Absolute Magnitude Calibration for Dwarfs Based on the Colour-Magnitude Diagrams of Galactic Clusters

Abstract

We present two absolute magnitude calibrations for dwarfs based on colour-magnitude diagrams of Galactic clusters. The combination of the $M_g$ absolute magnitudes of the dwarf fiducial sequences of the clusters M92, M13, M5, NGC 2420, M67 and NGC 6791 with the corresponding metallicities provides absolute magnitude calibration for a given $(g-r{)}_0$ colour. The calibration is defined in the colour interval $0.25\le (g-r{)}_0\le 1.25$ mag and it covers the metallicity interval $-2.15\le [Fe/H]\le +0.37$ dex. The absolute magnitude residuals obtained by the application of the procedure to another set of Galactic clusters lie in the interval $-0.15\le \Delta M_g\le +0.12$ mag. The mean and standard deviation of the residuals are $<\Delta M_g>=-0.002$ and $\sigma =0.065$ mag, respectively. The calibration of the $M_J$ absolute magnitude in terms of metallicity is carried out by using the fiducial sequences of the clusters M92, M13, 47 Tuc, NGC 2158 and NGC 6791. It is defined in the colour interval $0.90\le (V-J{)}_0\le 1.75$ mag and it covers the same metallicity interval of the $M_g$ calibration. The absolute magnitude residuals obtained by the application of the procedure to the cluster M5 ($[Fe/H]=-1.40$ dex) and 46 solar metallicity, $-0.45\le [Fe/H]\le +0.35$ dex, field stars lie in the interval -0.29 and +0.35 mag. However, the range of 87 per cent of them is rather shorter, $-0.2\le \Delta M_J\le +0.2$ mag. The mean and standard deviation of all residuals are $<\Delta M_J>=0.05$ and $\sigma$=0.13 mag, respectively. The derived relations are applicable to stars older than 4 Gyr for the $M_g$ calibration, and older than 2 Gyr for the $M_J$ calibration. The cited limits are the ages of the youngest calibration clusters in the two systems.

31 pages, including 13 figures and 20 tables, accepted for publication in PASA. arXiv admin note: text overlap with arXiv:1206.2752

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• The SEGUE Stellar Parameter Pipeline. IV. Validation with an Extended Sample of Galactic Globular and Open Clusters
• Setting UBVRI Photometric Zero-Points Using Sloan Digital Sky Survey ugriz Magnitudes