Asymptotic gian branch (AGB) stars produce most of the mass loss in our Galaxy, and produce copious amounts of dust and molecular gas that are crucial ingredients of many astrophysical systems. We used the Keck Interferometer with a grism providing low spectral dispersion to determine the angular size of an AGB star as a function of wavelength, in order to constrain the relative locations of different atmospheric layers (Eisner et al. 2007). Our data show stellar continuum emission as well as emission from molecules--mostly water--arising from a region with twice the radius of the stelllar continuum photosphere. These results constrain the location of molecule formation around these stars, as well as the physics behind the pulsations seen from such objects.

Atmospheric opacity inferred for the Mira star R Vir using spatially resolved spectroscopy with the Keck Interferometer.