Sohee Jang

TWO DISTINCT RED GIANT BRANCH POPULATIONS IN THE GLOBULAR CLUSTER NGC 2419 AS TRACERS OF A MERGER EVENT IN THE MILKY WAY

Recent spectroscopic observations of the outer halo globular cluster (GC) NGC 2419 show that it is unique among GCs, in terms of chemical abundance patterns, and some suggest that it was originated in the nucleus of a dwarf galaxy. Here we show, from the Subaru narrowband photometry employing a calcium filter, that the red giant branch (RGB) of this GC is split into two distinct subpopulations. Comparison with spectroscopy has confirmed that the redder RGB stars in the hk[=(Ca–b) – (b – y)] index are enhanced in [Ca/H] by ~0.2 dex compared to the bluer RGB stars. Our population model further indicates that the calcium-rich second generation stars are also enhanced in helium abundance by a large amount (ΔY = 0.19). Our photometry, together with the results for other massive GCs (e.g., ω Cen, M22, and NGC 1851), suggests that the discrete distribution of RGB stars in the hk index might be a universal characteristic of this growing group of peculiar GCs. The planned narrowband calcium photometry for the Local Group dwarf galaxies would help to establish an empirical connection between these GCs and the primordial building blocks in the hierarchical merging paradigm of galaxy formation.

Multiple populations in globular clusters and the origin of the Oosterhoff period groups

The presence of multiple populations is now well-established in most globular clusters in the Milky Way. In light of this progress, here we suggest a new model explaining the origin of the Sandage period-shift and the difference in mean period of type ab RR Lyrae variables between the two Oosterhoff groups. In our models, the instability strip in the metal-poor group II clusters, such as M15, is populated by second generation stars (G2) with enhanced helium and CNO abundances, while the RR Lyraes in the relatively metal rich group I clusters like M3 are mostly produced by first generation stars (G1) without these enhancements. This population shift within the instability strip with metallicity can create the observed period-shift between the two groups, since both helium and CNO abundances play a role in increasing the period of RR Lyrae variables. The presence of more metal-rich clusters having Oosterhoff-intermediate characteristics, such as NGC 1851, as well as of most metal-rich clusters having RR Lyraes with longest periods (group III) can also be reproduced, as more helium-rich third and later generations of stars (G3) penetrate into the instability strip with further increase in metallicity. Therefore, for the most general cases, our models predict that the RR Lyraes are produced mostly by G1, G2, and G3, respectively, for the Oosterhoff groups I, II, and III.

STAR FORMATION HISTORY OF THE MILKY WAY HALO TRACED BY THE OOSTERHOFF DICHOTOMY AMONG GLOBULAR CLUSTERS

In our recent investigation of the Oosterhoff dichotomy in the multiple population paradigm, we have suggested that the RR Lyrae variables in the globular clusters (GCs) of Oosterhoff groups I, II, and III are produced mostly by first, second, and third generation stars (G1, G2, and G3), respectively. Here we show, for the first time, that the observed dichotomies in the inner and outer halo GCs can be naturally reproduced when these models are extended to all metallicity regimes, while maintaining reasonable agreements in the horizontal-branch type versus [Fe/H] correlations. In order to achieve this, however, specific star formation histories are required for the inner and outer halos. In the inner halo GCs, the star formation commenced and ceased earlier with a relatively short formation timescale between the subpopulations (?0.5 Gyr), while in the outer halo, the formation of G1 was delayed by ?0.8 Gyr with a more extended timescale between G1 and G2 (?1.4 Gyr). This is consistent with the dual origin of the Milky Way halo. Despite the difference in detail, our models show that the Oosterhoff period groups observed in both outer and inner halo GCs are all manifestations of the ?population-shift? effect within the instability strip, for which the origin can be traced back to the two or three discrete episodes of star formation in GCs.

COMMON ORIGIN OF TWO RR LYRAE POPULATIONS AND THE DOUBLE RED CLUMP IN THE MILKY WAY BULGE

A recent survey toward the Milky Way bulge has discovered two sequences of RR Lyrae stars on the period-amplitude diagram with a maximum period-shift of {\Delta}log P = 0.015 between the two populations. Here we show, from our synthetic horizontal-branch models, that this period-shift is most likely due to the small difference in helium abundance ({\Delta}Y = 0.012) between the first and second-generation stars (G1 and G2), as is the case in our models for the inner halo globular clusters with similar metallicity ([Fe/H] = -1.1). We further show that the observed double red clump (RC) in the bulge is naturally reproduced when these models are extended to solar metallicity following {\Delta}Y/{\Delta}Z = 6 for G2, as would be expected from the chemical evolution models. Therefore, the two populations of RR Lyrae stars and the double RC observed in the bulge appear to be different manifestations of the same multiple population phenomenon in the metal-poor and metal-rich regimes respectively.

Assembling the Milky Way Bulge from Globular Clusters: Evidence from the Double Red Clump

The two red clumps (RCs) observed in the color-magnitude diagram of the Milky Way bulge is widely accepted as evidence for an X-shaped structure originated from the bar instability. A drastically different interpretation has been suggested, however, based on the He-enhanced multiple stellar population phenomenon as is observed in globular clusters (GCs). Because these two scenarios imply very different pictures on the formation of the bulge and elliptical galaxies, understanding the origin of the double RC is of crucial importance. Here we report our discovery that the stars in the two RCs show a significant (> 5.3 {\sigma}) difference in CN-band strength, in stark contrast to that expected in the X-shaped bulge scenario. The difference in CN abundance and the population ratio between the two RCs are comparable to those observed in GCs between the first- and later generation stars. Since CN-strong stars trace a population with enhanced N, Na, and He abundances originated in GCs, this is direct evidence that the double RC is due to the multiple population phenomenon, and that a significant population of stars in the Milky Way bulge were assembled from disrupted proto-GCs. Our result also calls for the major revision of the 3D structure of the Milky Way bulge given that the current view is based on the previous interpretation of the double RC phenomenon.

THE OOSTERHOFF PERIOD GROUPS AND MULTIPLE POPULATIONS IN GLOBULAR CLUSTERS

One of the long-standing problems in modern astronomy is the curious division of globular clusters (GCs) into two groups, according to the mean period (〈Pab〉) of type ab RR Lyrae variables. In light of the recent discovery of multiple populations in GCs, we suggest a new model explaining the origin of the Sandage period-shift and the difference in mean period of type ab RR Lyrae variables between the two Oosterhoff groups. In our models, the instability strip in the metal-poor group II clusters, such as M15, is populated by second generation stars (G2) with enhanced helium and CNO abundances, while the RR Lyraes in the relatively metal-rich group I clusters like M3 are mostly produced by first generation stars (G1) without these enhancements. This population shift within the instability strip with metallicity can create the observed period-shift between the two groups, since both helium and CNO abundances play a role in increasing the period of RR Lyrae variables. The presence of more metal-rich clusters having Oosterhoff-intermediate characteristics, such as NGC 1851, as well as of most metal-rich clusters having RR Lyraes with the longest periods (group III) can also be reproduced, as more helium-rich third and later generations of stars (G3) penetrate into the instability strip with further increase in metallicity. Therefore, although there are systems where the suggested population shift cannot be a viable explanation, for the most general cases, our models predict that RR Lyraes are produced mostly by G1, G2, and G3, respectively, for the Oosterhoff groups I, II, and III.