Two distinct red giant branches in the globular cluster NGC288

Abstract

We report the presence of two distinct red giant branches (RGBs) in the globular cluster NGC288 from the narrowband calcium and Strömgren b and y photometry obtained at the CTIO 4m Blanco telescope. The RGB of NGC288 is clearly split into two in the hk [=(Ca - b) - (b - y)] index, while the split is not shown in the b - y color. Unlike other globular clusters with multiple populations reported thus far, the horizontal branch of NGC288 is only mildly extended. Our stellar population models show that this and the presence of two distinct RGBs in NGC288 can be reproduced if slightly metal-rich (Δ[m/H] 0.16) second generation stars are also enhanced in helium by small amount (ΔY 0.03) and younger by 1.5 Gyr. The RGB split in the hk index is most likely indicating that the second generation stars were affected by supernovae enrichment, together with the pollution of lighter elements by intermediate-mass asymptotic giant branch stars or fast-rotating massive stars. In order to confirm this, however, spectroscopy of stars in the two distinct RGB groups is urgently required. © 2011. The American Astronomical Society. All rights reserved.