The Cluster Membership of Objects Lying above the Main Sequence in NGC 6530.

Abstract

view Abstract Citations References Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS The Cluster Membership of Objects Lying above the Main Sequence in NGC 6530. The, Pie Sin ; Blanco, V. M. Abstract NGC 6530 according to Walker (1957) shows a normal main sequence up to visual magnitude 12.0. Fainter stars, presumably contracting objects, fall above the zero-age main sequence as defined by the brighter stars. However, Wallenquist (1940) has found that these faint stars show no clustering, which suggests that they are field stars. To examine this further, colors and magnitudes within 1 of the cluster were determined photograpbically for 3000 stars. Walker's sequence and one in NGC 6531 were used. Two or three plates were used in each magnitude determination. The probable error of a magnitude is 0.12, which although high does not affect our conclusions. Checks for systematic errors show these to be negligible. The stars were divided into five groups located in areas near NGC 6530 showing strong absorption and in a nearby clear region. In all cases the diagrams show stars above the main sequence. The numbers of such stars observed/unit area are comparable with Walker's. The surface distributions of stars of different magnitudes near NGC 6530 show a definite clustering only for stars brighter than 12.5, in agreement with Wallenquist's results. Wolf diagrams constructed from the magnitude distributions in the dark cloud that forms a backdrop to M 8 and in a nearby clear region gave an estimate for the cloud's distance of 1.3 kpc. The total absorption of the dark cloud is 1.7 magnitudes. This is sufficiently low to permit the observation of background stars in appreciable numbers. The number of such stars expected to fall above the NGC 6530 main sequence is in rough agreement with observations. It appears from this study that in the case of NGC 6530 at least the majority of Walker's stars are highly reddened background stars of early type. Publication: The Astronomical Journal Pub Date: 1960 DOI: 10.1086/108108 Bibcode: 1960AJ.....65R..57T full text sources ADS |