-mass star formation and star formation processes are discussed.