We present results from a program of near-infrared diffraction-limited imaging of late-type stars which has been underway at the Keck Observatory. Utilizing the techniques of aperture masking, the 10 m Keck primary mirror is transformed into a separate-element, multiple aperture interferometer. For bright target objects, such as evolved giants and supergiants, numerous theoretical and experimental studies have underscored the dramatic gains in signal-to-noise to be obtained from the use of such sparse apertures over more conventional full-pupil speckle interferometry. With multi-wavelength images spanning the infrared J, H, K and L bands for a number of the brightest Mira Variables and Supergiants, we have been able to address some of the most pressing questions in the field of late stellar evolution. Amongst the most important of these is mass loss. Although theoretical models usually assume spherical symmetry, there has been a growing body of evidence (e.g. from molecular and polarization data, spectrophotometry, and observations of planetary nebulae) that mass loss is anisotropic. By imaging the hotter regions of dust shells at unprecedented angular resolution, we have been able to directly observed the morphology revealing anisotropic mass-loss in the inner regions nearer to the star where nucleation occurs. Furthermore, observations with the Keck telescope have yielded near-IR images of the stellar photospheres of some of our target stars. This is a particularly important wavelength region due to the low dust opacity, low conntamination from molecular lines and high stellar bolometric output. Our measurements of stellar photospheric diameters and surface morphologies thus yield a wealth of information on the fundamental stellar properties such as pulsational modes, atmospheric structure, giant convective cells and overall photospheric symmetry.
A Half Century of Stellar Pulsation Interpretation
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