AAS 198th Meeting, June 2001
Session 63. Optical Interferometry III
Display, Wednesday, June 6, 2001, 10:00am-7:00pm, Exhibit Hall

## [63.12] Stellar Diameters at 712 nm and 754 nm

A. Quirrenbach (University of California, San Diego), D. Mozurkewich, J.T. Armstrong (Naval Research Laboratory), D.F. Buscher (University of Cambridge, UK), C.A. Hummel (US Naval Observatory)

We have used the Mark\,III Interferometer on Mt.\ Wilson 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. These observations can be qualitatively explained by the different TiO opacities in the two filters: the \tau = 1 surface occurs at a higher layer within the TiO bands than in the continuum. The atmosphere of cool giant stars is so distended that this effect leads to a measurable difference (up to ~10% for M supergiants) in the observed stellar diameter. However, the observed 712\,nm\,/\,754\,nm diameter ratios are larger than predicted by standard model atmospheres. This indicates that the current models do not adequately describe the TiO opacity in the tenuous outer layers of the atmosphere or at the base of the wind. We suggest that wavelength-dependent measurements of stellar diameters with the new generation of optical interferometers will provide a powerful new tool for the study of stellar atmospheres.