Accretion in Action in an Angled Disk

How does material move through an accretion disk to the young star at its center? Surprising detections from a fortuitously angled disk have now provided new insights.

Driving Inflow

When stars are born from the collapse of a dense molecular cloud, they spend their early stages surrounded by circumstellar disks: disks of gas and dust that we understand to be accreting onto the young stars at their centers.

How do we know that the disk matter is flowing onto the stars? Evidence for accretion comes from the high-energy light emitted when inflowing material strikes the surface of young stars, producing accretion shocks. But, though these observations provide evidence that accretion is occurring, they don’t tell us much about the mechanisms that drive these flows within the disk.

For material to move inwards within a disk, it must first lose angular momentum — but where does that momentum go? What processes remove or redistribute it? In a new study led by Joan Najita (NSF’s NOIRLab), a team of scientists presents high-resolution observations of an unusual disk — one that happens to be angled in such a way as to help us answer these questions.

Lucky Alignment

Najita and collaborators used the TEXES spectrograph on the Gemini North 8-m telescope to conduct mid-infrared observations of GV Tau N, a young star surrounded by a nearly edge-on circumstellar disk. The authors’ observations revealed rare molecular absorption lines, a result of the nearly edge-on inclination of the disk.

The unique viewing angle for GV Tau N means that our sightline passes through the disk atmosphere in the inner few au of the disk — the region where planet formation is thought to occur. The molecules in this gas absorb some of the continuum light emitted by the interior disk, leaving signatures in the spectrum that provide valuable insight into the composition and motions of the gas at the surface of the inner disk.

Caught in the Act

Najita and collaborators found evidence for a variety of molecular species in the disk: acetylene (C₂H₂), hydrogen cyanide (HCN), water (H₂0), and even ammonia (NH₃), which has never before been detected in an inner accretion disk. But the especially interesting result is that these molecules’ absorption lines are redshifted, lying at longer wavelengths than expected if the gas were moving in a stable circular orbit.

This redshift is an indication that the gas observed is flowing rapidly (about 1 au per year) inward along the disk surface — direct evidence for accretion in action. The authors show that their observations match expected mass accretion rates for active T Tauri stars: roughly a few to a few tens of Earth masses per year. The observations fit neatly with a disk accretion model in which angular momentum is redistributed within the disk, causing surface gas to flow in and accrete while the midplane of the disk spreads outward.

GV Tau N is a lucky break — its orientation allowed us to make these unique measurements. But it’s surely not alone! With more observations of systems like GV Tau N, we’ll be able to further deepen our understanding of disk accretion.

Citation

“High-resolution Mid-infrared Spectroscopy of GV Tau N: Surface Accretion and Detection of NH3 in a Young Protoplanetary Disk,” Joan R. Najita et al 2021 ApJ 908 171. doi:10.3847/1538-4357/abcfc6