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The Milky Way's stellar disk exhibits a bimodality in the [Fe/H] vs.[$\alpha$/Fe] plane, showing a distinct high-$\alpha$ and low-$\alpha$ sequencewhose origin is still under debate. We examine the [Fe/H]-[$\alpha$/Fe]abundance plane in cosmological hydrodynamical simulations of Milky Way likegalaxies from the NIHAO-UHD project and show that the bimodal $\alpha$-sequenceis a generic consequence of a gas-rich merger at some time in the Galaxy'sevolution. The high-$\alpha$ sequence evolves first in the early galaxies,extending to high metallicities, while it is the low-$\alpha$ sequence that isformed after the gas-rich merger. The merger brings in fresh metal-poor gasdiluting the interstellar medium's metallicity while keeping the [$\alpha$/Fe]abundance almost unchanged. The kinematic, structural and spatial properties ofthe bimodal $\alpha$-sequence in our simulations reproduces that ofobservations. In all simulations, the high-$\alpha$ disk is old, radiallyconcentrated towards the galaxy's center and shows large scale heights. Incontrast, the low-$\alpha$ disk is younger, more radially extended andconcentrated to the disk mid-plane. Our results show that the abundance planeis well described by these two populations that have been distributed radiallyacross the disk by migration: at present-day in the solar neighbourhood,low-$\alpha$ stars originate from both the inner and outer disk while most ofthe high-$\alpha$ stars have migrated from the inner disk. We show that agedating the stars in the [Fe/H]-[$\alpha$/Fe] plane can constrain the time ofthe low-$\alpha$ sequence forming merger and conclude that $\alpha$-bimodalityis likely a not uncommon feature of disk galaxies.

On the origin of the chemical bimodality of disk stars: A tale of merger and migration