Photometry of star clusters

Abstract

The suitability of the Cassegrain Schmidt telescopes at St. Andrews University Observatory for the measurement of stellar magnitudes and colours by in-focus multicolour photography has been examined. A major requirement is that the photographic plate should coincide with the focal surface. Thermal effects in the Scott Lang Telescope and optical and mechanical problems in the James Gregory Telescope cause difficulty in, attaining this. These difficulties have been overcome in the case of the Scott Lang Telescope but no certain method for focussing the James Gregory Telescope was found. The photometric field limited by field error, is approximately one degree in diameter in each case. Colour equations between the instrumental and standard B, V systems depend on magnitude and, in the case of the Scott Lang Telescope, on exposure time as well. The methods used to measure UBV magnitudes and colours with the Radcliffe 74-inch reflector are described and the accuracy of the results discussed. Magnitudes and colours of stars brighter than V - 15.5 in the open cluster IC 2581 have been measured, together with MK spectral types for a few of the brighter stars. The interstellar absorption provides a criterion for the recognition of cluster members. A discrepancy between the shape of the cluster main sequence and that of the zero age main sequence is attributed to an error in the derivation of the standard zero age main sequence. The cluster is found to be at a distance of 2500 parsecs and may form part of the Carinae complex. The positions of the brightest stars in the colour magnitude diagram are discussed in the light of modern theories of stellar evolution and an age of approximately 10 million years is deduced. The colour magnitude diagram of the open cluster NGC 6383 has been obtained for stars brighter than V - 18.1; the limiting magnitudes in B and U are 19.7 and 17.9, respectively. MK spectral types have permitted the cluster membership of several bright B stars to be established; some stars of later type are non-members. The observation of this cluster are more complete than for most young clusters studied to date, but the poorness of the cluster and the unfavourable distribution of interstellar absorption with distance make it impossible to be certain of the membership of stars fainter than V = 13. The lack of stars fainter than V = 12.8 on the zero age main sequence indicates a contraction age of 5 million years. The distance is 1300 parsecs, like those of other young groups in the vicinity. The dense dust clouds which divide the Milky Way in Scorpius are immediately beyond this. Several faint variable stars may be of the T Tauri type.