1.3 mm ALMA Observations of the Fomalhaut Debris System

TL;DR

This paper presents high-resolution ALMA observations of Fomalhaut's debris disk at 1.3 mm, precisely constraining its structure, spectral index, and central emission, and discusses modeling approaches and implications for inner debris and stellar emission.

Contribution

The study provides the highest resolution millimetre observations of Fomalhaut's debris disk, accurately constrains its geometry and spectral index, and compares modeling techniques for debris disk analysis.

Findings

The debris ring is located at 139 au with a 13 au width.

The millimetre spectral index is -2.62.

Central emission is consistent with a point source, with flux 0.90 mJy.

Abstract

We present ALMA Band 6 observations (1.3 mm/233 GHz) of Fomalhaut and its debris disc. The observations achieve a sensitivity of 17 $\mu$Jy and a resolution of 0.28 arcsec (2.1 au at a distance of 7.66 pc), which are the highest resolution observations to date of the millimetre grains in Fomalhaut's main debris ring. The ring is tightly constrained to $139^{+2}_{-3}$ au with a FWHM of $13\pm3$ au, following a Gaussian profile. The millimetre spectral index is constrained to $\alpha_{mm} = -2.62\pm0.12$. We explore fitting debris disc models in the image plane, as well as fitting models using visibility data directly. The results are compared and the potential advantages/disadvantages of each approach are discussed. The detected central emission is indistinguishable from a point source, with a most probable flux of $0.90\pm 0.12$ mJy (including calibration uncertainties). This implies that any inner debris structure, as was inferred from far-Infrared observations, must contribute little to the total central emission. Moreover, the stellar flux is less than 70\% of that predicted by extrapolating a black body from the constrained stellar photosphere temperature. This result emphasizes that unresolved inner debris components cannot be fully characterized until the behaviour of the host star's intrinsic stellar emission at millimetre wavelengths is properly understood.