Work

Destructuring the disk of AB Aurigae: Dynamics and accretion

Year: 2026

Type: article

Abstract: Context . The young disk around AB Aur features a complex assembly of spiral arms, several compact structures, and a protoplanet candidate, AB Aur b, suggesting ongoing planet formation in this young system. Because of its brightness and spatial extent, AB Aur represents a perfect laboratory for investigating the conditions under which planets start to form around intermediate-mass stars. Aims . In this paper, we present near-IR polarized images of the AB Aur disk at three epochs spanning 3.85 years with SPHERE/IRDIS, as well as H α images obtained with SPHERE/ZIMPOL at a single epoch. The purpose of this study is to analyze the dynamics of the entire disk and of the various structures in near-IR polarimetry, and to identify sources of H α emission to derive constraints on their mass accretion rate. Methods . We developed a method to measure the rotation of the disk as a function of the radius, covering physical separations from as close as ~25 au up to 400 au. We applied this method to the global structure of the disk as well as to specific features of interest, including both extended or compact sources. For the compact sources, we performed orbital analyses. We also studied the variability of shadows seen as thin radial streaks. For the H α data, we extracted photometric measurements of several features and derived estimations of the accretion luminosities and mass accretion rates, assuming three different accretion models. Results . The dynamical study in the near-IR shows that the disk globally follows Keplerian rotation, but we observe a departure from this behavior at radii smaller than ~60 au. At the smallest radius of ~25 au, we measure a deviation from Keplerian rotation as large as ~12° over 3.85 years, demonstrating sub-Keplerian rotation. The two bright spirals within the millimeter cavity have different dynamic trends, and we discuss their possible link with the identified planet candidates. We also discuss the implications of the non-Keplerian behavior, and we posit that it could be related to interactions with multiple protoplanets orbiting out of the disk plane on elliptical orbits. Furthermore, the orbital analysis of the compact sources (labeled f1 , f2 , and f3 ) suggests that their orbital planes are significantly inclined with respect to the disk plane by several tens of degrees. The variability of the shadows suggests that they are produced by optically thick regions located within ~60 au. For the photometric analysis in H α , we derive a flux of about 8.22 × 10 −15 erg/s/cm 2 for the entire feature f1 , but only 6.46 × 10 −16 erg/s/cm 2 at the location of AB Aur b, consistent with non-detection. If f1 were a point source and the accretion remained constant for 1 Myr, it would correspond to ~5-20 Jupiter masses according to the magnetospheric accretion model or ~6-10 Jupiter masses according to the boundary layer accretion model. We further discuss the non-detection of H α emission on AB Aur b. Finally, we discuss the binarity of the host star, in particular using Gaia measurements. Conclusions . AB Aur is a rare system in which the morphology and dynamics can be studied at a very high level of detail, contrasting with the generic picture of a young planet-forming disk. The excellent image quality of SPHERE, both in the near-IR and in the visible, allows us to track the disk rotation with unprecedented precision thanks to the stability of the instrument across several years and to study localized H α emissions in the disk. Overall, these observations strongly argue for an active and complex phase of planet formation in this system.

Source: Astronomy and Astrophysics

DOI: 10.1051/0004-6361/202659736