High-Resolution Mid-Infrared Spectroscopy of GV Tau N: Surface Accretion and Detection of Ammonia in a Young Protoplanetary Disk

TL;DR

This study uses high-resolution mid-infrared spectroscopy to detect surface accretion flows and ammonia in a young protoplanetary disk, providing insights into disk dynamics and nitrogen chemistry relevant to planet formation.

Contribution

It presents the first detection of ammonia in a protoplanetary disk and offers observational evidence for surface accretion flows consistent with MHD simulations.

Findings

Detection of redshifted molecular absorption indicating rapid inflow.

First observation of ammonia in a planet formation region.

Estimated accretion rate comparable to stellar accretion rates.

Abstract

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 water. From the properties of the molecular absorption, we can infer that it carries a significant accretion rate (~ 1e-8 to 1e-7 Msun/yr), 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 ammonia in the planet formation region of a protoplanetary disk. With ammonia 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.