1997A&AS..121..223D

For the search of Herbig Ae/Be objects in the extremely young open cluster NGC 6611 we have selected a sample of 52 pre-main sequence candidates, discovered by Walker (1961ApJ...133..438W), Sagar & Joshi (1979Ap&SS..66....3S), Chini & Wargau (1990A&A...227..213C) and The et al. (1990A&AS...82..319T). We continue the approach of the last paper by studying each star individually with new and unpublished Walraven WULBV, Johnson/Cousins UBV(RI)_C and Johnson JHKLM photometric data as well as low resolution spectroscopy. Each object is shown to have its own extinction law, which is investigated using their spectral energy distribution (SED). There does not seem to be a clear relationship between the location of a star and the extinction law. This means that the extinction is generated locally and its correction must be taken individually. For each object accurate astrophysical parameters are then derived. Plotting the objects in an HR-diagram, together with the values for the E(B-V), the probability of membership value P and the extinction characteristics, helps to discriminate between cluster members and non-cluster members. Most foreground stars are of late spectral type and are labeled as Group III objects. Group I, to which most members of this cluster belong, contains objects of early spectral type. Part of them seem to be in their post-ZAMS phase and the other part in their pre-ZAMS stage. By comparing the evolutionary tracks of Palla & Stahler (1993ApJ...418..414P) for pre-MS objects and of Maeder & Meyenet (1988A&AS...76..411M) for post-MS stars we have concluded that the cluster contains objects of a few 0.1Myr as well as objects of about 6Myr. As most of the Group I objects do not show well-known Herbig Ae/Be characteristics, the time scale of clearing the disk material must be typically less than about 0.1Myr for the more massive objects. Objects that show an IR-excess are found among the less luminous ones. They could still be in their pre-ZAMS phase, having an age of about 1Myr. Such an age is appropriate for the Group II objects, which are of intermediate spectral type. As they are located close to the stellar birthline they should have been formed recently. It seems that an efficient clearing mechanism must have taken place, because not many of these objects are embedded and show only some IR-anomalies. This could be the reason that we have found no more than four typical Herbig Ae/Be candidates. We support the hypothesis of Hillenbrand et al. (1993AJ....106.1906H) that there is an age spread in NGC 6611. The stars with the highest A_V values are located in the centre of the cluster, somewhat to the northwest. This coincides with the location of many embedded sources more towards the northwest, a region in which star-formation is probably still taking place. The age of the most evolved objects is about 6Myr. It would be interesting to study these regions and the Group II objects since investigating the youngest objects which are still partly embedded may help us to understand the first phases of star formation and the rapid cleaning of the circumstellar material. Young ``naked'' stars can also be formed by evaporating gaseous globules (EGGs) as was recently discovered in dark regions of this cluster Hester & Scowen (1995). The lack of angular momentum of such objects could explain the paucity of HAeBe candidates.