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A. K. Dupree, A. Lobel (Harvard-Smithsonian CfA), R. L. Gilliland (STScI)
Seven ultraviolet (\lambda 2500) images of the ultraviolet surface of the bright M supergiant star Betelgeuse (Alpha Orionis, HD 39801) have been obtained from March 1995 to March 1999 using the Faint Object Camera (COSTAR-corrected) on HST. Four dithered exposures were generally obtained at each visit with a total exposure time of 2000 s. The point spread function from the FOC with our filter set is \approx38 mas. Because the extended size of Betelgeuse in the ultraviolet continuum is about 125 mas -- more than twice the size of the 55 mas optical disk - approximately 10 resolution elements are achieved over the stellar image. Bright areas are found, similar to those first identified in March 1995 images (Gilliland and Dupree, 1996, ApJ, 463, L29), however the appearance of the bright asymmetry is not the same in all images. Characteristics of the ultraviolet chromosphere, and the apparent asymmetric structures will be discussed.
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