A sub-AU outwardly truncated accretion disk around a classical T Tauri star

TL;DR

This study analyzes the infrared spectrum of the SR20 binary T Tauri system, revealing an unusually small, outwardly truncated disk with significant grain growth, likely caused by gravitational perturbations in the 1990s.

Contribution

It provides the first detailed spectral analysis showing a sub-AU truncated disk around a T Tauri star, with evidence of grain growth and past gravitational disturbance.

Findings

Disk is truncated at about 0.39 AU, smaller than binary orbit predictions.

Spectral features indicate significant grain growth (>1 μm).

Mid-infrared flux increased notably between 1993 and 1996.

Abstract

We present the Spitzer Infrared Spectrograph (IRS) spectrum of SR20, a 5--10 AU binary T Tauri system in the $\rho$ Ophiuchi star forming region. The spectrum has features consistent with the presence of a disk; however, the continuum slope is steeper than the $\lambda^{-4/3}$ 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$_{+0.03}^{-0.01}$ AU, much smaller than predicted by models of binary orbits. Using a two temperature $\chi^2$ minimization model to fit the dust composition of the IRS spectrum, we find the disk has experienced significant grain growth: its spectrum is well-fit using opacities of grains larger than 1 $\mu$m. We conclude that the system experienced a significant gravitational perturbation in the 1990s.