A&A 380, 212-220 (2001)
DOI: 10.1051/0004-6361:20011369

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

K. Ohnaka¹, Y. Balega², T. Blöcker¹, Y. S. Efimov³, K.-H. Hofmann¹, N. R. Ikhsanov¹, V. I. Shenavrin⁴, G. Weigelt¹ and B. F. Yudin<sup>4

1</sup>  Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, 53121 Bonn, Germany
²  Special Astrophysical Observatory, Nizhnij Arkhyz, Zelenchuk region, 35147 Karachai-Cherkesia, Russia
³  Crimean Astrophysical Observatory, Nauchny, 98409 Crimea, Ukraine, and Isaac Newton Institute of Chile, Crimean Branch, Russia
⁴  Sternberg Astronomical Institute, Universitetskii pr. 13, 119899 Moscow, Russia

(Received 31 January 2001 / Accepted 27 September 2001)

Abstract
We present the first speckle interferometric observations of R CrB , the prototype of a class of peculiar stars which undergo irregular declines in their visible light curves. The observations were carried out with the 6 m telescope 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 resolution 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 frequency 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 maximum light can be simultaneously fitted with a model consisting of the central star and an optically thin dust shell with density proportional to r^-2. The inner boundary of the shell is found to be 82 (19 mas) with a temperature of 920 K. However, this simple model fails to simultaneously reproduce the visibility and spectral energy distribution obtained at minimum light. We show that this discrepancy can be attributed to thermal emission from a newly formed dust cloud.

Key words: stars: carbon -- stars: circumstellar matter -- stars: mass-loss -- stars: individual: R CrB -- stars: variable: general -- infrared: stars

Offprint request: K. Ohnaka, kohnaka@mpifr-bonn.mpg.de

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© ESO 2001

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