The star formation region NGC 6530: Distance, ages and Initial Mass Function

Abstract

We present astrometry and BVI photometry, down to V ≃ 22, of the very young open cluster NGC 6530, obtained from observations taken with the Wide Field Imager camera at the MPG/ESO 2.2 m Telescope. Both the V vs. B - V and the V vs. V - I color-magnitude diagrams (CMD) show that the upper main sequence is dominated by very bright cluster stars, while, because of the high obscuration of the giant molecular cloud surrounding the cluster, the blue envelopes of the diagrams at V ≃ 14 are limited to the main sequence stars at the distance of NGC 6530. This particular structure of the NGC 6530 CMD allows us to conclude that its distance is about d ≃ 1250pc, significantly lower than the previous determination of d = 1800pc. We have positionally matched our optical catalog with the list of X-ray sources found in a Chandra-ACIS observation, finding a total of 828 common stars, 90% of which are pre-main sequence stars in NGC 6530. Using evolutionary tracks of Siess et al. (2000), mass and age values are inferred for these stars. The median age of the cluster is about 2.3 Myr; in the mass range (0.6-4.0) M·, the Initial Mass Function (IMF) shows a power law index x = 1.22 ± 0.17, consistent with both the Salpeter index (1.35), and with the index derived for other young clusters; towards smaller masses the IMF shows a peak and then it starts to decrease.