Jong-Myung Joo

Multiple stellar populations in the globular cluster omega Centauri as tracers of a merger event

The discovery of the Sagittarius dwarf galaxy, which is being tidally disrupted by and merging with the Milky Way, supports the view that the halo of the Galaxy has been built up at least partially by the accretion of similar dwarf systems. The Sagittarius dwarf contains several distinct populations of stars, and includes M54 as its nucleus, which is the second most massive globular cluster associated with the Milky Way. The most massive globular cluster is ω Centauri, and here we report that ω Centauri also has several distinct stellar populations, as traced by red-giant-branch stars. The most metal-rich red-giant-branch stars are about 2 Gyr younger than the dominant metal-poor component, indicating that ω Centauri was enriched over this timescale. The presence of more than one epoch of star formation in a globular cluster is quite surprising, and suggests that ω Centauri was once part of a more massive system that merged with the Milky Way, as the Sagittarius dwarf galaxy is in the process of doing now. Mergers probably were much more frequent in the early history of the Galaxy and ω Centauri appears to be a relict of this era.

CCD Photometry of the Globular Cluster ω Centauri. I. Metallicity of RR Lyrae Stars from Caby Photometry

We present new measurements of the metallicity of 131 RR Lyrae stars in the globular cluster ω Centauri, using the hk index of the Caby photometric system. The hk method has distinct advantages over ΔS and other techniques in determining the metallicity of RR Lyrae stars and has allowed us to obtain the most complete and homogeneous metallicity data to date for the RR Lyrae stars in this cluster. For RR Lyrae stars in common with the ΔS observations of Butler, Dickens, &Epps and Gratton, Tornambe, &Ortolani, we have found that our metallicities, [Fe/H]hk, deviate systematically from their ΔS metallicity, while our [Fe/H]hk for well-observed, field RRab stars are consistent with previous spectroscopic measurements. We conclude that this is because of the larger errors associated with the previous ΔS observations for this cluster. The MV(RR)-[Fe/H] and period shift–[Fe/H] relations obtained from our new data are consistent with the evolutionary models predicted by Y.-W. Lee, confirming that the luminosity of RR Lyrae stars depends on evolutionary status as well as metallicity. Using the period-amplitude diagram, we have also identified highly evolved RRab stars in the range - 1.9 ≤ [Fe/H] < -1.5, as predicted from the synthetic horizontal-branch models.