Broad - band photometric colors and effective temperature calibrations for late - type giants . II . 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 (Teff = 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 ∼ 3800K, but the effects grow larger at lower Teff , 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 Teff–color and color–color planes (which is done at two metallicities, [M/H] = −1.0 and −2.0), we derive a set of new Teff–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 Teff–log g scales that we derive employing literature data for 152 latetype 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 Teff–log g–color relations at [M/H] = 0.0 (Kučinskas et al. 2005), the set of new relations covers metallicities [M/H] = 0.0 . . .−2.0 (∆ [M/H] = 0.5), effective temperatures Teff = 3500 . . . 4800K (∆Teff = 100K), and gravities log g = −0.5 . . . 3.0. The new Teff–log g–color relations are in good agreement with published Teff–color relations based on observed properties of late-type giants, both at [M/H] = −1.0 and −2.0. The differences in all Teff–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 Teff–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 Teff–(B − V ) and Teff–(V − K) planes. The consistency between Teff–log g–color scales based on synthetic colors calculated with different stellar atmosphere codes is very good, with typical differences being well within ∆Teff ∼ 70K at [M/H] = −1.0 and ∆Teff ∼ 40K at [M/H] = −2.0.