A team of astronomers using the Atacama Large Millimeter/submillimeter Array (ALMA), of which the U.S. National Science Foundation National Radio Astronomy Observatory is a partner, has captured a massive stream of gas—one trillion miles, or .2 light years long—feeding the young triple-star system GW Orionis. These new observations provide the clearest evidence yet for how such “streamers” can tilt and twist the disks where planets are born.
These new ALMA observations focus on GW Orionis, a very young system located about 1,300 light-years away in the Orion constellation, that hosts three stars encircled by multiple rings of planet-forming material. The rings in this system are famously tilted at different angles instead of lying in a single flat plane, making GW Orionis a natural laboratory for studying the formation of unusual planetary architecture.
A team led by Maria Galloway-Sprietsma, a PhD candidate at the University of Florida, measured how the streamer is moving and compared its motion, known as its angular momentum, to the orientations of the system’s rings. They found that the streamer’s trajectory lines up closely with the outer dust ring but is strongly misaligned with the inner ring, pointing to a likely cause-and-effect connection between the infalling material and the outer ring’s tilted state.
“Previous studies of GW Orionis revealed that the system’s inner, middle and outer rings are misaligned, with each ring tilted at a different angle. When our team modeled the infall of this streamer, we found that the angle at which it impacts the disk is closely aligned with the outer ring,” explains Galloway-Sprietsma.
For decades, textbook diagrams have shown young planetary systems forming quietly from flat, orderly disks of gas and dust. This new ALMA result supports a more dynamic picture, in which clumpy, turbulent streamers from the surrounding environment can reshape disks late in their evolution and potentially set planets on orbits that are tilted or even opposite to the spin of their host star.
“None of this would have been possible without ALMA. ALMA has the highest resolution offered for these wavelengths. Archival data allowed astronomers to see these high-resolution dust rings to model their relative misalignments, and now our observations use all three ALMA arrays – the 12-meter, 7-meter, and Total Power – to zoom out and see the full extent of the streamer, so really the observations with ALMA have been building, year after year,” Galloway-Sprietsma says.
ALMA’s unique capabilities allowed Galloway-Sprietsma and her team to dig deeper into the data. “Because ALMA is such a sensitive instrument, we were able to study kinematics of the streamer with the molecular line data,” Galloway-Sprietsma adds. By observing molecular lines of 12CO and 13CO with ALMA , Galloway-Sprietsma and her team found that the total angular momentum of the streamer is much less than that of the GW Orionis disk. This means that the dynamics of this system likely represent the later stages of this infall phenomenon. “The estimated angular momentum of the streamer is less than that of the outer disk, so this would mean that the streamer should not significantly misalign the disk and further. In the past it likely had a greater angular momentum, and that allowed the disk to become misaligned,” points out Jaehan Bae, Assistant Professor Astronomy with the University of Florida, a co-author of the research and Galloway-Sprietsma’s PhD advisor.
Astronomers hope to survey more young systems with streamers to see how common this mechanism is and whether it can explain other puzzling features of known exoplanet systems, such as wild orbital tilts and unusual chemical signatures. Future ALMA observations of GW Orionis will search for shock-tracing molecules, including sulfur-bearing species, to pinpoint exactly where the streamer slams into the disk and how that impact alters the raw material for planet formation.