Photometric calibration of the [\(\alpha\)/Fe] element: II. Calibration with SDSS photometry

We present the calibration of the [\(\alpha\)/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, \(0.1<(g-r)_{0} \leq 0.6\) mag, and high frequency in the blueward of the spectrum which minimize the guillotine effect. Our calibration provides [\(\alpha\)/Fe] elements in the range \((-0.05, 0.35]\) dex. We applied the procedure to a high-latitude field, \(85^{\circ }\leq b \leq 90^{\circ }\) with size 78 deg2 and we could estimate the [\(\alpha\)/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, \(d[\alpha \text{/Fe}]/dz=+0.032 \pm 0.002~\mbox{dex}\,\mbox{kpc}^{-1}\), for the range \(0< z<5\) kpc, while the distribution of the [\(\alpha\)/Fe] element is flat for further \(z\) distances.