Galactic clusters with associated Cepheid variables. I. NGC 6087 AND S Normae

Abstract

New photoelectric UBVphotometry is presented for stars in NGC 6087, the cluster containing the 9[d over dot]75 Cepheid S Normae. The data are used with published photoelectric observations for cluster stars to redetermine the cluster reddening and distance by means of a careful application of the ZAMS fitting technique. The derived distance modulus, V[subscript 0] — M[subscript V] = 9.78 [plus or minus] 0.03 (internal error), [plus or minus] 0.13 (total estimated uncertainty), is shown to be in very good agreement with the cluster distance modulus obtained from Walraven photometry and from the analysis of Strömgren and H[Beta] photometry for cluster stars. However, the hydrogen-line data are found to produce more accurate and precise results if evolutionary corrections are ignored and replaced by due consideration of the cosmic scatter in H[Beta] distance moduli introduced by the effects of stellar rotation. The excellent agreement of the various distance estimates for NGC 6087 presented in this study results from the demonstrably identical zero points shared by the various luminosity calibrations used in the analysis. Internal consistency of this type has tended to be disregarded in some recent analyses of the Cepheid calibration clusters.