ALMA observations of the narrow HR 4796A debris ring

TL;DR

This study uses high-resolution ALMA imaging to analyze the structure and composition of the narrow debris ring around HR 4796A, providing insights into its vertical extent, potential planetary influences, and dust properties.

Contribution

First high-resolution ALMA continuum images of HR 4796A's debris ring, revealing its radial and vertical structure and constraining the presence of CO gas and potential planetary perturbers.

Findings

The dust ring is approximately 80au in radius with a width of about 10au.

The disk is likely vertically thin, consistent with a dynamically cold state.

No CO J=3-2 emission detected, implying low ice content in planetesimals.

Abstract

The young A0V star HR 4796A is host to a bright and narrow ring of dust, thought to originate in collisions between planetesimals within a belt analogous to the Solar System's Edgeworth-Kuiper belt. Here we present high spatial resolution 880$\mu$m continuum images from the Atacama Large Millimeter Array. The 80au radius dust ring is resolved radially with a characteristic width of 10au, consistent with the narrow profile seen in scattered light. Our modelling consistently finds that the disk is also vertically resolved with a similar extent. However, this extent is less than the beam size, and a disk that is dynamically very cold (i.e. vertically thin) provides a better theoretical explanation for the narrow scattered light profile, so we remain cautious about this conclusion. We do not detect $^{12}$CO J=3-2 emission, concluding that unless the disk is dynamically cold the CO+CO$_2$ ice content of the planetesimals is of order a few percent or less. We consider the range of semi-major axes and masses of an interior planet supposed to cause the ring's eccentricity, finding that such a planet should be more massive than Neptune and orbit beyond 40au. Independent of our ALMA observations, we note a conflict between mid-IR pericenter-glow and scattered light imaging interpretations, concluding that models where the spatial dust density and grain size vary around the ring should be explored.