We investigate the effects of metallicity on the broad-band photometric colors of late-type giants , and make a comparison of synthetic colors with observed photometric properties of late-type giants over a wide range of effective temperatures ( T _ { eff } = 3500 - 4800 K ) and gravities ( \log g = 0.0 - 2.5 ) , at \left [ \mathrm { M } / \mathrm { H } \right ] = -1.0 and -2.0 .
The influence of metallicity on the synthetic photometric colors is small at effective temperatures above \sim 3800 K , but the effects grow larger at lower T _ { eff } , due to the changing efficiency of molecule formation which reduces molecular opacities at lower \left [ \mathrm { M } / \mathrm { H } \right ] .
To make a detailed comparison of the synthetic and observed photometric colors of late type giants in the T _ { eff } –color and color–color planes ( which is done at two metallicities , \left [ \mathrm { M } / \mathrm { H } \right ] = -1.0 and -2.0 ) , we derive a set of new T _ { eff } – \log g –color relations based on synthetic photometric colors , at \left [ \mathrm { M } / \mathrm { H } \right ] = -0.5 , -1.0 , -1.5 , and -2.0 .
These relations are based on the T _ { eff } – \log g scales that we derive employing literature data for 152 late-type giants in 10 Galactic globular clusters ( with metallicities of the individual stars between \left [ \mathrm { M } / \mathrm { H } \right ] = -0.7 and -2.5 ) , and synthetic colors produced with the PHOENIX , MARCS and ATLAS stellar atmosphere codes .
Combined with the T _ { eff } – \log g –color relations at \left [ \mathrm { M } / \mathrm { H } \right ] = 0.0 ( Kučinskas et al . 2005 ) , the set of new relations covers metallicities \left [ \mathrm { M } / \mathrm { H } \right ] = 0.0 \dots - 2.0 ( \Delta \left [ \mathrm { M } / \mathrm { H } \right ] = 0.5 ) , effective temperatures T _ { eff } = 3500 \dots 4800 K ( \Delta T _ { eff } = 100 K ) , and gravities \log g = -0.5 \dots 3.0 .
The new T _ { eff } – \log g –color relations are in good agreement with published T _ { eff } –color relations based on observed properties of late-type giants , both at \left [ \mathrm { M } / \mathrm { H } \right ] = -1.0 and -2.0 .
The differences in all T _ { eff } –color planes are typically well within \sim 100 K. We find , however , that effective temperatures predicted by the scales based on synthetic colors tend to be slightly higher than those resulting from the T _ { eff } –color relations based on observations , with the offsets up to \sim 100 K. This is clearly seen both at \left [ \mathrm { M } / \mathrm { H } \right ] = -1.0 and -2.0 , especially in the T _ { eff } – ( B - V ) and T _ { eff } – ( V - K ) planes .
The consistency between T _ { eff } – \log g –color scales based on synthetic colors calculated with different stellar atmosphere codes is very good , with typical differences being well within \Delta T _ { eff } \sim 70 K at \left [ \mathrm { M } / \mathrm { H } \right ] = -1.0 and \Delta T _ { eff } \sim 40 K at \left [ \mathrm { M } / \mathrm { H } \right ] = -2.0 .