Letter to the Editor

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

¹ Leiden Observatory, PO Box 9513, 2300 RA Leiden, The Netherlands e-mail: vcgeers@strw.leidenuniv.nl
² Hubble Fellow, Division of GPS, Mail Code 150-21, California Institute of Technology, Pasadena, CA 91125, USA
³ Max-Plank-Institut für Astronomie, Koenigstuhl 17, 69117 Heidelberg, Germany
⁴ Laboratoire d'Astrophysique de l'Observatoire de Grenoble, BP 53, 38041 Grenoble Cedex 9, France
⁵ Research and Scientific Support Department (ESTEC/ESA), Keplerlaan 1, 2200 AG Noordwijk, The Netherlands
⁶ Kapteyn Astronomical Institute, Landleven 12, 9747 AD Groningen, The Netherlands

Received: 22 March 2007
Accepted: 28 April 2007

Abstract

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 μm, 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 μm 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.

Results.The 18.7 μm ring peaks at a diameter of 110 AU, with a gap of ~60 AU. The shape of the ring is consistent with an inclination of i = 48° ± 8°. In contrast, the 7.5–13 μm 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 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.