Images of the Extended Outer Regions of the Debris Ring Around HR 4796 A

TL;DR

This study uses high-contrast imaging to analyze the debris disk around HR 4796 A, revealing detailed disk structure, asymmetries, and potential signs of unseen planets through advanced imaging techniques and modeling.

Contribution

First high-contrast H-band images of HR 4796 A's debris disk with detailed analysis of its structure and potential planetary influences, using specialized ADI methods.

Findings

Resolved the debris disk's inner edge with high precision.

Detected streamers extending from the disk ansae.

Identified offsets in the disk suggestive of unseen planets.

Abstract

We present high-contrast images of HR 4796 A taken with Subaru/HiCIAO in H-band, resolving the debris disk in scattered light. The application of specialized angular differential imaging methods (ADI) allows us to trace the inner edge of the disk with high precision, and reveals a pair of "streamers" extending radially outwards from the ansae. Using a simple disk model with a power-law surface brightness profile, we demonstrate that the observed streamers can be understood as part of the smoothly tapered outer boundary of the debris disk, which is most visible at the ansae. Our observations are consistent with the expected result of a narrow planetesimal ring being ground up in a collisional cascade, yielding dust with a wide range of grain sizes. Radiation forces leave large grains in the ring and push smaller grains onto elliptical, or even hyperbolic trajectories. We measure and characterize the disk's surface brightness profile, and confirm the previously suspected offset of the disk's center from the star's position along the ring's major axis. Furthermore, we present first evidence for an offset along the minor axis. Such offsets are commonly viewed as signposts for the presence of unseen planets within a disk's cavity. Our images also offer new constraints on the presence of companions down to the planetary mass regime (~9 Jupiter masses at 0.5", ~3 Jupiter masses at 1").