We combine the estimated metallicities [ Fe/H ] , abundances [ \alpha /Fe ] , positions and motions of a sample of 27,500 local ( 7 < R / \mathrm { kpc } < 9 , 0.5 < |z| / \mathrm { kpc } < 2.5 ) SDSS / SEGUE G-type dwarf stars to investigate the chemo-orbital properties of the Milky Way ’ s disk around the Sun .
When we derive the orbital properties reflecting angular momentum , circularity , and thickness as function of [ \alpha /Fe ] vs. [ Fe/H ] , we find that there is a smooth variation with [ \alpha /Fe ] , a proxy for age .
At the same time , the orbital properties of the old stars with [ \alpha /Fe ] \gtrsim 0.25 do show a transition with [ Fe/H ] : below [ Fe/H ] \simeq -0.6 the orbital angular momentum decreases , and the orbits become significantly non-circular and thicker .
Radial migration of stars into the Solar neighborhood would naturally result in a smooth variation in the orbital properties , but the latter old metal-poor stars form a clear challenge , in particular because a basic feature of radial migration is that stars remain on near-circular orbits .
When we next select stars on near-circular orbits , we indeed find besides the \alpha -young ’ thin-disk ’ stars a significant contribution to the \alpha -old ’ thick-disk ’ metal-rich stars .
However , the remaining \alpha -old ’ thick-disk ’ stars on eccentric orbits , including nearly all old metal-poor stars , are difficult to explain with radial migration alone , but might have formed through early-on gas-rich mergers .
We thus find chemo-orbital evidence that the thicker component of the Milky Way disk is not distinct from the thin component as expected from smooth internal evolution through radial migration , except for the old metal-poor stars with different orbital properties which could be part of a distinct thick-disk component formed through an external mechanism .