Berkeley 87, a heavily-obscured young cluster associated with the ON2 star-formation complex and containing the WO star Stephenson 3

Abstract

UBV photometry is given for 39 photoelectrically observed and 66 photographically observed stars lying in the open cluster Berkeley 87. A cluster distance of 946 + or - 26 pc and an evolutionary age of about one to two million years, are derived from star counts delineating cluster boundaries and from the ZAMS fitting for faint cluster stars and MK spectral types for four bright members. A physical association is suggested for Berkeley 87 with the molecular clouds, compact H II regions and maser sources belonging to the ON2 star formation complex. The nature of the reddening and obscuration within the cluster can be used to demonstrate that Berkeley 87 is located on the western edge of one of the giant molecular clouds belonging to the Cygnus X complex. The BC Cyg and HDE 229059 supergiants are found to be likely members of the cluster corona.