SIMBAD references

As part of a program to understand disk dispersal and the interplay between circumstellar disks and X-ray emission, we present new high-resolution mid-infrared (IR) imaging, high-resolution optical spectroscopy, and Chandra grating X-ray spectroscopy of the weak-lined T Tauri star DoAr 21. DoAr 21 (age <10⁶ yr and mass ∼2.2 M_☉ based on evolutionary tracks) is a strong X-ray emitter, with conflicting evidence in the literature about its disk properties. It shows weak but broad Hα emission (reported here for the first time since the 1950s); polarimetric variability; polycyclic aromatic hydrocarbon (PAH) and H₂emission; and a strong, spatially resolved 24 µm excess in archival Spitzer photometry. Gemini sub-arcsecond-resolution 9-18 µm images show that there is little or no excess mid-IR emission within 100 AU of the star; the excess emission is extended over several arcseconds and is quite asymmetric. The extended emission is bright in the ultraviolet (UV)-excited λ = 11.3 µm 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, typical of hard coronal sources, and shows no evidence of unusually high densities. A flare during the X-ray observation shows a temperature approaching 10⁸ K. We argue that the far-UV emission from the transition region is sufficient to excite the observed extended PAH and continuum emission, and that the H₂ emission may be similarly extended and excited. While this extended emission may be a disk in the final stages of clearing, it also could be more akin to a small-scale photodissociation region than a protoplanetary disk, highlighting both the very young ages (<10⁶ yr) at which some stars are found without disks and the extreme radiation environment around even late-type pre-main-sequence stars.