A High Sensitivity Exploration of the Distribution of Circumstellar Dust in Optical Light based on the Development of a Stellar Coronagraph

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This study aims at detecting and exploring the circumstellar dust shells of bright red giant stars in faint scattered optical light by dust grains with an unprecedented sensitivity. We investigate the time variability and geometry of mass様oss in red giants on the basis of observed dust distribution revealed by the scattered optical light. For this purpose we use existing observing facilities at first hand, while we develop a new coronagraph which will be dedicated to this study. With the coronagraph we will search bright red giants in the optical for their extended dust shells produced by their mass様oss systematically. We intend to depict their dust shells far out into a region beyond 0.1 pc from the central stars as two dimensional images. From the analysis of the structure we learn the history of mass様oss into unprecedented details. We have used Kiso Observatory that belongs to the Institute of Astronomy, The University of Tokyo as our observations with existing facilities. We have detected a circular dust shell with 120" radius of which brightness is about 25^<th> magnitude per square arcseconds in U Hya. We have monitored the brightness of the shell for 4 months but we have not found any significant variability. Furthermore, we have obtained a possible micro structure in the shell. It should be related to a variability or asymmetry of mass様oss, worthy for a further scrutiny. We have also developed a coronagraph equipped with a focal plane mask and a pupil plane mask.. We have installed it to the 115cm reflector at Rikubetsu Space and Earth Science Museum and performed the first observations. As expected, ghost and scattered light in the optics are well suppressed and we can get least affected images, In particular, we are able to detect the dust shell of U Hya in R-band relatively easily, which confirms the effectiveness of R-band in our dust shell survey. We decide to begin with R band for our survey with the coronagraph.