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Session 114 - Circumstellar Material & Nebulae.
Oral session, Saturday, January 10

[114.05] Near-Infrared Imaging of Stellar Outflows with Keck I

J. D. Monnier, P. G. Tuthill, W. C. Danchi (UC, Berkeley), C. Haniff (Cambridge U.)

We have transformed the 10-m primary mirror of the Keck I telescope into a multi-element Michelson interferometric array using an aperture mask which admits starlight through small, separated sub-apertures spread over the telescope pupil. Non-redundant aperture masking of this sort has been shown on other telescopes to significantly improve the signal-to-noise ratio of high-resolution information for bright sources. This is especially important for reconstructing images of complex, extended structures with diffraction-limited resolution, \sim63 milli-arcseconds at K-band (2.2\,\micron) with the Keck Telescopes.

We present near-infrared images with unprecedented detail of the inner dust shells (within a few stellar radii) of carbon stars IRC +10216 and CIT 6, and red supergiants VY CMa and NML Cyg. Our observations clearly indicate the dust outflow is often not spherically symmetric and probably not even axisymmetric. The asymmetric, clumpy nature of the mass outflow may be due to the chaotic nature of dust formation itself, the large scale convective elements of the photosphere, the presence of an unseen companion, or some other process. With continued observations over the coming years, inhomogeneities in the dust outflows should show proper motions, directly revealing the gas dynamics in the dust-formation region.

We have also measured multi-wavelength stellar diameters of six nearby Mira variable stars. Because the infrared spectrum is not contaminated by the many molecular transitions found in the visible, near-infrared diameters are especially useful for comparison with stellar atmospheric model predictions. Furthermore, photospheric temperature variations, previously observed as visible ``hotspots'' on the stellar surface, have little effect on near-infrared diameter measurements. Monitoring the diameters over a pulsational period (typically 1-2 years) will allow us to literally watch the stars change size.

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