We report the presence of two distinct red giant branches ( RGBs ) in the globular cluster NGC 288 from the narrow-band calcium and Strömgren b & y photometry obtained at the CTIO 4m Blanco telescope .
The RGB of NGC 288 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 NGC 288 is only mildly extended .
Our stellar population models show that this and the presence of two distinct RGBs in NGC 288 can be reproduced if slightly metal-rich ( \Delta [ m / H ] \approx 0.16 ) second generation stars are also enhanced in helium by small amount ( \Delta Y \approx 0.03 ) and younger by \sim 1.5 Gyrs .
The RGB split in hk index is most likely indicating that the second generation stars were affected by supernovae enrichment , together with the pollutions 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 .