Variations of the 10 um Silicate Features in the Actively Accreting T Tauri Stars: DG Tau and XZ Tau

TL;DR

This study used multi-epoch Spitzer observations to analyze the variability of the 10 um silicate features in T Tauri stars DG Tau and XZ Tau, revealing significant temporal changes likely caused by disk-related processes.

Contribution

It provides the first multi-epoch analysis of silicate feature variability in T Tauri stars, suggesting self-absorption effects and disk phenomena as causes of observed changes.

Findings

Silicate features vary significantly over months and years.

Pure emission models poorly fit the observed features.

Two-temperature, two-slab models suggest self-absorption causes variability.

Abstract

Using the Infrared Spectrograph aboard the Spitzer Space Telescope, we observed multiple epochs of 11 actively accreting T Tauri stars in the nearby Taurus-Auriga star forming region. In total, 88 low-resolution mid-infrared spectra were collected over 1.5 years in Cycles 2 and 3. The results of this multi-epoch survey show that the 10 um silicate complex in the spectra of two sources - DG Tau and XZ Tau - undergoes significant variations with the silicate feature growing both weaker and stronger over month- and year-long timescales. Shorter timescale variations on day- to week-long timescales were not detected within the measured flux errors. The time resolution coverage of this data set is inadequate for determining if the variations are periodic. Pure emission compositional models of the silicate complex in each epoch of the DG Tau and XZ Tau spectra provide poor fits to the observed silicate features. These results agree with those of previous groups that attempted to fit only single-epoch observations of these sources. Simple two-temperature, two-slab models with similar compositions successfully reproduce the observed variations in the silicate features. These models hint at a self-absorption origin of the diminution of the silicate complex instead of a compositional change in the population of emitting dust grains. We discuss several scenarios for producing such variability including disk shadowing, vertical mixing, variations in disk heating, and disk wind events associated with accretion outbursts.