We present new measurements of the diameter of o Cet (Mira) as a function of wavelength in the 2.2 micron atmospheric window using the adaptive optics system and the infrared camera and spectrograph mounted on the Subaru Telescope. We found that the angular size of the star at the wavelengths of CO and H2O absorption lines were up to twice as large as the continuum photosphere. This size difference is attributable to the optically thick CO and H2O molecular layers surrounding the photosphere. This measurement is the first direct differential spectroscopic imaging of stellar extension that resolves individual molecular lines with high spectral-resolution observations. This observation technique is extremely sensitive to differences in spatial profiles at different wavelengths; we show that a difference in diameter much smaller than the point spread function can be measured.
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