2021ApJ...908..171N

Physical processes that redistribute or remove angular momentum from protoplanetary disks can drive mass accretion onto the star and affect the outcome of planet formation. Despite ubiquitous evidence that protoplanetary disks are engaged in accretion, the process(es) responsible remain unclear. Here we present evidence for redshifted molecular absorption in the spectrum of a Class I source that indicates rapid inflow at the disk surface. High-resolution mid-infrared spectroscopy of GV Tau N reveals a rich absorption spectrum of individual lines of C₂H₂, HCN, NH₃, and H₂O. From the properties of the molecular absorption, we can infer that it carries a significant accretion rate {dot}M_abs ∼ 10^–8-10^–7 M_☉ yr^–1, comparable to the stellar accretion rates of active T Tauri stars. Thus, we may be observing disk accretion in action. The results may provide observational evidence for supersonic "surface accretion flows," which have been found in MHD simulations of magnetized disks. The observed spectra also represent the first detection of NH₃ in the planet formation region of a protoplanetary disk. With NH₃ only comparable in abundance to HCN, it cannot be a major missing reservoir of nitrogen. If, as expected, the dominant nitrogen reservoir in inner disks is instead N₂, its high volatility would make it difficult to incorporate into forming planets, which may help to explain the low nitrogen content of the bulk Earth.