EDP Sciences
Free Access
Volume 430, Number 3, February II 2005
Page(s) 941 - 957
Section Galactic structure, stellar clusters and populations
DOI https://doi.org/10.1051/0004-6361:20040432

A&A 430, 941-957 (2005)
DOI: 10.1051/0004-6361:20040432

The star formation region NGC 6530: Distance, ages and initial mass function

L. Prisinzano1, 2, F. Damiani2, G. Micela2 and S. Sciortino2

1  Dipartimento di Scienze Fisiche ed Astronomiche, Università di Palermo, Piazza del Parlamento 1, 90134 Palermo, Italy
    e-mail: loredana@astropa.unipa.it
2  INAF - Osservatorio Astronomico di Palermo, Piazza del Parlamento 1, 90134 Palermo, Italy

(Received 12 March 2004 / Accepted 28 September 2004)

We present astrometry and BVI photometry, down to $V\simeq22$, 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 $V\gtrsim 14$ 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 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) $\,M_\odot$, the Initial Mass Function (IMF) shows a power law index $x=1.22\pm0.17$, 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.

Key words: Galaxy: open clusters and association: individual: NGC 6530 -- techniques: photometric -- astrometry -- stars: pre-main sequence stars -- stars: Hertzprung-Russel (HR) and C-M diagrams

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