U BV I imaging photometry of NGC 6231

Abstract

CCD UBVI photometry in the field of the open cluster NGC 6231 was obtained for 1060 stars down to V ≈ 19 mag. Memberships, reddening, distance and age of this cluster were investigated. Its lower sequence displays a notorious bend at V ≈ 13.5 mag followed by a high number of faint stars showing a large magnitude spread at constant colour. This distribution of stars does not seem to be produced by a mere accumulation of field stars seen in the direction of the cluster but by a real star excess in the zone. The evidence suggests these stars are serious candidates to be cluster members caught in their way towards the ZAMS. Fitting them with pre main sequence isochrones an age spread of about 10 Myr is obtained. The luminosity and mass functions were found to be flat for -7.5 < MV < 1.5 and from 3 to 80 M⊙ respectively.