Luminosity function and star formation history

The stellar content of the young open cluster NGC 6611 was studied on the basis of the data of the Compiled Catalogue published in Belikov et al. (1999b). Using photomet- ric and proper motion criteria as well as taking into account the cluster structure, we selected 376 probable members down to MV 3m (a limit never reached before) from 2185 cata- logue stars. The observed luminosity function was constructed and compared with the theoretical luminosity function com- puted with the log-normal and power-law IMFs and under the assumption of a non-coeval star formation. From this compar- ison, the following main conclusions can be drawn, also sup- ported by the analysis of the cluster color-magnitude diagram: (1) the expected H-feature due to the Pre-Main-Sequence - Main-Sequence transition is marginally present in the observed luminosity function at MV 1m (i.e., at the supposed turn-on point of the cluster's CMD); (2) in the mass range (2.1, 85) m the mass function of cluster stars could be better represented by a power-law IMF with a logarithmic slope = 1:2 than by a log-normal IMF; (3) a cluster age of 6 Myr derived both from the LF and CMD analysis coincides well with present-day find- ings. A considerable (of the order of the cluster age) spread of individual ages of cluster stars was confirmed, but no evidence of a sequential star formation was found from the data.