THE INNER DEBRIS STRUCTURE IN THE FOMALHAUT PLANETARY SYSTEM*

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 μ m (345 GHz) observations targeted at the inner part of the Fomalhaut system with a synthesized beam of 0.″45 × 0.″37 (∼3 AU linear resolution at the distance of Fomalhaut) and an rms of 26 μ Jy beam −1 . 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 σ are detected in the central 15 AU × 15 AU region. Modeling the spectral energy distribution using parameters expected for a dust-producing planetesimal belt indicates a radial location in the range of ∼8–15 AU. This is consistent with the location where ice sublimates in Fomalhaut, i.e., an asteroid-belt analog. The 3 σ upper limit for such a belt is <1.3 mJy at 870 μ m. We also interpret the 2 and 8–13 μ 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 nanograins.