Spatial separation of small and large grains in the transitional disk around the young star IRS 48

Aims. We present spatially resolved mid-infrared images of the disk surrounding the young star IRS 48 in the Ophiuchus cloud complex. The disk exhibits a ring-like structure at 18.7 mu m, and is dominated by very strong emission from polycyclic aromatic hydrocarbons at shorter wavelengths. This allows a detailed study of the relative distributions of small and large dust grains. Methods. Images of IRS 48 in 5 mid-infrared bands from 8.6 to 18.7 mu m as well as a low resolution N-band spectrum are obtained with VLT-VISIR. Optical spectroscopy is used to determine the spectral type of the central star and to measure the strength of the H alpha line. Results. The 18.7 mu m ring peaks at a diameter of 110 AU, with a gap of similar to 60 AU. The shape of the ring is consistent with an inclination of i = 48 degrees +/- 8 degrees. In contrast, the 7.5 - 13 mu m PAH emission bands are centered on the source and appear to fill the gap within the ring. The measured PAH line strengths are 10 - 100 x stronger than those typically measured for young M0 stars and can only be explained with a high PAH abundance and/or strong excess optical/UV emission. The morphology of the images, combined with the absence of a silicate emission feature, imply that the inner disk has been cleared of micron-sized dust but with a significant population of PAHs remaining. We argue that the gap can be due to grain growth and settling or to clearing by an unseen planetary or low-mass companion. IRS 48 may represent a short-lived transitional phase from a classical to a weak-line T Tauri star.