No Transition Disk? Infrared Excess, PAH, H2, and X-rays from the Weak-Lined T Tauri Star DoAr 21

TL;DR

This study investigates the properties of the weak-lined T Tauri star DoAr 21, revealing extended PAH and H2 emission likely excited by UV radiation, suggesting a small-scale photodissociation region rather than a traditional protoplanetary disk.

Contribution

It provides detailed multi-wavelength observations of DoAr 21, demonstrating the presence of extended UV-excited emission features and challenging the assumption of a disk around such young stars.

Findings

Extended PAH emission is spatially resolved and asymmetric.

X-ray emission is very hard and dominated by continuum.

No significant excess mid-infrared emission within 100 AU.

Abstract

We present new high-resolution mid-infrared imaging, high-resolution optical spectroscopy, and Chandra grating X-ray spectroscopy of the weak-lined T Tauri star DoAr 21. DoAr 21 (age < 10^6 yr and mass ~ 2.2 M_sun) is a strong X-ray emitter, with conflicting evidence in the literature about its disk properties. It shows weak but broad H-alpha emission; polarimetric variability; PAH and H2 emission; and a strong, spatially-resolved 24-micron excess in archival Spitzer photometry. Gemini sub-arcsecond-resolution 9--18 micron images show that there is little or no excess mid-infrared emission within 100 AU of the star; the excess emission is extended over several arcseconds, is quite asymmetric, and is bright in the UV-excited 11.3 micron PAH emission feature. A new high-resolution X-ray grating spectrum from Chandra shows that the stellar X-ray emission is very hard and dominated by continuum emission; it is well-fit by a multi-temperature thermal model and shows no evidence of unusually high densities. We argue that the far-ultraviolet emission from the star's transition region is sufficient to excite the observed extended PAH and continuum emission, and that the H2 emission may be similarly extended and excited. This extended emission may be more akin to a small-scale photodissociation region than a protoplanetary disk, highlighting both the very young ages (< 10^6 yr) at which some stars are found without disks, and the extreme radiation environment around even late-type pre--main-sequence stars.