Two Distinct Red Giant Branches in the Globular Cluster NGC 288

TL;DR

This study reveals two distinct red giant branches in NGC 288 using narrow-band photometry, indicating multiple stellar populations with slight metallicity and helium differences, suggesting complex formation history.

Contribution

It demonstrates the presence of two RGBs in NGC 288 through specific photometric indices, linking them to different stellar populations with distinct chemical enrichment histories.

Findings

Two RGBs are visible in the hk index but not in b - y color.

Second generation stars are slightly more metal-rich and helium-enhanced.

The RGB split suggests supernovae and AGB star pollution effects.

Abstract

We report the presence of two distinct red giant branches (RGBs) in the globular cluster NGC 288 from the narrow-band calcium and Str\"{o}mgren 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 ~ 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.