What happens when a newly formed star captures material from a nearby cloud? A new study provides a glimpse into how the planet-forming system AB Aurigae is under renovation.
Late Infall and Planet Formation
Gas, dust, and rocky materials swirl around recently formed stars, creating a circumstellar disk that seeds the formation of planets. Recent observations of some of these disks reveal filamentary structures of material falling into the disk, feeding the planet-forming environment with new ingredients in a process known as “late-stage infall.” According to models, this infall can significantly impact the chemical composition and dynamics within the system — shock-heating materials, forming of pressure bumps or spiral arms, and inciting gravitational instability — which complicates the picture of planet formation.
One such system is AB Aurigae, a 3.9–4.4 million-year-old young stellar object whose circumstellar disk hosts several candidate protoplanets. Observations revealing spiral arm structures that extend outside of the disk and motions suspected to be associated with gravitational instability have led researchers to believe that AB Aurigae may be experiencing late-stage infall. However, the nature of the infall and its potential consequences remain unclear.
Finding Infall in AB Aurigae
ALMA observations showing intensity maps of the gas and dust in the circumstellar disk of AB Aurigae. Each panel shows a different wavelength with the distribution of the material changing depending on what emission is being observed. Click to enlarge. [Speedie et al 2025]
Through modeling the motions within the disk, the authors successfully separate the regularly rotating disk component from an infalling “exo-disk” component that shows distinct motions consistent with material falling into the disk. The team identified three exo-disk streams, with S1 and S2 being in front of the disk moving away from us into the system, and S3 being smaller and behind the disk moving toward us into the system. S1 and S2 seem to meet the disk in a “merging zone” where their emission becomes indistinguishable from the main disk. The authors note that this merging zone aligns with where planet candidates have been identified. These findings support the idea that AB Aurigae is undergoing a late-stage infall event.
The paths of the three identified infalling streams overlaid on the dust emission intensity maps. The merging zone is shown where S1 and S2 meet the disk, which aligns with an intensity peak in the ring of dusty emission. Click to enlarge. [Speedie et al 2025]
Implications and Impacts for AB Aurigae
Where does this infalling material come from, and how will it impact AB Aurigae? Previous optical observations show a reflection nebula of dusty material nearby, and the ALMA observations of this study show a faint emission structure that appears to align with the reflection nebula. Though further kinematic analysis is required, this suggests that the infall is coming from a small cloud that has been gravitationally captured by the star.
As the infalling material enters the disk, it can cause perturbations that create vortices and pressure bumps within the disk that trap material, facilitating the growth of protoplanets. Additionally, as this late-stage infall dumps gas and dust into the system, more material is available for planet formation. Continued studies of objects like AB Aurigae will reveal more about the dynamical impacts of late-stage infall on circumstellar disks and how those changes influence planet formation.
Citation
“Mapping the Merging Zone of Late Infall in the AB Aur Planet-forming System,” Jessica Speedie et al 2025 ApJL 981 L30. doi:10.3847/2041-8213/adb7d5