Absolute Magnitude Calibration for Giants Based on the Colour-Magnitude Diagrams of Galactic Clusters. II. Calibration with SDSS

We present an absolute magnitude calibration for red giants with the colour-magnitude diagrams of six Galactic clusters with different metallicities, i.e. M92, M13, M3, M71, NGC 6791, and NGC 2158. The combination of the absolute magnitudes of the red giant sequences with the corresponding metallicities provides calibration for absolute magnitude estimation for red giants for a given (g - r)₀ colour. The calibration is defined in the colour interval 0.45 ≤ (g - r)₀ ≤ 1.30 mag and it covers the metallicity interval -2.15≤[Fe/H]≤ +0.37 dex. The absolute magnitude residuals obtained by the application of the procedure to another set of Galactic clusters lie in the interval -0.28 < ΔM ≤ +0.43 mag. However, the range of 94% of the residuals is shorter, -0.1 < ΔM ≤ +0.4 mag. The mean and the standard deviation of (all) residuals are 0.169 and 0.140 mag, respectively. The derived relations are applicable to stars older than 2 Gyr, the age of the youngest calibrating cluster.