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Z. Wahhaj, D.W. Koerner (University of Pennsylvania), D.E. Backman (Franklin and Marshall College), M.W. Werner, E. Serabyn, M.E. Ressler (Jet Propulsion Laboratory)
We present new high-dynamic-range images of circumstellar dust around HR 4796A that were obtained with MIRLIN at the Keck II telescope at 8.8, 10.3, 12.5 and 24.5 microns. We have also obtained a new continuum measurement at 350 microns with the Caltech Submillimeter Observatory. Emission is resolved in 12.5 and 24.5 micron images at ~0.25'' and ~0.4'' resolution, respectively, and confirms both the outer ring and the presence of a warm, tenuous, exo-zodiacal component of dust close to the star. The flux of unresolved emission at the stellar position (within a linear separation of 13 AU) was determined by fitting the central peak emission with an image of a nearby standard star imaged within a half hour of HR 4796A. Point source emission determined in this way is in excess of the photosphere by 59.1 ±17.6 mJy at 12.5 microns and 258 ±50.7 mJy at 24.5 microns, implying a color temperature of 163±17 K and radial distance of 11±3 AU. To refine this estimate, we are carrying out modeling which simultaneously fits mid-infrared images and continuum flux measurements with simulations generated by a model of an inner disk with an outer ring. Preliminary model-fitting to all data confirms the presence of an inner zodiacal dust component centered at a radial distance of approximately 9 AU from the star.