2007A&A...469L..35G

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 18.7µm, 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. Images of IRS 48 in 5 mid-infrared bands from 8.6 to 18.7µm 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 Hα line. The 18.7µm ring peaks at a diameter of 110AU, with a gap of ∼60AU. The shape of the ring is consistent with an inclination of i=48°±8°. In contrast, the 7.5-13µm 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 times 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.