2006A&A...452.1021K

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=3500-4800K) and gravities (logg=0.0-2.5), at [M/H]=-1.0 and -2.0. The influence of metallicity on the synthetic photometric colors is small at effective temperatures above ∼3800K, but the effects grow larger at lower T_eff, due to the changing efficiency of molecule formation which reduces molecular opacities at lower [M/H]. 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, [M/H]=-1.0 and -2.0), we derive a set of new T_eff-logg-color relations based on synthetic photometric colors, at [M/H]=-0.5, -1.0, -1.5, and -2.0. These relations are based on the T_eff-logg scales that we derive employing literature data for 178 late-type giants in 10 Galactic globular clusters (with metallicities of the individual stars between [M/H]=-0.7 and -2.5), and synthetic colors produced with the PHOENIX, MARCS and ATLAS stellar atmosphere codes. Combined with the T_eff-logg-color relations at [M/H]=0.0 Kuinskas et al. (2005A&A...442..281K), the set of new relations covers metallicities [M/H]=0.0...-2.0 (Δ[M/H]=0.5), effective temperatures T_eff=3500...4800K (ΔT_eff=100K), and gravities logg=-0.5...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 [M/H]=-1.0 and -2.0. The differences in all T_eff-color planes are typically well within ∼100K. 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 ∼100K. This is clearly seen both at [M/H]=-1.0 and -2.0, especially in the T_eff-(B-V) and T_ eff_-(V-K) planes. The consistency between T_ eff_-logg-color scales based on synthetic colors calculated with different stellar atmosphere codes is very good, with typical differences being well within ΔT_eff∼70K at [M/H]=-1.0 and ΔT_eff∼40K at [M/H]=-2.0.