PRE-MAIN-SEQUENCE CANDIDATES IN THE VERY YOUNG OPEN CLUSTER NGC-6611

For the search of Herbig Ae/Be objects in the extremely young open clu ster NGC 6611 we have selected a sample of 52 pre-main sequence candid ates, discovered by Walker (1961), Sagar & Joshi (1979), Chini & Warga u (1990) and The et al. (1990). We continue the approach of the last p aper by studying each star individually with new and unpublished Walra ven WULBV, Johnson/Cousins UBV(RI)(C) and Johnson JHKLM photometric da ta as well as low resolution spectroscopy. Each object is shown to hav e its own extinction law, which is investigated using their spectral e nergy distribution (SED). There does not seem to be a clear relationsh ip between the location of a star and the extinction law. This means t hat the extinction is generated locally and its correction must be tak en individually. For each object accurate astrophysical parameters are then derived. Plotting the objects in an HR-diagram, together with th e values for the E(B - V), the probability of membership value P and t he extinction characteristics, helps to discriminate between cluster m embers and non-cluster members. Most foreground stars are of late spec tral type and are labeled as Group III objects. Group I, to which most members of this cluster belong, contains objects of early spectral ty pe. Part of them seem to be in their post ZAMS phase and the other par t in their pre-ZAMS stage. By comparing the evolutionary tracks of Pal la & Stahler (1993) for pre-MS objects and of Maeder & Meyenet (1988) for post-MS stars we have concluded that the cluster contains objects of a few 0.1 Myr as well as objects of about 6 Myr. As most of the Gro up I objects do not show well-known Herbig Ae/Be characteristics, the time scale of clearing the disk material must be typically less than a bout 0.1 Myr for the more massive objects. Objects that show an IR-exc ess are found among the less luminous ones. They could still be in the ir pre-ZAMS phase, having an age of about 1 Myr. Such an age is approp riate for the Group II objects, which are of intermediate spectral typ e. As they are located close to the stellar birthline they should have been formed recently. It seems that an efficient clearing mechanism m ust have taken place, because not many of these objects are embedded a nd 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 th e hypothesis of Hillenbrand et al. (1993) that there is an age spread in NGC 6611. The stars with the highest Av values are located in the c entre of the cluster, somewhat to the northwest. This coincides with t he location of many embedded sources more towards the northwest, a reg ion in which star-formation is probably still taking place. The age of the most evolved objects is about 6 Myr. It would be interesting to s tudy these regions and the Group II objects since investigating the yo ungest objects which are still partly embedded may help us to understa nd the first phases of star formation and the rapid cleaning of the ci rcumstellar material. Young ''naked'' stars can also be formed by evap orating gaseous globules (EGGs) as was recently discovered in dark reg ions of this cluster (Hester & Scowen 1995). The lack of angular momen tum of such objects could explain the paucity of HAeBe candidates.