We present astrometry and BVI photometry , down to V \simeq 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 the upper main sequence dominated by very bright cluster stars , while , due to the high obscuration of the giant molecular cloud surrounding the cluster , the blue envelopes of the diagrams at V \gtrsim 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 \simeq 1250 pc , significantly lower than the previous determination of d=1800 pc . 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 , 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 _ { \odot } , the Initial Mass Function ( IMF ) shows a power law index x = 1.22 \pm 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 .