  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.
