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Register a new userOn the origin of the chemical bimodality of disk stars: A tale of merger and migration
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Tobias Buck
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The Milky Ways stellar disk exhibits a bimodality in the [Fe/H] vs. [$alpha$/Fe] plane, showing a distinct high-$alpha$ and low-$alpha$ sequence whose origin is still under debate. We examine the [Fe/H]-[$alpha$/Fe] abundance plane in cosmological hydrodynamical simulations of Milky Way like galaxies from the NIHAO-UHD project and show that the bimodal $alpha$-sequence is a generic consequence of a gas-rich merger at some time in the Galaxys evolution. The high-$alpha$ sequence evolves first in the early galaxies, extending to high metallicities, while it is the low-$alpha$ sequence that is formed after the gas-rich merger. The merger brings in fresh metal-poor gas diluting the interstellar mediums metallicity while keeping the [$alpha$/Fe] abundance almost unchanged. The kinematic, structural and spatial properties of the bimodal $alpha$-sequence in our simulations reproduces that of observations. In all simulations, the high-$alpha$ disk is old, radially concentrated towards the galaxys center and shows large scale heights. In contrast, the low-$alpha$ disk is younger, more radially extended and concentrated to the disk mid-plane. Our results show that the abundance plane is well described by these two populations that have been distributed radially across the disk by migration: at present-day in the solar neighbourhood, low-$alpha$ stars originate from both the inner and outer disk while most of the high-$alpha$ stars have migrated from the inner disk. We show that age dating the stars in the [Fe/H]-[$alpha$/Fe] plane can constrain the time of the low-$alpha$ sequence forming merger and conclude that $alpha$-bimodality is likely a not uncommon feature of disk galaxies.
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Radial migration is an important process in the Galactic disk. A few open clusters show some evidence on this mechanism but there is no systematic study. In this work, we investigate the role of radial migration on the Galactic disk based on a large sample of 146 open clusters with homogeneous metallicity and age from Netopil et al. and kinematics calculated from Gaia DR2. The birth site Rb, guiding radius Rg and other orbital parameters are calculated, and the migration distance |Rg-Rb| is obtained, which is a combination of metallicity, kinematics and age information. It is found that 44% open clusters have |Rg-Rb|< 1 kpc, for which radial migration (churning) is not significant. Among the remaining 56% open clusters with |Rg-Rb|> 1 kpc, young ones with t<1.0 Gyr tend to migrate inward, while older clusters usually migrate outward. Different mechanisms of radial migration between young and old clusters are suggested based on their different migration rates, Galactic locations and orbital parameters. For the old group, we propose a plausible way to estimate migration rate and obtain a reasonable value of 1.5(+-0.5) kpc/Gyr based on ten intermediate-age clusters at the outer disk, where the existence of several special clusters implies its complicate formation history.
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Stellar migration, whether due to trapping by transient spirals (churning), or to scattering by non-axisymmetric perturbations, has been proposed to explain the presence of stars in outer disks. After a review of the basic theory, we present compelling, but not yet conclusive, evidence that churning has been important in the outer disks of galaxies with type II (down-bending) profiles, while scattering has produced the outer disks of type III (up-bending) galaxies. In contrast, field galaxies with type I (pure exponential) profiles appear to not have experienced substantial migration. We conclude by suggesting work that would improve our understanding of the origin of outer disks.
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