A Sub-AU Outwardly Truncated Accretion Disk around a Classical T Tauri Star

Abstract

We present the Spitzer Infrared Spectrograph (IRS) spectrum of SR 20, a 5-10 AU binary T Tauri system in the ρ Ophiuchi star-forming region. The spectrum has features consistent with the presence of a disk; however, the continuum slope is steeper than the λ−4/3 slope of an infinite geometrically thin, optically thick disk, indicating that the disk is outwardly truncated. Comparison with photometry from the literature shows a large increase in the mid-infrared flux from 1993 to 1996. We model the spectral energy distribution and IRS spectrum with a wall+optically thick irradiated disk, yielding an outer radius of 0.39−0.01+ 0.03 AU, much smaller than predicted by models of binary orbits. Using a two-temperature χ2 minimization model to fit the dust composition of the IRS spectrum, we find that the disk has experienced significant grain growth: its spectrum is well fitted using opacities of grains larger than 1 μm. We conclude that the system experienced a significant gravitational perturbation in the 1990s.