A Stellar Flyby Makes Some Waves


The gaseous, dusty disks surrounding newly born stars can reveal a wealth of information about how distant stellar systems form and evolve. In a new study, scientists have now watched the interaction of two such disks in a stellar flyby.

Spotting Spirals

In the past decade, new instrumentation has led to a dramatic improvement in our views of circumstellar environments. We’ve spotted remarkable structure in the dusty disks that surround newborn stars — including, in many cases, pronounced spiral arms.

MWC 758

An example of spiral arms detected in a protoplanetary disk, MWC 758. [NASA/ESA/ESO/M. Benisty et al]

The presence of these spiral arms has provoked much discussion and debate. Are they caused by gravitational instabilities in the gas and dust? Or are they produced by perturbations from unseen, newborn planets orbiting within the disks? While both of these explanations could be at play in different systems, there’s an additional possibility to consider: the arms could be excited by tidal interactions with another star.

In a new study led by Luis Zapata (UNAM Radio Astronomy and Astrophysics Institute, Mexico), a team of scientists has used the sensitive and high-angular-resolution observations of the Atacama Large Millimeter/submillimeter Array (ALMA), located in Chile, to understand how tidal interactions with an orbiting star might be responsible for spiral arms observed in UX Tauri.

A New Disk Found

Located ~450 light-years away, the UX Tauri system consists of four stars: UX Tau A (the main star), UX Tau B (a binary star), and UX Tau C (a close companion that lies just to UX Tau A’s south). Past observations have revealed a disk of gas and dust around UX Tau A exhibiting distinct spiral arms.

ALMA obs of UX Tau

The intensity (top) and radial velocities (bottom) of the molecular gas observed in UX Tauri reveals a disk around both UX Tau A (top star in both images) and UX Tau C (bottom star), as well as a stream of gas connecting the two. Curves tracing the spiral arms in the disk surrounding UX Tau A are overlaid in the top image. [Adapted from Zapata et al. 2020]

Zapata and collaborators have now followed up with detailed ALMA observations to explore the structure of the molecular gas and dust in UX Tau. In addition to further resolving the disk around UX Tau A, the team was also able to detect — for the first time — molecular gas swirling in a disk around UX Tau C. What’s more, the observations reveal tidal interactions between the two disks that surround these stars.

Drama in UX Tau

What do these findings mean? Zapata and collaborators suggest that we’re witnessing a close flyby of UX Tau C as it progresses on a wide, evolving, and eccentric orbit around the disk of UX Tau A. As UX Tau C plowed through UX Tau A’s circumstellar disk, it captured some of the gas, forming its own disk. Through its motion and this tidal interaction, UX Tau C also excited the observed spiral arms in UX Tau A’s disk.

The drama spotted in UX Tauri represents one of the few cases of binary disk interactions that have been mapped out in molecular gas — but this is likely a common occurrence, since stars often occur in multiple-star systems. Sensitive observations like the ALMA detections presented here will likely reveal more such interactions in the future, shining additional light on the process of star and planet formation.


“Tidal Interaction between the UX Tauri A/C Disk System Revealed by ALMA,” Luis A. Zapata et al 2020 ApJ 896 132. doi:10.3847/1538-4357/ab8fac