Resolving the Planetesimal Belt of HR 8799 with ALMA

TL;DR

This study uses ALMA observations to resolve the inner edge of the debris disc around HR 8799, revealing a broad ring that challenges simple planetary interaction models.

Contribution

First direct resolution of the inner edge of HR 8799's planetesimal belt using ALMA band 6 data and MCMC fitting.

Findings

Disc extends from ~145 AU to ~429 AU.

Disc inclination is approximately 40 degrees.

Inner edge at 145 AU suggests complex dynamics or additional planets.

Abstract

The star HR 8799 hosts one of the largest known debris discs and at least four giant planets. Previous observations have found evidence for a warm belt within the orbits of the planets, a cold planetesimal belt beyond their orbits and a halo of small grains. With the infrared data, it is hard to distinguish the planetesimal belt emission from that of the grains in the halo. With this in mind, the system has been observed with ALMA in band 6 (1.34 mm) using a compact array format. These observations allow the inner edge of the planetesimal belt to be resolved for the first time. A radial distribution of dust grains is fitted to the data using an MCMC method. The disc is best fit by a broad ring between $145^{+12}_{-12}$ AU and $429^{+37}_{-32}$ AU at an inclination of $40^{+5}_{-6}${\deg} and a position angle of $51^{+8}_{-8}${\deg}. A disc edge at ~145 AU is too far out to be explained simply by interactions with planet b, requiring either a more complicated dynamical history or an extra planet beyond the orbit of planet b.