The Electronographic Color-Magnitude Diagram of Hodge 11 [chapter]

J. Andersen, A. Blecha, M. F. Walker
1984 Structure and Evolution of the Magellanic Clouds  
A program of electronographic photometry of star clusters in the Magellanic Clouds is being carried out using the McMullan camera on the Danish 1.5-m telescope at La Silla. The observations are being reduced using the Geneva microdensitometer and data reduction programs. Results are shown for the LMC cluster Hodge 11 (=SL 868), based on measurements of two exposures in both yellow and blue-light, each three hours in length, recorded on Ilford L4 emulsion. In the reductions, areas of 250x250p,
more » ... reas of 250x250p, 340x340p or 380x380y centered on each star were raster-scanned with a scanning aperture of 14 or 20y. X-Y raster steps of 12, 16 or 18p were used, producing a 21x21-point matrix. For each area, the local sky background level was determined by fitting a third-degree polynomial. Standard star profiles were deter mined for each film using images of bright stars, fitted to modified Gaussian profiles having three shape parameters allowing for seeing and image elongation and orientation. A multiple-peak profile, capable of resolving up to nine stars per scan-area, was then fitted to each subfield scan and the volume of the density solid of each star image found by numerical integration. Measures were made of 413 sub-fields in and near the cluster. Volumes of the density solids of the measured stars were converted to the BV systems using a least-squares fit to the previous electronogra phic photometry by Walker (1979) for those stars of V < 19.50 in yellow and B < 20.00 in blue light. Analysis of other data indicates no signi ficant divergence of the McMullan camera color system from B-V colors; color errors are < + 0.02 B-V/mag. Eliminating double stars and stars in badly crowded regions and considering only the 108 uncrowded stars measured on all four exposures and having radial distances of 38" <_ R £ 80" from the cluster center, the color-magnitude diagram shown in Figure 1 was obtained. This diagram, extending to V -22, indicates that Hodge 11 is an old, metal-poor globular cluster, similar to M 92 and thus having [Fe/H] --2.1, in agreement with the integral photometry by Danziger (1973) and by Searle, et al. (1980) . Fitting the horizontal branch of Hodge 11 to that of M 92 and assuming E R _ V = 0.06, the 41 S. van den Bergh and K. S. de Boer (eds.J, Structure and Evolution of the Magellanic Clouds, 41-42.
doi:10.1007/978-94-010-9801-4_12 fatcat:jcnuobldqzabjibucplmjcqkvi