SEARCH FOR LONG-TERM LUMINOSITY CHANGE OF GIANT STARS IN M 3

Abstract

Walker has found1 that the globular cluster giants show small luminosity fluctuations which increase in amplitude with increasing absolute magnitude to a maximum fluctuation of 0.04 mag. (pv) at the bright end of the giant branch. Direct plates of M 3 taken in 1900 with the Crossley reflector are available for comparison with recent Crossley plates in an attempt to detect any significant progressive luminosity changes over this 62-year period. Seven 1900 Crossley plates with images of sufficient photometric quality were selected from a collection of 20 plates of M 3 taken by J. E. Keeler on May 20 and 21, 1900. In each case the plate was centered 10' north of the cluster center. These plates were 10-minute exposures at the Newtonian focus (two silver reflections) on an unknown type of emulsion. The Newtonian focus is no longer equipped for direct photography, hence the 1962 comparison plates were taken of the same field at the prime focus (one aluminum reflection). Two exposures of 3 minutes on Kodak 33 and two exposures of 1.6 minutes on Kodak Ila-O (without filter) were used to approximate the image density and quality of the 1900 plates. The current-epoch Crossley plates were taken on June 26, 1962. Sixty-five stars on the giant and horizontal branches of the cluster were chosen within 16' of the plate center, where the comatic diameter of the image is less than 4.2 No stars were chosen within 3' of the cluster center, where background effects were found to be appreciable. All stars were measured with the Sartorius iris diaphragm astrophotometer. A plot of the results for each star as a function of its distance from the plate center revealed no systematic trend. The astrophotometer readings were plotted as a function of the photographic magnitudes of these