Cool giant stars are bigger at 712 nm than at 754 nm

Details Cool giant stars are bigger at 712 nm than at 754 nm Cool giant stars are bigger at 712 nm than at 754 nm

Jan 2001

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2001iaus..205..304q&link_type=abstract

Galaxies and their Constituents at the Highest Angular Resolutions, Proceedings of IAU Symposium #205, held 15-18 August 2000 at

Physics

2

Scientific paper

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 τ = 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.

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