POPULATION TYPES AND THE MAGELLANIC CLOUDS

Abstract

According to W. Baade,1 the characteristics of population I include the presence of interstellar dust and of classical cepheids, and these occur together or not at all. H. Shapley's2 recent announcement, that because galaxies can be photographed through it, the Small Magellanic Cloud must be effectively transparent, seems therefore inconsistent with the presence in the Cloud of so many classical cepheids. The purpose of this report is to show that recent photoelectric observations of the Clouds remove this inconsistency, since they confirm that the Large Magellanic Cloud is a member of type I, and suggest that the Small Cloud is predominantly of type II because the cepheids in it are related more to the W Virginis variables than to the classical galactic cepheids. The observations comprise fairly complete two-color lightcurves for eight Small Cloud variables, obtained during the latter part of 1951 with a refrigerated RCA-type C-7073B multiplier attached to the Reynolds reflecting telescope at Mount Stromlo, Australia. A summary of the data is given in Table I, and the features essential to the present discussion the amplitudes and the colors at maximum are plotted as open circles in Figures 1 and 2. Through the courtesy of O. J. Eggen we have been able to include in the table his unpublished data for the three type II galactic variables W Vir, AL Vir, and ti Pav ; these points are plotted as filled circles. Also in the figures are Eggen's3 data for the classical galactic cepheids. The full lines refer to his types A and B, to which twenty-four of his thirty-two stars belong, and the crosses to the six members of his type C (the remaining two stars are anomalous). All data, including Eggen's, have been reduced to the P,V System.4 We have also determined the colors of the Clouds themselves