Data from our compiled catalog of spectroscopically determined magnesium abundances in stars with accurate parallaxes are used to select thin-disk dwarfs and subgiants according to kinematic criteria .
We analyze the relations between the relative magnesium abundances in stars , [ Mg/Fe ] , and their metallicities , Galactic orbital elements , and ages.The [ Mg/Fe ] ratios in the thin disk at any metallicity in the range ( -1.0 < [ Fe / H ] < -0.4 Â dex ) are shown to be smaller than those in the thick disk implying that the thin-disk stars are on average younger than the thick-disk stars .
The relative magnesium abundances in such metal-poor thin-disk stars have been found to systematically decrease with increasing stellar orbital radii in such a way that magnesium overabundances ( [ Mg / Fe ] > 0.2 Â dex ) are essentially observed only in the stars whose orbits lie almost entirely within the solar circle .
At the same time , the range of metallicities in magnesium-poor stars is displaced from ( -0.5 < [ Fe / H ] < +0.3 Â dex ) to ( -0.7 < [ Fe / H ] < +0.2 Â dex ) as their orbital radii increase .
This behavior suggests that , first , the star formation rate decreases with increasing Galactocentric distance and , second , there was no star formation for some time outside the solar circle while this process was continuous within the solar circle .
The decrease in the star formation rate with increasing Galactocentric distance is responsible for the existence of a negative radial metallicity gradient ( grad _ { R } [ Fe/H ] = ( -0.05 \pm 0.01 ) Â kpc ^ { -1 } ) in the disk , which shows a tendency to increase with decreasing age .
At the same time the relative magnesium abundance exhibits no radial gradient .
We have confirmed the existence of a steep negative vertical metallicity gradient ( grad _ { Z } [ Fe/H ] = ( -0.29 \pm 0.06 ) Â kpc ^ { -1 } ) and detected a significant positive vertical gradient in relative magnesium abundance ( grad _ { Z } [ Mg/Fe ] = ( 0.13 \pm 0.02 ) Â kpc ^ { -1 } ) ; both gradients increase appreciably in absolute value with decreasing age .
We have found that there is not only an age–metallicity relation , but also an age–magnesium abundance relation in the thin disk .
We surmise that the thin disk has a multicomponent structure but the existence of a negative trend in the star formation rate along the Galactocentric radius does not allow the stars of its various components to be identified in the immediate solar neighborhood .

Keywords : Galaxy ( Milky Way ) , stellar chemical composition , thin disk , Galactic evolution .