The Inner Debris Structure in the Fomalhaut Planetary System

TL;DR

This study uses high-resolution ALMA observations to investigate the inner debris structure of the Fomalhaut system, constraining the location and nature of potential asteroid-belt analogs and inner dust components.

Contribution

It provides the first high-resolution ALMA constraints on the inner debris structure of Fomalhaut, identifying a potential asteroid-belt region and connecting inner dust features to the outer belt.

Findings

Detected a point source consistent with stellar photosphere emission.

No bright structures detected within 15 AU, setting upper limits on dust emission.

Identified a potential asteroid-belt analog at 8-15 AU.

Abstract

Fomalhaut plays an important role in the study of debris disks and small bodies in other planetary systems. The proximity and luminosity of the star make key features of its debris, like the water ice-line, accessible. Here we present ALMA cycle 1, 870 \mu m (345 GHz) observations targeted at the inner part of the Fomalhaut system with a synthesized beam of 0.45"x0.37" (~3 AU linear resolution at the distance of Fomalhaut) and a rms of 26 \mu Jy/beam. The high angular resolution and sensitivity of the ALMA data enable us to place strong constraints on the nature of the warm excess revealed by Spitzer and Herschel observations. We detect a point source at the star position with a total flux consistent with thermal emission from the stellar photosphere. No structures that are brighter than 3\sigma\ are detected in the central 15 AU x 15 AU region. Modeling the spectral energy distribution using parameters expected for a dust-producing planetesimal belt indicates a radial location in the range ~8-15 AU. This is consistent with the location where ice sublimates in Fomalhaut, i.e., an asteroid-belt analog. The 3\sigma\ upper limit for such a belt is <1.3 mJy at 870 \mu m. We also interpret the 2 and 8-13 \mu m interferometric measurements to reveal the structure in the inner 10 AU region as dust naturally connected to this proposed asteroid belt by Poynting-Robertson drag, dust sublimation, and magnetically trapped nano grains.