Absolute Magnitude Calibration for Red Giants based on the Colour-Magnitude Diagrams of Galactic Clusters. I- Calibration with V and (B-V)

TL;DR

This paper develops a calibration method for estimating the absolute magnitude of red giants using their colour and metallicity, based on data from six Galactic clusters with varying metallicities, applicable to stars older than 4 Gyr.

Contribution

The study introduces a new calibration for red giant absolute magnitudes using (B-V) colour and metallicity, validated across multiple clusters with minimal residuals.

Findings

91% of residuals within -0.40 to +0.40 mag

Mean residual of 0.05 mag, standard deviation 0.19 mag

Age can be omitted in the calibration for stars older than 4 Gyr

Abstract

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, M5, 47 Tuc, M67, and NGC 6791. The combination of the absolute magnitude offset from the fiducial of giant sequence of the cluster M5 with the corresponding metallicity offset provides calibration for absolute magnitude estimation for red giants for a given (B-V)o colour. The calibration is defined in the colour interval 0.75<=(B-V)o<=1.50 mag and it covers the metallicity interval -2.15<[Fe/H]<=+0.37 dex. 91% of the absolute magnitude residuals obtained by the application of the procedure to another set of Galactic clusters lie in the interval -0.40<\Delta M<=+0.40 mag. The mean and the standard deviation of the residuals are 0.05 and 0.19 mag, respectively. We fitted the absolute magnitude also to metallicity and age for a limited sub-sample of (B-V)o colour, just to test the effect of age in absolute magnitude calibration. Comparison of the mean and the standard deviation of the residuals evaluated by this procedure with the corresponding ones provided by the procedure where the absolute magnitude fitted to a third degree polynomial of metallicity show that the age parameter may be omitted in absolute magnitude estimation of red giants. The derived relations are applicable to stars older than 4 Gyr, the age of the youngest calibrating cluster.