Broad-band photometric colors and effective temperature calibrations for late-type giants

II. Z 0.02

¹ Lund Observatory, Lund University, Box 43, 221 00, Lund, Sweden
² National Astronomical Observatory of Japan, Mitaka, Tokyo, 181-8588, Japan
³ Institute of Theoretical Physics and Astronomy, Goštauto 12, Vilnius 01108, Lithuania e-mail: ak@itpa.lt
⁴ Hamburger Sternwarte, Gojenbergsweg 112, 21029 Hamburg, Germany
⁵ INTEGRAL Science Data Centre, Chemin d'Ecogia 16, 1290 Versoix, Switzerland
⁶ Institute of Physics, Savanoriu 231, Vilnius 02300, Lithuania
⁷ Institute of Astronomy, The University of Tokyo, Mitaka, Tokyo, 181-0015, Japan
⁸ Centre de Recherche Astronomique de Lyon, École Normale Supérieure, 69364 Lyon Cedex 07, France

Received: 28 October 2005
Accepted: 7 March 2006

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 ( K) and gravities (), at 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 T_eff, due to the changing efficiency of molecule formation which reduces molecular opacities at lower . To make a detailed comparison of the synthetic and observed photometric colors of late type giants in the -color and color-color planes (which is done at two metallicities, and -2.0), we derive a set of new -color relations based on synthetic photometric colors, at , -1.0, -1.5, and -2.0. These relations are based on the scales that we derive employing literature data for 178 late-type giants in 10 Galactic globular clusters (with metallicities of the individual stars between and -2.5), and synthetic colors produced with the PHOENIX, MARCS and ATLAS stellar atmosphere codes. Combined with the -color relations at (Kučinskas et al. 2005), the set of new relations covers metallicities (), effective temperatures  K ( K), and gravities . The new -color relations are in good agreement with published -color relations based on observed properties of late-type giants, both at and -2.0. The differences in all T_eff-color planes are typically well within ~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 ~100 K. This is clearly seen both at and -2.0, especially in the and planes. The consistency between -color scales based on synthetic colors calculated with different stellar atmosphere codes is very good, with typical differences being well within  K at and  K at .