JWST-MIRI Spectroscopy of Warm Molecular Emission and Variability in the AS 209 Disk

We present MIRI MRS observations of the large, multi-gapped protoplanetary disk around the T-Tauri star AS 209. The observations reveal hundreds of water vapor lines from 4.9 to 25.5 μm towards the inner ∼1 au in the disk, including the first detection of ro-vibrational water emission in this disk. The spectrum is dominated by hot (∼800 K) water vapor and OH gas, with only marginal detections of CO_2, HCN, and a possible colder water vapor component. Using slab models with a detailed treatment of opacities and line overlap, we retrieve the column density, emitting area, and excitation temperature of water vapor and OH, and provide upper limits for the observable mass of other molecules. Compared to MIRI spectra of other T-Tauri disks, the inner disk of AS 209 does not appear to be atypically depleted in CO_2 nor HCN. Based on Spitzer IRS observations, we further find evidence for molecular emission variability over a 10-year baseline. Water, OH, and CO_2 line luminosities have decreased by factors 2-4 in the new MIRI epoch, yet there are minimal continuum emission variations. The origin of this variability is yet to be understood.