A study of two open clusters containing Wolf-Rayet stars

Abstract

The results of UBV CCD photometry are presented for a newly discovered open cluster, as well as new photometry for thirty-seven members of the open cluster HM 1. The new open cluster, to be designated OCL 1104-610, has a distance modulus of V[subscript o]-M[subscript v]= 15.5[plus-minus sign]0.2, corresponding to a distance of 12.6[plus-minius sign][1.2 over 1.1] kpc, and is several Myr old. Members of the distant cluster include two Wolf-Rayet (WR) stars, WR 38 and WR 38a. WR 38 is the first WC4 star identified in an open cluster, and has been found to have an absolute magnitude M[subscript v] = -5.1, two magnitudes brighter than has been suspected for WC4 stars. WR 38a has an absolute magnitude M[subscript v] = -4.4, in agreement with other WN6 stars. Intrinsic colours of both WR stars are uncertain given the faintness of cluster members. HM 1 has been determined to have a distance modulus of V[subscript o]-M[subscript v] = 13.19[plus-minus sign]0.16, corresponding to a distance of 4.34[plus-minus sign] [0.33 over 0.31] kpc. Its WR members, WR 87 and WR 89, have been found to be intrinsically slightly brighter and bluer than on average for WN7 stars, but not unreasonably so. In addition, photometry is presented for a third group of stars which possibly comprise an open cluster. The third cluster, to be designated OCL 1103-610, lies at a distance of V[subscript o] - M[subscript v] = 12.02 [plus-minus sign] 0.06 and is 77 Myr old.