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

A. Kučinskas; P. H. Hauschildt; I. Brott; V. Vansevičius; L. Lindegren; T. Tanabé; F. Allard

doi:doi:10.1051/0004-6361:20054431

Free access

Issue		A&A Volume 452, Number 3, June IV 2006


Page(s)		1021 - 1038
Section		Stellar atmospheres
DOI		http://dx.doi.org/10.1051/0004-6361:20054431

A&A 452, 1021-1038 (2006)
DOI: 10.1051/0004-6361:20054431

II. Z < 0.02

A. Kucinskas^{1, 2, 3}, P. H. Hauschildt⁴, I. Brott^{4, 5}, V. Vansevicius⁶, L. Lindegren¹, T. Tanabé⁷ and F. Allard⁸

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, Gostauto 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 ( $T_{\rm eff}=3500{-}4800$ K) and gravities ( $\log g=0.0{-}2.5$ ), at $\ensuremath{\left[{\rm M}/{\rm H}\right]} =-1.0$ 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 $T_{\rm eff}$ , due to the changing efficiency of molecule formation which reduces molecular opacities at lower $\ensuremath{\left[{\rm M}/{\rm H}\right]}$ . To make a detailed comparison of the synthetic and observed photometric colors of late type giants in the $T_{\rm eff}$ -color and color-color planes (which is done at two metallicities, $\ensuremath{\left[{\rm M}/{\rm H}\right]} =-1.0$ and -2.0), we derive a set of new $T_{\rm eff}{-}\log g$ -color relations based on synthetic photometric colors, at $\ensuremath{\left[{\rm M}/{\rm H}\right]} =-0.5$ , -1.0, -1.5, and -2.0. These relations are based on the $T_{\rm eff}{-}\log g$ scales that we derive employing literature data for 178 late-type giants in 10 Galactic globular clusters (with metallicities of the individual stars between $\ensuremath{\left[{\rm M}/{\rm H}\right]} =-0.7$ and -2.5), and synthetic colors produced with the PHOENIX, MARCS and ATLAS stellar atmosphere codes. Combined with the $T_{\rm eff}{-}\log g$ -color relations at $\ensuremath{\left[{\rm M}/{\rm H}\right]} =0.0$ (Kucinskas et al. 2005), the set of new relations covers metallicities $\ensuremath{\left[{\rm M}/{\rm H}\right]} =0.0\dots-2.0$ ( $\Delta\ensuremath{\left[{\rm M}/{\rm H}\right]} =0.5$ ), effective temperatures $T_{\rm eff}=3500\dots4800$ K ( $\Delta T_{\rm eff}=100$ K), and gravities $\log g=-0.5\dots3.0$ . The new $T_{\rm eff}{-}\log g$ -color relations are in good agreement with published $T_{\rm eff}$ -color relations based on observed properties of late-type giants, both at $\ensuremath{\left[{\rm M}/{\rm H}\right]} =-1.0$ and -2.0. The differences in all $T_{\rm eff}$ -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 $T_{\rm eff}$ -color relations based on observations, with the offsets up to ~100 K. This is clearly seen both at $\ensuremath{\left[{\rm M}/{\rm H}\right]} =-1.0$ and -2.0, especially in the $T_{\rm eff}{-}(B-V)$ and $T_{\rm eff}{-}(V-K)$ planes. The consistency between $T_{\rm eff}{-}\log g$ -color scales based on synthetic colors calculated with different stellar atmosphere codes is very good, with typical differences being well within $\Delta T_{\rm eff} \sim 70$ K at $\ensuremath{\left[{\rm M}/{\rm H}\right]} =-1.0$ and $\Delta T_{\rm eff} \sim 40$ K at $\ensuremath{\left[{\rm M}/{\rm H}\right]} =-2.0$ .

Key words: stars: atmospheres -- stars: late-type -- stars: fundamental parameters -- techniques: photometric

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