We analyze a sample of solar neighborhood stars which have high-quality abundance determinations and show that there are two distinct regimes of [ \alpha /Fe ] versus age which we identify as the epochs of the thick and thin disk formation .
A tight correlation between metallicity and [ \alpha /Fe ] versus age is clearly identifiable for thick disk stars , implying that this population formed from a well mixed interstellar medium , probably initially in starburst and then more quiescently , over a time scale of 4-5 Gyr .
Thick disk stars have vertical velocity dispersions which correlate with age , with the youngest objects of this population having small scale heights similar to that of thin disk stars .
A natural consequence of these two results is that a vertical metallicity gradient is expected in this population .
We suggest that the youngest thick disk set the initial conditions from which the inner thin disk started to form about 8 Gyr ago , at [ Fe/H ] in the range of ( -0.1 , +0.1 ) dex and [ \alpha /Fe ] \sim 0.1 dex .
This also provides an explanation of the apparent coincidence between the existence of a step in metallicity at 7-10 kpc in the thin disk and the confinement of the thick disk within R < 10 kpc .
We suggest that the outer thin disk developed outside the influence of the thick disk , giving rise to a separate structure , but also that the high alpha-enrichment of those regions may originate from a primordial pollution of the outer regions by the gas expelled from the forming thick disk .
Metal-poor thin disk stars ( [ Fe/H ] < -0.4 dex ) in the solar vicinity , whose properties are best explained by them originating in the outer disk , are shown to be as old as the youngest thick disk ( 9-10 Gyr ) .
This implies that the outer thin disk started to form while the thick disk was still forming stars in the inner parts of the Galaxy .
Hence , while the overall inner ( thick+thin ) disks is comprised of two structures with different scale lengths and whose combination may give the impression of an inside-out formation process , the local thin disk itself probably formed its first stars in its outskirts .
Moreover , we point out that , given the tight age-metallicity and age- [ \alpha /Fe ] relations that exist in the thick disk , an inside-out process would give rise to a radial gradient in metallicity and \alpha -elements in this population which is not observed .
Finally , we argue that our results leave little room for radial migration ( in the sense of churning ) either to have contaminated the solar vicinity , or , on a larger scale , to have redistributed stars in significant proportion across the solar annulus .