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

Abstract

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 = 3500-4800 K) 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 ∼3800 K, 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 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 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 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 reff- 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 (Δ [M/H] = 0.5), effective temperatures Teff = 3500... 4800 K (ΔTeff = 100 K), 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 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 ∼100 K. 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 ΔTeff ∼ 70 K at [M/H] = -1.0 and ΔTeff ∼ 40 K at [M/H] = -2.0. © ESO 2006.