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D. R. Alexander, J. W. Ferguson (Wichita St. U.), France Allard (Ecole Normale Superieure - Lyon), P. H. Hauschildt (U. Georgia - Athens)
As part of a larger project to compute accurate model atmospheres for cool giant stars (Hauschildt et al, ApJ, 525, 871, 1999), we have used the PHOENIX program to compute models in spherical geometry with a detailed treatment of both the equation of state and the opacity. The equation of state includes atoms, molecules, and the condensation of solid grains in thermodynamic equilibrium. Monochromatic atomic and molecular opacities are included via the opacity sampling technique at 20,000 frequencies. The opacity of 26 grain species is computed using the Mie theory.
Despite the very low densities found in red giant atmospheres, we find that models of oxygen-rich stars with effective temperatures below 3000 K include dust grains. The presence of grains in an atmosphere has two important consequences that are potentially observable. First, the temperature of the surface layers of the star is raised by the increased opacity of the grains, compared to a model with identical parameters in which grain condensation is not included. Second, the depletion of TiO molecules from the gas phase, as titanium condenses into CaTiO3, reduces the strength of the visible absorption features of TiO in the emergent spectra of the models.
Low temperature astrophysics at Wichita State University is supported by NASA EPSCoR grant NCC5-168 and NASA LTSA grant NAG5-3435.
The author(s) of this abstract have provided an email address for comments about the abstract: dra@twsuvm.uc.twsu.edu