Changes in apparent size of giant stars with wavelength due to electron-hydrogen collisions

Interferometric measurements of stellar sizes in frequency bands ranging from the near-infrared to longer wavelengths give different results. Various explanations have been proposed to account for these variations in apparent size with wavelength, but none have been entirely consistent. We propose that thermal ionization in the stellar atmosphere and resulting opacity, primarily due to free-free electron-hydrogen collisions, play a significant role. Such an opacity has a quadratic dependence on photon wavelength and produces variations in the opacity of the atmosphere with wavelength, consistent with pertinent measurements. This may be particularly important for Mira-type stars, and two examples, o Ceti and W Hya, are analyzed as examples. For stars that are much smaller or with more concentrated mass, it is not likely to be significant.