Destructuring the disk of AB Aurigae: Dynamics and accretion

¹ LIRA, Observatoire de Paris, Université PSL, CNRS, Sorbonne Université, Univ. Paris Diderot, Sorbonne Paris Cité, CY Cergy Paris Université, 5 place Jules Janssen, 92195 Meudon, France
² Laboratoire d’Astrophysique de Bordeaux, Université de Bordeaux, CNRS, B18N, Allée Geoffroy Saint-Hilaire, 33615 Pessac, France
³ Academia Sinica, Institute of Astronomy and Astrophysics, 11F of AS/NTU Astronomy-Mathematics Building, No.1, Sec. 4, Roosevelt Rd, Taipei, Taiwan
⁴ Laboratoire CEA, IRFU/DAp, AIM, Université Paris-Saclay, Université Paris Diderot, Sorbonne Paris Cité, CNRS, 91191 Gif-sur-Yvette, France
⁵ Colgate University, 13 Oak Dr., Hamilton, NY 13346, USA
⁶ Centro de Astrobiología (CAB), CSIC-INTA, ESAC Campus, Camino bajo del Castillo s/n, 28692 Madrid, Spain
⁷ Konkoly Observatory, HUN-REN Research Centre for Astronomy and Earth Sciences, MTA Centre of Excellence, Konkoly-Thege Miklós út 15-17, 1121 Budapest, Hungary
⁸ CNRS, IPAG, Univ. Grenoble Alpes, 38000 Grenoble, France
⁹ IRAM, 300 rue de la piscine, Domaine Universitaire, 38406 SaintMartin d’Hères, France
¹⁰ NASA Goddard Space Flight Center, Astrophysics Division, Greenbelt, MD, 20771, USA
¹¹ Observatoire de la Côte d’Azur, 96 Bd de l’Observatoire, 06304 Nice, France
¹² Department of Astronomy, Xiamen University, Xiamen, China
¹³ IRAP, Université de Toulouse, CNRS, Université Paul Sabatier, CNES, Toulouse, France
¹⁴ Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD, 21218, USA
¹⁵ Centre de recherche astrophysique de Lyon (CRAL), Université Claude Bernard Lyon 1, CNRS, ENS, 9 avenue Charles Andre, 69561 Saint Genis Laval, France
¹⁶ European Southern Observatory, Karl-Schwarzschild-Straße 2, 85748 Garching, Germany

^★ Corresponding author: This email address is being protected from spambots. You need JavaScript enabled to view it.

Received: 6 March 2026
Accepted: 27 April 2026

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 Hα 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 Hα 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 Hα 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 Hα, we derive a flux of about 8.22 × 10⁻¹⁵ erg/s/cm² for the entire feature f1, but only 6.46 × 10⁻¹⁶ erg/s/cm² 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 Hα 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 Hα emissions in the disk. Overall, these observations strongly argue for an active and complex phase of planet formation in this system.