Hubble space telescope reveals multiple sub-giant branch in eight globular clusters

Abstract

In the last few years many globular clusters (GCs) have revealed complex color-magnitude diagrams, with the presence of multiple main sequences (MSs), broad or multiple sub-giant branches (SGBs) and MS turnoffs, and broad or split red giant branches (RGBs). After a careful correction for differential reddening, high-accuracy photometry with the Hubble Space Telescope (HST) presented in this paper reveals a broadened or even split SGB in five additional Milky Way GCs: NGC362, NGC5286, NGC6656, NGC6715, and NGC7089. In addition, we confirm (with new and archival HST data) the presence of a split SGB in 47 Tuc, NGC1851, and NGC6388. The fraction of faint SGB stars with respect to the entire SGB population varies from one cluster to another and ranges from 0.03 for NGC362 to 0.50 for NGC6715. The average magnitude difference between the bright SGB and the faint SGB is almost the same at different wavelengths. This peculiarity is consistent with the presence of two groups of stars with either an age difference of about 1-2Gyr or a significant difference in their overall C+N+O content. Show more