STIS Imaging of the HR 4796A Circumstellar Debris Ring

TL;DR

This study uses Hubble Space Telescope imaging to analyze the HR 4796A debris ring, revealing asymmetries, radial confinement, and potential planetary influences on the disk's structure and composition.

Contribution

First high-resolution optical imaging of HR 4796A's debris ring, providing detailed geometrical, brightness, and compositional analysis, suggesting planetary perturbations.

Findings

Detected brightness asymmetries and forward scattering in the debris ring.

Constrained the ring's radial extent to within 14 AU with steep inner truncation.

Indications of possible unseen planetary-mass perturbers affecting the disk.

Abstract

We have obtained high spatial resolution imaging observations of the HR 4796A circumstellar debris dust ring using the broad optical response of the Hubble Space Telescope Imaging Spectrograph in coronagraphic mode. We use our visual wavelength observations to improve upon the earlier measured geometrical parameters of the ring-like disk. Two significant flux density asymmetries are noted: (1) preferential forward scattering by the disk grains and (2) an azimuthal surface brightness anisotropy about the morphological minor axis of the disk with corresponding differential ansal brightness. We find the debris ring offset from the location of the star by ~1.4 AU, a shift insufficient to explain the differing brightnesses of the NE and SW ansae simply by the 1/$r^2$ dimmunition of starlight. The STIS data also better quantify the radial confinement of the starlight-scattering circumstellar debris, to a characteristic region <14 AU in photometric half-width, with a significantly steeper inner truncation than outward falloff in radial surface brightness. The inferred spatial distribution of the disk grains is consistent with the possibility of one or more unseen co-orbital planetary-mass perturbers, and the colors of the disk grains are consistent with a collisionally evolved population of debris, possibly including ices reddened by radiation exposure to the central star.