  We have measured the CH G band (CH(G)) index for evolved stars in the
globular cluster M3 based on the Sloan Digital Sky Survey (SDSS) spectroscopic
survey. It is found that there is a useful way to select red giant branch (RGB)
stars from the contamination of other evolved stars such as asymptotic giant
branch (AGB) and red horizontal branch (RHB) stars by using the CH(G) index
versus (g-r) diagram if the metallicity is known from the spectra. When this
diagram is applied to field giant stars with similar metallicity, we establish
a calibration of CH(G)=1.625(g-r)-1.174(g-r)^2-0.934. This method is confirmed
by stars with [Fe/H]~2.3 where spectra of member stars in globular clusters M15
and M92 are available in the SDSS database. We thus extend this kind of
calibration to every individual metallicity bin ranging from [Fe/H] ~ -3.0 to
[Fe/H] ~ 0.0 by using field red giant stars with 0.4 < (g-r) < 1.0. The
metallicity-dependent calibrations give CH(G) =1.625(g-r)-1.174(g-r)^2+
0.060[Fe/H]-0.830 for -3.0 < [Fe/H] <-1.2 and
CH(G)=0.953(g-r)-0.655(g-r)^2+0.060[Fe/H]-0.650 for -1.2 < [Fe/H] < 0.0. The
calibrations are valid for the SDSS spectroscopic data set, and they cannot be
applied blindly to other data sets. With the two calibrations, a significant
number of the contaminating stars (AGB and RHB stars) were excluded and thus a
clear sample of red giant stars is obtained by selecting stars within 0.05 mag
of the calibration. The sample is published online and it is expected that this
large and clean sample of RGB stars will provide new information on the
formation and evolution of the Galaxy.
