Continuum maps at 850 μm showing the results of the fits of the visibilities. The top left panel shows the observations, the two panels in the center the best-fit model, and the two panels on the right the residuals. The red solid curve represents the CO snowline at 90 AU. Panel a): we use a classical two-layer disk model that implements a monotonically decreasing surface brightness. Ring-shaped residuals are clearly visible. Panel b): we use the two layer disk model with an additional Gaussian peak. We fit the peak location (R_p = 96 AU) and width FWHM_p = 58 AU. The residuals are substantially reduced. The parameters used for the CLEAN are the same as discussed in Sect. 2, and the contour levels are the same as those used in Figs. 1 and 9.