Broad-band photometric colors and effective temperature calibrations for late-type giants. I. Z = 0.02

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 (log g = 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 similar to 3800K, but the effects grow larger at lower T-eff,T- due to the changing effciency 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-log g-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- log g 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- log g-color relations at [M/H] = 0.0 (Kucinskas et al. 2005), the set of new relations covers metallicities [M/H] = 0.0... -2.0 ([M/H] = 0.5), effective temperatures T-eff = 3500... 4800 K (T-eff = 100K), and gravities log g = - 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 similar to 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 similar to 100 K. 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- 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 similar to 70 K at [M/H] = - 1.0 and. T-eff similar to 40 K at [M/H] = -2.0. (Less)

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