JHK'-band IOTA interferometry of the circumstellar environment of R CrB

We report the first long-baseline interferometry of the circumstellar dust environment of R CrB. The observations were carried out with the Infrared Optical Telescope Array (IOTA), using our new JHK' beam combiner which enables us to record fringes in the J, H, and K' bands simultaneously. The circumstellar dust envelope of R CrB is resolved at a baseline of 21 m along a position angle of ~ 170deg, and the visibilities in the J, H, and K' bands are 0.97 +/- 0.06, 0.78 +/- 0.06, and 0.61 +/- 0.03, respectively. These observed visibilities, together with the K'-band visibility obtained by speckle interferometry with baselines of up to 6 m, and the spectral energy distribution are compared with predictions from spherical dust shell models which consist of the central star and an optically thin dust shell. The comparison reveals that the observed J- and H-band visibilities are in agreement with those predicted by these models, and the inner radius and inner boundary temperature of the dust shell were derived to be 60-80 Rstar and 950-1050 K, respectively. However, the predicted K'-band visibilities are found to be ~ 10% smaller than the one obtained with IOTA. Given the simplifications adopted in our models and the complex nature of the object, this can nevertheless be regarded as rough agreement. As a hypothesis to explain this small discrepancy, we propose that there might be a group of newly formed dust clouds, which may appear as a third visibility component.