ALMA Traces Trillion-Mile Gas Streamer Feeding GW Orionis

A gas streamer roughly 1 trillion miles long — about 0.2 light-years — is feeding material onto the misaligned protoplanetary disk of GW Orionis, according to new observations from the Atacama Large Millimeter/submillimeter Array (ALMA). The streamer’s trajectory closely matches the tilt of the system’s outer dust ring, giving astronomers their clearest evidence yet that gas falling in from outside a young star system can warp the disk where planets form.

A Triple Star System With a Twisted History

GW Orionis, a young triple-star system roughly 1,300 light-years away in the constellation Orion, has puzzled astronomers since ALMA and the European Southern Observatory’s Very Large Telescope first resolved its protoplanetary disk into three separate dust rings, each tilted at a different angle rather than lying flat as in most planet-forming systems.

The new study, led by Maria Galloway-Sprietsma, a Ph.D. candidate at the University of Florida, used all three of ALMA’s antenna arrays — the 12-meter, 7-meter, and Total Power arrays — to capture the streamer’s full extent for the first time, building on years of archival ALMA data that had previously resolved only the high-resolution rings close to the stars.

Matching the Streamer to the Tilted Ring

Galloway-Sprietsma’s team measured the streamer’s angular momentum — the physical quantity describing how its gas is moving and rotating — and compared it to the orientation of GW Orionis’s rings. The streamer’s trajectory lines up closely with the disk’s outer dust ring but is strongly misaligned with the inner ring.

“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,” Galloway-Sprietsma said, pointing to a likely cause-and-effect relationship between the infalling gas and the outer ring’s tilt.

What the Molecular Line Data Shows

The team traced the streamer using two molecular tracers, 12CO and 13CO, which let them measure the gas’s motion in three dimensions rather than relying on dust images alone. Key figures from the study:

• Streamer length: approximately 1 trillion miles, or 0.2 light-years
• Molecular tracers used: 12CO and 13CO
• Angular momentum of the streamer: lower than that of the GW Orionis disk itself

That lower angular momentum indicates the streamer represents a later stage of the infall process. Earlier in the system’s history, said Jaehan Bae, a professor of astronomy at the University of Florida and Galloway-Sprietsma’s Ph.D. advisor, the streamer likely carried more angular momentum — enough to tilt the disk into its current misaligned configuration.

ALMA and the Case for Dynamic Disk Formation

The finding challenges a long-standing picture in which planet-forming disks evolve quietly and symmetrically around a single star. “None of this would have been possible without ALMA,” Galloway-Sprietsma said, noting that the observatory’s resolution let the team combine archival dust-ring imaging with new, wide-field gas kinematics to reconstruct the streamer’s extent.

“If these streamers are common, then we can naturally explain why planets may not necessarily end up in very orderly systems,” said Bae. “They can have much more random orientations.” Astronomers had previously attributed GW Orionis’s misaligned rings mainly to the gravitational pull of its three stars; the new data indicate the external gas streamer better explains the outer ring’s tilt.

The findings, which included co-authors from the Max-Planck Institute for Astronomy in Germany, the University of Galway in Ireland, and Queen Mary University of London, were published August 6 in The Astronomical Journal.

What Comes Next

The team plans further ALMA observations of GW Orionis to search for shock-tracing molecules, including sulfur-bearing species, that would pinpoint exactly where the streamer collides with the disk. Astronomers also intend to survey additional young star systems for similar streamers. “What we need next is a systematic survey of young stars to see how many have streamers and how many don’t,” Galloway-Sprietsma said. “That will tell us how important they are in shaping planetary systems.”

The GW Orionis result adds to a growing body of evidence that protoplanetary disks are not static formation environments but continue to be reshaped by material falling in from their surrounding cloud well into their evolution. That reshaping, astronomers say, may help explain why some exoplanets are found on orbits tilted relative to, or even opposite, the spin of their host star.

Source: National Radio Astronomy Observatory (NRAO) / University of Florida. Published in The Astronomical Journal. Primary source: news.ufl.edu. Additional source: newswise.com (NRAO press release).