AAS 199th meeting, Washington, DC, January 2002
Session 92. Circumstellar Material and Atmospheres: Cooler
Display, Wednesday, January 9, 2002, 9:20am-6:30pm, Exhibit Hall

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[92.07] Diameters of Cool Giant Stars at 712 nm & 754 nm: Theory vs. Observations

J. P. Aufdenberg (CfA), A. Quirrenbach (UCSD), P. H. Hauschildt (UGA), E. Baron (OU)

As reported by Quirrenbach et al. (2000, IAU Symposium No. 205), the Mark III Interferometer on Mt. Wilson was used to measure the diameters of 47 cool giant stars in two filters centered at 712 nm and at 754 nm. These filters probe the stellar atmosphere in a strong TiO band (712 nm) and in a ``continuum'' band relatively free of TiO absorption (754 nm). The measured diameters are systematically larger at 712 nm than at 754 nm. The diameter ratio increases with decreasing effective temperature, and it is larger for luminosity class I than for luminosity class II and III stars. We have been successful in quantitatively reproducing these trends with the latest set of cool giant models from the general-purpose stellar atmosphere code {\tt PHOENIX}. Spherical, hydrostatic, massively line-blanketed atmosphere models have been constructed using a depth-impact parameter coordinate system where parallel rays intersect nested shells. From these models, synthetic center-to-limb intensity variations in the 712 nm and 754 nm bands have been extracted. From these synthetic limb profiles, we predict the uniform-disk ratio of the two bands as a function of model effective temperature and surface gravity. With the exception of Mira (o Ceti), the diameter ratios of all of the observed oxygen-rich giants fall within the limits predicted by the models. We find that the computation of the model atmospheres in a spherical geometry, with complete as possible atomic and molecular line blanketing, is required to generate the geometrical extension of the atmosphere and the limb profiles inferred from the observations. Most of the stars have well constrained bolometric flux measurements and IR interferometric angular diameters which provide nearly model-independent effective temperatures. These measured effective temperatures, along with surface gravity estimates, provide a consistency check of the models.

JPA is supported by a Harvard-Smithsonian CfA postdoctoral fellowship. This work was supported in part by NSF grants AST-9720704 and AST-0086246, NASA grants NAG5-8425, NAG5-9222, as well as NASA/JPL grant 961582 to the University of Georgia and in part by NSF grants AST-97314508, by NASA grant NAG5-3505 and an IBM SUR grant to the University of Oklahoma.

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The author(s) of this abstract have provided an email address for comments about the abstract: jaufdenberg@cfa.harvard.edu

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