Mid-infrared interferometric variability of DG Tauri: Implications for the inner-disk structure

Abstract

Context. DG Tau is a low-mass pre-main sequence star, whose strongly accreting protoplanetary disk exhibits a so-far enigmatic behavior: its mid-infrared thermal emission is strongly time-variable, even turning the 10 µm silicate feature from emission to absorption temporarily. Aims. We look for the reason for the spectral variability at high spatial resolution and at multiple epochs. Methods. Infrared interferometry can spatially resolve the thermal emission of the circumstellar disk, also giving information about dust processing. We study the temporal variability of the mid-infrared interferometric signal, observed with the VLTI/MIDI instrument at six epochs between 2011 and 2014. We fit a geometric disk model to the observed interferometric signal to obtain spatial information about the disk. We also model the mid-infrared spectra by template fitting to characterize the profile and time dependence of the silicate emission. We use physically motivated radiative transfer modeling to interpret the mid-infrared interferometric spectra. Results. The inner disk (r < 1 − 3 au) spectra exhibit a 10 µm absorption feature related to amorphous silicate grains. The outer disk (r > 1 − 3 au) spectra show a crystalline silicate feature in emission, similar to the spectra of comet Hale-Bopp. The striking difference between the inner and outer disk spectral feature is highly unusual among T Tauri stars. The mid-infrared variability is dominated by the outer disk. The strength of the silicate feature changed by more than a factor of two. Between 2011 and 2014 the half-light radius of the mid-infrared-emitting region decreased from 1.15 to 0.7 au. Conclusions. For the origin of the absorption we discuss four possible explanations: a cold obscuring envelope, an accretion heated inner disk, a temperature inversion on the disk surface and a misaligned inner geometry. The silicate emission in the outer disk can be explained by dusty material high above the disk plane, whose mass can change with time, possibly due to turbulence in the disk.

VLTI/MIDI atlas of disks around low- and intermediate-mass young stellar objects

“…Signs of dust evolution were later observed in low-mass systems as well (Schegerer et al 2008(Schegerer et al , 2009Ratzka et al 2009). Varga et al (2017), however, found that the outer disk of DG Tau shows a crystalline silicate emission feature, while the inner disk shows amorphous absorption.…”

“…Examples are Schegerer et al (2008) on RY Tau, Schegerer et al (2009) on a sample consisting of DR Tau, RU Lup, S CrA N, S CrA S, HD 72106, HBC 639, and GW Ori, Juhász et al (2012) on EX Lup, Mosoni et al (2013) on V1647 Ori, Schegerer et al (2013) on HD 142666, AS 205 N, and AS 205 S, Menu et al (2014) on TW Hya, Scicluna et al (2016) on VV CrA, and Brunngräber et al (2016) on DR Tau. Varga et al (2017) applied both geometric and radiative transfer modeling to study the variable silicate emission of DG Tau. The presence of gaps in the inner disks of several objects were also revealed (Ratzka et al 2007;Schegerer et al 2009Schegerer et al , 2013.…”

Planetesimals around stars with TESS (PAST) – I. Transient dimming of a binary solar analogue at the end of the planet accretion era

“…(Vural et al 2014;Davies et al 2018a,b). Alternative explanations for dips include dust lofted in disk winds, for which there is indirect support in other systems (Varga et al 2017;Fernandes et al 2018;Ellerbroek et al 2014), gravitationally bound clumps of planetesimals (Ansdell et al 2016b), or disintegrating comet-like planetesimals (Kennedy et al 2017).…”

Grain growth in newly discovered young eruptive stars

“…We adopted a model fitting procedure similar to that used by Varga et al (2017) for the T Tauri-type star DG Tau. We model the spectra as a combination of a linear continuum and one of the three template features which can be either in absorption or in emission.…”