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

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The extremely young open cluster NGC 6611

Luminosity function and star formation history

A.N. Belikov 1, N.V. Kharchenko 2, A.E. Piskunov 1 and E. Schilbach 3

1 Institute of Astronomy of the Russian Academy of Sciences, 48 Pyatnitskaya street, Moscow 109017, Russia (abelikov@inasan.rssi.ru; piskunov@inasan.rssi.ru)
2 Main Astronomical Observatory, Golosiiv, 252127 Kiev, Ukraine (nkhar@mao.kiev.ua)
3 Astrophysikalisches Institut Potsdam, An der Sternwarte 16, 14482 Potsdam, Germany (eschilbach@aip.de)

Received 4 June 1999 / Accepted 29 November 1999


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 photometric and proper motion criteria as well as taking into account the cluster structure, we selected 376 probable members down to [FORMULA] (a limit never reached before) from 2185 catalogue stars. The observed luminosity function was constructed and compared with the theoretical luminosity function computed with the log-normal and power-law IMFs and under the assumption of a non-coeval star formation. From this comparison, the following main conclusions can be drawn, also supported 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 [FORMULA] (i.e., at the supposed turn-on point of the cluster's CMD); (2) in the mass range [2.1, 85] [FORMULA] the mass function of cluster stars could be better represented by a power-law IMF with a logarithmic slope [FORMULA] 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 findings. 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.

Key words: Galaxy: open clusters and associations: individual: NGC 6611 – stars: luminosity function, mass function – stars: pre-main sequence

Send offprint requests to: E. Schilbach

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© European Southern Observatory (ESO) 2000

Online publication: June 20, 2000