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Low-alpha Metal-Rich Stars with Sausage Kinematics in the LAMOST Survey: Are they from the Gaia-Sausage-Enceladus Galaxy?

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 Added by Yuqin Chen
 Publication date 2020
  fields Physics
and research's language is English




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We search for metal-rich Sausage-kinematic (MRSK) stars with [Fe/H]> -0.8 and -100<Vphi<50 km/s in LAMOST DR5 in order to investigate the influence of the Gaia-Sausage-Enceladus (GSE) merger event on the Galactic disk. For the first time, we find a group of low-alpha MRSK stars, and classify it as a metal-rich tail of the GSE galaxy based on the chemical and kinematical properties. This group has slightly larger Rapo, Zmax and Etot distributions than a previously-reported high-alpha group. Its low-alpha ratio does not allow for an origin resulting from the splash process of the GSE merger event, as is proposed to explain the high-alpha group. A hydrodynamical simulation by Amarante et al. provides a promising solution, in which the GSE galaxy is a clumpy Milky-Way analogue that develops a bimodal disk chemistry. This scenario explains the existence of MRSK stars with both high-alpha and low-alpha ratios found in this work. It is further supported by another new feature that a clump of MRSK stars is located at Zmax=3-5 kpc, which corresponds to the widely adopted disk-halo transition at |Z|~4 kpc. We suggest that a pile-up of MRSK stars at Zmax contributes significantly to this disk-halo transition, an interesting imprint left by the GSE merger event. These results also provide an important implication on the connection between the GSE and the Virgo Radial Merger.



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Identifying stars found in the Milky Way as having formed in situ or accreted can be a complex and uncertain undertaking. We use Gaia kinematics and APOGEE elemental abundances to select stars belonging to the Gaia-Sausage-Enceladus (GSE) and Sequoia accretion events. These samples are used to characterize the GSE and Sequoia population metallicity distribution functions, elemental abundance patterns, age distributions, and progenitor masses. We find that the GSE population has a mean [Fe/H] $sim -1.15$ and a mean age of $10-12$ Gyr. GSE has a single sequence in [Mg/Fe] vs [Fe/H] consistent with the onset of SN Ia Fe contributions and uniformly low [Al/Fe] of $sim -0.25$ dex. The derived properties of the Sequoia population are strongly dependent on the kinematic selection. We argue the selection with the least contamination is $J_{phi}/J_{mbox{tot}} < -0.6$ and $(J_z - J_R)/J_{mbox{tot}} < 0.1$. This results in a mean [Fe/H] $sim -1.3$ and a mean age of $12-14$ Gyr. The Sequoia population has a complex elemental abundance distribution with mainly high [Mg/Fe] stars. We use the GSE [Al/Fe] vs [Mg/H] abundance distribution to inform a chemically-based selection of accreted stars, which is used to remove possible contaminant stars from the GSE and Sequoia samples.
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