Abstract
Although there is consensus that metal-rich stars in the Milky Way bulge are formed via secular evolution of the thin disc, the origin of their metal-poor counterparts is still under debate. Two different origins have been invoked for metal-poor stars: they might be classical bulge stars or stars formed via internal evolution of a massive thick disc. We use N-body simulations to calculate the kinematic signature given by the difference in the mean Galactocentric radial velocity (ΔVGC) between metal-rich stars ([Fe/H] ≥ 0) and moderately metal-poor stars (–1.0 ≤ [Fe/H] < 0) in two models, one containing a thin disc and a small classical bulge (B/D = 0.1), and the other containing a thin disc and a massive centrally concentrated thick disc. We reasonably assume that thin-disk stars in each model may be considered as a proxy of metal-rich stars. Similarly, bulge stars and thick-disc stars may be considered as a proxy of metal-poor stars. We calculate ΔVGC at different latitudes (b = 0°, − 2°, − 4°, − 6°, − 8° and − 10°) and longitudes (l = 0°, ± 5°, ± 10° and ± 15°) and show that the ΔVGC trends predicted by the two models are different. We compare the predicted results with ARGOS data and APOGEE DR13 data and show that moderately metal-poor stars are well reproduced with the co-spatial stellar discs model, which has a massive thick disc. Our results give more evidence against the scenario that most of the metal-poor stars are classical bulge stars. If classical bulge stars exists, most of them probably have metallicities [Fe/H] < –1 dex, and their contribution to the mass of the bulge should be a small percentage of the total bulge mass.
Highlights
In recent years, major progress has been made in our understanding of the Galactic bulge
We here explored the origin of moderately metal-poor stars (–1.0 ≤ [Fe/H] < 0) by comparing a kinematic signature given by the difference in the mean Galactocentric radial velocity between metal-rich and metal-poor stars in two different N-body models with ARGOS data and APOGEE DR13 data
One model consists of a disc and a small classical bulge (B/D = 0.1), and the other consists of a composite stellar disc where the intermediate and thick discs together represent 50% of the stellar mass
Summary
Major progress has been made in our understanding of the Galactic bulge. Using N-body simulations, we have calculated the difference in the mean Galactocentric radial velocity (∆VGC) between metal-rich and metal-poor stars in two models; one containing a small classical bulge, and the other consisting of only disc components.
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