Diffraction-limited speckle interferometry and modeling of the circumstellar envelope of R CrB at maximum and minimum light

We present the first speckle interferometric observations of R CrB, the pro totype of a class of peculiar stars which undergo irregular declines in the ir visible light curves. The observations were carried out with the 6 m tel escope at the Special Astrophysical Observatory near maximum light (V = 7, 1996 Oct. 1) and at minimum light (V = 10.61, 1999 Sep. 28). A spatial reso lution of 75 mas was achieved in the K-band. The dust shell around R CrB is partially resolved, and the visibility is approximately 0.8 at a spatial f requency of 10 cycles/arcsec. The two-dimensional power spectra obtained at both epochs do not show any significant deviation from circular symmetry. The visibility function and spectral energy distribution obtained near maxi mum light can be simultaneously fitted with a model consisting of the centr al star and an optically thin dust shell with density proportional to r(-2) . The inner boundary of the shell is found to be 82 R-* (19 mas) with a tem perature of 920 K. However, this simple model fails to simultaneously repro duce the visibility and spectral energy distribution obtained at minimum li ght. We show that this discrepancy can be attributed to thermal emission fr om a newly formed dust cloud.