2017MNRAS.466.4201W

We present Atacama Large Millimeter/submillimeter Array (ALMA) Band 6 observations (1.3 mm/233 GHz) of Fomalhaut and its debris disc. The observations achieve a sensitivity of 17 µJy 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 full width at half-maximum of 13 ± 3 au, following a Gaussian profile. The millimetre spectral index is constrained to α_mm = -2.73 ± 0.13. 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 ± 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 per cent of that predicted by extrapolating a blackbody 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.