Star formation in NGC 6531 - Evidence from the age spread and initial mass function

The results of a photometric UBV study of the young open cluster NGC 6531 are presented. The cluster is found to have a mean reddening E(B-V)=0.28+/-0.04 (s.d.) and distance modulus (V-0-M(V))=10.70+/-0.13 (s.e.), and 105+/-11 likely cluster members have been identified within the cluster coronal radius of 9 arcmin. A comparison of the high-luminosity end of the cluster color-magnitude diagram to the evolutionary models by Maeder & Meynet [A&AS, 76, 411 (1988)] suggests a nuclear age of (8+/-2) Myr. The very clear gap in the distribution of stars with 0 less than or equal to(B-V)(0) less than or equal to 0.20, corresponding to the ''burn-off' of He-3 in stars contracting to the main sequence [Ulrich, ApJ, 168, 57 (1971)], implies a contraction age of (8+/-3) Myr. There would seem to be no evidence of a spread in the ages of cluster stars, as has been observed in several other young open clusters [Herbst & Miller, AJ, 87, 1478 (1982)]. The initial mass function (IMF) constructed from the cluster luminosity function and the mass-luminosity relation given by Scale (1986) shows good agreement with the field star IMF, and with the IMFs of a number of clusters of similar age and richness. The relative deficiency of low-mass stars seen by Herbst and Miller in NGC 3293 (a cluster of quite similar age and reddening) is not evident in NGC 6531. (C) 1996 American Astronomical Society.