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Astron. Astrophys. 358, 886-896 (2000)

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6. Summary and conclusions

This study was aiming at the construction of the luminosity function and color-magnitude diagram of the young open cluster NGC 6611 and at the analysis of their relation to the star formation history and, especially, to the cluster mass function and age spread in this star-, gas- and dust aggregate.

From the proper motion and photometric data of the Compiled Catalogue published in Paper I, we selected members of NGC 6611 down to [FORMULA] taking into account its general structure and very irregular distribution of absorption across the cluster. This sample is about one order of magnitude larger than the samples previously studied. The observed and theoretical luminosity functions were constructed and compared by taking into account incompleteness of the data at faint magnitudes due to variable absorption. Both log-normal and power-law IMFs were tested and an arbitrary spread of stellar ages in the cluster (at a constant star formation rate) was allowed.

The comparison of the observed and theoretical luminosity functions led us to the following conclusions supported by the analysis of the cluster color-magnitude diagram:

  • a. The obtained data allow to find out the H-maximum of the cluster LF, i.e. a feature in the LF structure which is related to the turn-on point of the cluster CMD and can be used as an independent age indicator of the cluster.

  • b. Within the studied mass range [FORMULA], the power-law IMF is sufficient to represent the mass function of the cluster. A slope of [FORMULA] determined for the mass function of NGC 6611 is in good agreement with the slope assumed for the IMF of field stars.

  • c. The cluster age of 6 Myr derived both from the LF and CMD analysis is compatible with previous estimations. We also confirm a considerable (of the order of the cluster age) spread of individual ages of cluster stars. No evidence of sequential star formation was found from our data.

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© European Southern Observatory (ESO) 2000

Online publication: June 20, 2000