Photometric calibration of the [alpha/Fe] element: I. Calibration with U BV photometry

Abstract

We present the calibration of the [alpha/Fe] element in terms of the ultra-violet excess for 469 dwarf stars with 0.325 < (B -V)(0) <= 0.775 mag corresponding to the spectral type range F0-K2. The star sample is separated into nine sub-samples with equal range in (B -V)(0) colour, Delta(B-V)(0) = 0.05 mag, and a third degree polynomial is fitted to each dataset. Our calibrations provide [alpha/Fe] elements in the range [0.0, 0.4]. We applied the procedure to two sets of field stars and two sets of clusters. The mean and the corresponding standard deviation of the residuals for 43 field stars taken from the Hypatia catalogue are [alpha/Fe] = -0.090 and sigma = 0.102 dex, while for the 39 ones taken from the same catalogue of stars used in the calibration these quantities are [alpha/Fe] = -0.009 and sigma = 0.079 dex, respectively. We showed that the differences between the mean of the residuals and standard deviations for two sets of clusters ([alpha/Fe] = 0.073 and sigma = 0.91 dex; [alpha/Fe] = -0.012 and sigma = 0.053 dex) originate from the (B - V)(0) and (U - B)(0) colour indices of the clusters which are taken from different sources. The differences between the original [alpha/Fe] elements and the estimated ones (the residuals) are compatible with the uncertainties in the literature. Also, there is good agreement between the distribution of the synthetic alpha elements versus ultra-violet excesses and the ones obtained via our calibrations.