High-Resolution Polarimetry of AB Aurigae: Insights into Protoplanetary Disk Structures
The paper presents a detailed examination of the protoplanetary disk around the Herbig Ae star AB Aurigae utilizing high-resolution polarized intensity imaging at the Subaru Telescope. Researchers employed the HiCIAO instrument, coupled with dual-beam polarimetry, to capture complex structures within the disk spanning radial distances from 22 to 554 AU. The analysis reveals intricate and non-axisymmetric formations in the inner disk regions, alongside previously identified spiral structures in the outer parts. This study provides essential insights into the disk’s architecture and its potential implications for giant planet formation mechanisms far from the central star.
Key Observations and Findings
The imaging results exhibit several significant features of AB Aurigae's disk, which are crucial for advancing our understanding of planet formation scenarios:
Double Ring Structure: The observations uncovered two distinct ellipse rings at radii approximately 40 AU and 100 AU. These rings display differing inclination angles, suggesting potential warping within the disk. An offset between the outer ring’s geometric center and the central star further indicates geometrical disk effects.
Ring-like Gap: Between the rings, a pronounced gap was observed, with characteristics reminiscent of disk gaps reported in other Herbig Be stars. This gap, especially its far-side wall, may correspond to previously noted mid-infrared structure, posing intriguing questions about its origins.
Non-Axisymmetric Dips and Peaks: Seven small dips within the rings were identified, along with three prominent polarized intensity peaks. Notably, Dip A aligns with features reported in earlier studies and is confirmed in both polarized and total intensity imaging. These localized structures support the presence of disk perturbations potentially induced by unseen planetary bodies.
Absence of Point Sources: The study did not detect any point-like sources within the dips, contrary to previous claims. The derived 5σ upper mass limits for potential companions are consistent with simulations predicting unseen planetary masses within the disk.
Implications and Theoretical Considerations
The complex structures observed in AB Aurigae's disk may originate from several theoretical processes:
Gravitational Instability (GI): While initially considered, optically thin sub-millimeter observations suggest that GI is unlikely in the current phase of the disk due to high Toomre's Q-parameter values.
Embedded Planetary Perturbations: The presence of one or more unseen planets could generate the observed anomalies through gravitational interactions, forming gaps and inducing disk warps. The constraints on the planet mass required to open such gaps are aligned with limits derived by the study, supporting this hypothesis.
Magneto-Rotational Instability (MRI): This mechanism may drive perturbations across the disk surface and contribute to the observed non-axisymmetric formations. MRI-induced variability would occur on the order of the local rotation timescale, distinguishing it from planet-induced patterns.
Future Directions
To refine our understanding of AB Aurigae’s disk and its evolution, ongoing monitoring to detect time variability in the disk structures is essential. Such observations could clarify the formation mechanisms of wide-orbit companions recently imaged around various stellar types. Additionally, they could probe the dynamics of ongoing processes within active protoplanetary disks, offering valuable insights into planet-formation timescales and conditions in such environments.
This paper significantly contributes to the discourse on circumstellar disk dynamics and the intricate mechanisms governing planet formation beyond traditional orbital zones, enhancing our comprehension of the processes shaping celestial systems.