Resolved Millimeter Observations of the HR 8799 Debris Disk

TL;DR

This study uses millimeter observations from SMA and VLA to analyze the HR 8799 debris disk, revealing its structure, constraining the outer planet's mass, and characterizing dust grain properties consistent with collisional cascade models.

Contribution

It provides the first combined 1.3 mm and 9 mm observations of HR 8799's debris disk, constrains the inner belt edge, and estimates the outer planet's mass based on debris disk features.

Findings

The debris belt is broad and located at about 104 AU.

The outer planet's mass is estimated at approximately 5.8 Jupiter masses.

The grain size distribution follows a power-law index of about 3.27.

Abstract

We present 1.3 millimeter observations of the debris disk surrounding the HR 8799 multi-planet system from the Submillimeter Array to complement archival ALMA observations that spatially filtered away the bulk of the emission. The image morphology at $3.8$ arcsecond (150 AU) resolution indicates an optically thin circumstellar belt, which we associate with a population of dust-producing planetesimals within the debris disk. The interferometric visibilities are fit well by an axisymmetric radial power-law model characterized by a broad width, $\Delta R/R\gtrsim 1$. The belt inclination and orientation parameters are consistent with the planet orbital parameters within the mutual uncertainties. The models constrain the radial location of the inner edge of the belt to $R_\text{in}= 104_{-12}^{+8}$ AU. In a simple scenario where the chaotic zone of the outermost planet b truncates the planetesimal distribution, this inner edge location translates into a constraint on the planet~b mass of $M_\text{pl} = 5.8_{-3.1}^{+7.9}$ M$_{\rm Jup}$. This mass estimate is consistent with infrared observations of the planet luminosity and standard hot-start evolutionary models, with the uncertainties allowing for a range of initial conditions. We also present new 9 millimeter observations of the debris disk from the Very Large Array and determine a millimeter spectral index of $2.41\pm0.17$. This value is typical of debris disks and indicates a power-law index of the grain size distribution $q=3.27\pm0.10$, close to predictions for a classical collisional cascade.