Spectra and Photometric Colors of Late-type Giants: Theoretical Predictions Versus Observations
A. Kucinskas^1,2, P.H. Hauschildt³, H.-G. Ludwig⁴, T. Tanabé^5
1 National Astronomical Observatory of Japan, Tokyo, Japan ² Institute of Theoretical Physics and Astronomy, Vilnius, Lithuania ³ Hamburger Sternwarte, Hamburg, Germany ⁴ Lund Observatory, Lund, Sweden ⁵ Institute of Astronomy, The University of Tokyo, Tokyo, Japan
The main objective of our presentation is to assess the current status in the theoretical modeling of the spectral properties of late-type giants with current stellar atmosphere models. With this aim, we will present an extensive comparison of synthetic broad-band photometric colors of late-type giants (produced with PHOENIX, MARCS and ATLAS model atmospheres) with observations, both at solar and sub-solar metallicities. We will show that there is a good agreement between the synthetic and observed photometric colors, and synthetic colors and published T_eff-color and color-color relations, especially in the T_eff-(V-K), T_eff-(J-K) and (J-K)-(V-K) planes, which makes these colors indeed well suited for use as diagnostics in late-type giants. Deviations from the observed trends in T_eff-color planes are generally within ±100-150 K in the effective temperature range of T_eff=3500-4800 K. The comparison of the observed and synthetic spectra of late-type giants shows that discrepancies result from the differences both in the strengths of various spectral lines/bands (especially those of molecular bands, such as TiO, H₂O, CO) and the continuum level. Synthetic colors calculated with different stellar atmosphere models agree remarkably well, typically to ~ ±50 K within a large range of effective temperatures, gravities and metallicities. We will show that individual broad-band photometric colors are strongly affected by various model parameters such as molecular opacities, gravity, microturbulent velocity, and stellar mass. Our exploratory 3D modeling of a prototypical late-type giant shows that convection has a noticeable effect on the photometric colors too, as it alters significantly both the vertical and horizontal thermal structures in the outer atmosphere. The differences between colors calculated with full 3D hydrodynamical and 1D model atmospheres are significant (e.g., D(V-K) ~ 0.2 mag), translating into offsets in effective temperature of up to ~ 70 K. We will give a discussion on the implications of our findings, especially in the light of using the future VLTs for the studies of distant stellar populations with late-type giants.

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