Chemical Inhomogeneity in Red Giant Branch Stars and Rr Lyrae Variables in NGC 1851: Two Subpopulations in Red Giant Branch

We investigate the red giant branch (RGB) subpopulations of NGC 1851 using Ca uvby photometry. Our color–magnitude diagrams show that the RGB stars have two subpopulations and the RGB stars in NGC 1851 appear to have distinct elemental abundance patterns with the [Ca/H] abundance. We discuss that the elemental abundance patterns can be explained by the contributions from the asymptotic giant stars, confirming the previous studies by others. The RR Lyrae variables in NGC 1851 appear to have a large metallicity spread and, perhaps, a bimodal metallicity distribution. Our period shift analysis of the RR Lyrae variables shows that the helium enhancement appears to be insignificant in the NGC 1851 RR Lyrae population. However, the helium enrichment scenario in the blue or the red parts of horizontal branch (HB) cannot be completely ruled out. Our results show that about 18% of the bright RGB stars have enhanced CNO abundances, sharply in contrast to previous estimates by others (≈38%), making it difficult to explain the double subgiant, RGB sequences, and the bimodal HB distribution by their number ratios.