We have studied 23 long-lived G dwarfs that belong to the thin disk and thick disk stellar populations .
The stellar data and analyses are identical , reducing the chances for systematic errors in the comparisons of the chemical abundance patterns in the two populations .
Abundances have been derived for 24 elements : O , Na , Mg , Al , Si , Ca , Ti , Sc , V , Cr , Mn , Fe , Co , Ni , Cu , Zn , Sr , Y , Zr , Ba , La , Ce , Nd , and Eu .
We find that the behavior of [ \alpha /Fe ] and [ Eu/Fe ] vs. [ Fe/H ] are quite different for the two populations .
As has long been known , the thin disk O , Mg , Si , Ca , and Ti ratios are enhanced relative to iron at the lowest metallicities , and decline toward solar values as [ Fe/H ] rises above -1.0 .
For the thick disk , the decline in [ \alpha /Fe ] and [ Eu/Fe ] does not begin at [ Fe/H ] = -1.0 , but at -0.4 .
Other elements share this same behavior , including Sc , Co , and Zn , suggesting that at least in the chemical enrichment history of the thick disk , these elements were manufactured in similar-mass stars .
The heavy s -process elements Ba , La , Ce , and Nd are over-abundant in the thin disk stars relative to the thick disk stars .
On the other hand , the constancy of the [ Ba/Y ] ratio suggests that only one s -process site was manufacturing these elements , or , possibly , that the r -process was responsible for the bulk of the nucleosynthesis of these elements .
We combine our results with other studies ( Edvardsson et al. , Prochaska et al. , Bensby et al. , and Reddy et al .
) , who had already found very similar trends , in order to further explore the origin of the thick disk .
The signs for an independent ( parent galaxy ) evolution of the thick disk are clear , in terms of the different metallicities at which the [ \alpha /Fe ] ratios begin to decline , as well as “ step function ” behavior of some elements , including [ Eu/Y ] , [ Ba/Fe ] , and , possibly , [ Cu/Fe ] , at [ Fe/H ] \approx - 0.2 .