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G. Schneider (Steward Observatory, University of Arizona)
Space-based coronagraphy with the HST second generation instruments (NICMOS and STIS) provides a unique venue for high contrast imaging of both spatially diffuse and faint point sources (eg. circumstellar debris disks and sub-stellar companions) in very close angular proximity to bright stars. During HST cycle 7 we conducted a NICMOS coronagraphic imaging survey of young stars searching for, finding, and spatially resolving dusty debris disks. The existence of a dusty disk about the young main-sequence star HR 4796A had been inferred from its thermal infrared excess (Jura, 1991, ApJ 503, L83), and its likely global morphology and structure from modeling of mid-IR imaging (Koerner et al., 1998, ApJ, 503, L83; Jayawardhana et al., 1998, ApJ 503, 79)). Our HST/NICMOS PSF-subtracted coronagraphic imaging of the HR 4796A disk indicated a remarkable correlation in the spatial distribution of the dust seen in scattered light and the regions of thermal emission reported at longer wavelengths. These observations clearly revealed its ring-like structure and better defined its geometric and photometric properties (Schneider et al, 1999, ApJ 513, L127). Now, we have coronagraphically re-imaged the HR 4796A ring with the higher spatial resolution afforded by STIS. With these data we refine the systemic parameters elucidated by the NICMOS images. In addition we find, and discuss, compelling evidence for two brightness anisotropies in the ring from the STIS data (which were only hinted at, at larger spatial scales, in the NICMOS data). The anisotropies indicate a non-homogeneous spatial distribution of the debris particles and/or directionally preferential scattering by the circumstellar grains. High-contrast, high spatial scale observations such as these remain technically challenging from the ground. Currently, HST continues to set the benchmark in the arena of disk imaging which ground-based adaptive optics imaging systems are striving to meet.
This work is based on observations with the NASA/ESA Hubble Space Telescope, obtained at the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Inc. under NASA contract NAS5-26555 and supported by NASA grant NAG5-3042 to the NICMOS IDT and HST/GO grant 8624.