Metallicity Dependent Transformations for Red Giants with Synthetic Colours of UBV and ugr

TL;DR

This paper develops metallicity-dependent transformation equations between UBV and SDSS ugr colours for red giants using synthetic data, enabling better comparison of observational data across different photometric systems.

Contribution

It introduces new transformation equations for red giants that account for metallicity, covering a wide colour range and validated on multiple clusters.

Findings

Residuals have a mean of -0.01 mag with a standard deviation of 0.07 mag.

Transformations are effective for both clusters and field stars.

Interstellar reddening significantly affects derived colours.

Abstract

We present metallicity dependent transformation equations between UBV and SDSS ugr colours for red giants with synthetic data. The ranges of the colours used for the transformations are 0.400 $\leq(B-V)_{0}\leq$ 1.460, -0.085$\leq(U - B)_{0}\leq$ 1.868, 0.291$\leq(g-r)_{0}\leq$ 1.326, and 1.030$\leq(u - g)_{0}\leq$ 3.316 mag, and cover almost all the observational colours of red giants. We applied the transformation equations to six clusters with different metallicities and compared the resulting $(u - g)_{0}$ colours with the ones estimated by the calibration of the fiducial sequences of the clusters. The mean and standard deviation of the residuals for all clusters are $<\Delta(u - g)_0> =$-0.01 and $\sigma(u - g)_{0} =$0.07 mag, respectively. We showed that interstellar reddening plays an important role on the derived colours. The transformations can be applied to clusters as well as to field stars. They can be used to extend the colour range of the red giants in the clusters which are restricted due to the saturation of SDSS data.