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

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1. Introduction

For the last two decades, the young open cluster NGC 6611 has attracted the attention of different researchers. Its large distance, heavy obscuration, rich stellar fore- and background, however, make the observations of this cluster extremely difficult. The first photometric study of NGC 6611 was carried out by Walker (1961) who found massive stars in the cluster color-magnitude diagram (CMD) above the Main Sequence (MS). During the following years, several photometric, spectroscopic, and proper motion surveys (e.g., van Schewick 1962, Kamp 1974, Sagar & Joshi 1979, Tucholke et al. 1986, Thé et al. 1990, Hillenbrand et al. 1993, de Winter et al. 1997) were obtained in this region. The data enabled careful investigations of the brightest population of the cluster, especially, the construction of its luminosity function (LF) and CMD which were used for study of star formation in the cluster. The comparison of Hertzsprung-Russell diagram of the cluster with theoretical models showed that not one single age could be assigned for the whole cluster. The cluster age corresponding to the "oldest" cluster members was determined to be about 5 Myr (see e.g., Hillenbrand et al. 1993, Massey et al. 1995, and de Winter et al. 1997).

Since NGC 6611 is richly populated by early-type stars, the luminosity or mass functions of this cluster can tell us much on the initial mass function (IMF) of the galactic disk stars. In this respect, NGC 6611 seems to be a very peculiar cluster. For the first time, an extremely flat luminosity function of the NGC 6611 stars was derived by Sagar & Joshi (1979). Comparing the cluster CMD with stellar models, Sagar et al. (1986) found that the logarithmic slope [FORMULA] of the cluster mass function (MF) within the range [FORMULA] is about [FORMULA]. A similar value ([FORMULA]) for the same mass range was also determined by Massey et al. (1995) for NGC 6611. However, based on the same data but a slightly different mass calibration, Hillenbrand et al. (1993) found [FORMULA].

From these results, the question arises whether the IMF flattening with decreasing galactocentric distance is a real property of the disk population or if it is rather an artifact coming from the incompleteness of the data due to the large distance of the cluster and its heavy visual obscuration. Since NGC 6611 is one of a few young distant clusters observed towards the Galactic Center, each effort to improve the data base for NGC 6611 can make an important contribution by answering this fundamental question.

An additional reason for our special interest in this cluster can be described as following. As it was found in the previous theoretical investigations by Piskunov & Belikov (1996), the presence of Pre-Main- Sequence (Pre-MS) stars in a cluster affects its luminosity function by producing an additional detail (we call it H-feature) in its LF. The H-feature reveals as a local inhomogeneity in a cluster LF consisting of a bump with a trailing depletion. The H-feature results from a non-uniform behaviour of the mass-luminosity relation in the vicinity of the turn-on point of a cluster's HRD. Since the location of the H-feature on the absolute magnitude axis depends on cluster age, LFs of young clusters can be used for independent age estimation (H-calibration according to Belikov & Piskunov 1997). This feature was found in the luminosity functions published for several young clusters (Belikov & Piskunov 1997) as well as in the Pleiades LF (Belikov et al. 1998, Belikov et al. 1999a). In contrast to the Pleiades, NGC 6611 belongs to the youngest end of the H-calibration. According to the results of the H-calibration, the H-detail should occur at [FORMULA] for a 5 Myr old cluster. Due to the large distance of NGC 6611 (with distance modulus being [FORMULA]), the data available so far were not sufficiently complete to construct a reliable luminosity function down to this limit. The situation was improved with the Compiled Catalogue of photometric and astrometric data in the field of NGC 6611 (Belikov et al. 1999b, Paper I hereafter).

The Compiled Catalogue (CC hereafter) covers a circular area with a diameter of [FORMULA] which is centered at the NGC 6611 cluster. The survey contains astrometric (positions, absolute proper motions) and photometric (photoelectric, CCD, and photographic UBV magnitudes as well as color excesses) data for 2185 stars brighter than [FORMULA]. The catalogue is based on the Tautenburg Schmidt survey supplemented by the relevant data published in this region which were reduced to a common homogeneous system.

Since the CC has a completeness limit which should allow to reach the H-feature in the LF of NGC 6611, the present study is aiming at the following major goals. First, the construction of a confident and deep luminosity function of cluster stars based on a careful analysis of proper motion and photometric membership, as well as by taking into account the cluster structure and distribution of absorption across the cluster. Secondly, the comparison of the empirical and theoretical LFs by considering different assumptions on star formation history of the cluster. Third, the determination of the cluster parameters describing this process i.e., onset and duration of star formation, cluster age, and MF slope.

In Sect. 2 the results on the cluster's structure and stellar content based on the the CC data are presented. In Sect. 3 we use these results for a better selection of cluster members yielding a sample down to [FORMULA]. Sect. 4 describes how the empirical and theoretical luminosity function were constructed for the NGC 6611 cluster. In Sect. 5 we compare these LFs and discuss the results on star formation history of the cluster. The conclusions from this study are presented in Sect. 6.

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Online publication: June 20, 2000