Photometric Calibration of the $[\alpha \mathrm{/}$Fe] Element: II. Calibration with SDSS Photometry

We present the calibration of the [$\alpha \mathrm{/}$Fe] element in terms of ultra-violet excess for 465 dwarf stars with spectral type F0-K2. We used a single calibration, fitted to a third degree polynomial with a square of the correlation coefficient 0.74 and standard deviation 0.05 mag, for all stars due to their small colour range, $0.1<(g-r{)}_0\le 0.6$ mag, and high frequency in the blueward of the spectrum which minimize the guillotine effect. Our calibration provides [$\alpha \mathrm{/}$Fe] elements in the range $(-0.05,0.35]$ dex. We applied the procedure to a high-latitude field, $85^\circ \leq b \leq 90^\circ$withsize78deg$^2$andwecouldestimatethe[$\alpha/$Fe] elements of 23,414 dwarf stars which occupy a Galactic region up to a vertical distance of $z=9$ kpc. We could detect a small positive gradient, $d[\alpha/{\rm Fe}]/dz=+0.032 \pm0.002$dexkpc$^{-1}$,fortherange$0<z<5$ kpc, while the distribution of the [$\alpha \mathrm{/}$Fe] element is flat for further $z$ distances.