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Description
Using data from the GALAH pilot survey, we determine properties of the Galactic thin and thick discs near the solar neighbourhood. The data cover a small range of Galactocentric radius (7.9<~R_GC_<~9.5kpc), but extend up to 4kpc in height from the Galactic plane, and several kpc in the direction of Galactic anti-rotation (at longitude 260{deg}<=l<=280{deg}). This allows us to reliably measure the vertical density and abundance profiles of the chemically and kinematically defined 'thick' and 'thin' discs of the Galaxy. The thin disc (low-{alpha} population) exhibits a steep negative vertical metallicity gradient, at d[M/H]/dz=-0.18+/-0.01dex/kpc, which is broadly consistent with previous studies. In contrast, its vertical {alpha}-abundance profile is almost flat, with a gradient of d[{alpha}/M]/dz=0.008+/-0.002dex/kpc. The steep vertical metallicity gradient of the low-{alpha} population is in agreement with models where radial migration has a major role in the evolution of the thin disc. The thick disc (high-{alpha} population) has a weaker vertical metallicity gradient d[M/H]/dz=-0.058+/-0.003dex/kpc. The {alpha}-abundance of the thick disc is nearly constant with height, d[{alpha}/M]/dz=0.007+/-0.002dex/kpc. The negative gradient in metallicity and the small gradient in [{alpha}/M] indicate that the high-{alpha} population experienced a settling phase, but also formed prior to the onset of major Type Ia supernova enrichment. We explore the implications of the distinct {alpha}-enrichments and narrow [{alpha}/M] range of the sub-populations in the context of thick disc formation.
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