Image of Fomalhaut Dust Ring at 350 Microns: Relative Column Density Map Shows Pericenter-Apocenter Asymmetry

Abstract

We have imaged the circumstellar disk of Fomalhaut at 350 microns wavelength, using SHARC II at the Caltech Submillimeter Observatory. The spatial resolution of the raw images (9 arcsec) has been enhanced by a factor of three using the HiRes deconvolution procedure. We find that at this wavelength and signal to noise ratio (~ 12), the observed morphology is that of a simple inclined ring (i ~ 70 deg), with little or no other apparent structure; this is the first observation that shows clearly the ring morphology of the disk. We have combined our 350 micron data with Spitzer Space Telescope images at 24, 70, and 160 microns in order to estimate the 2-dimensional spatial variation of relative column density ("tau map") using our DISKFIT procedure. The tau map is based on the following physical assumptions: (1) the wavelength variation of opacity is the same throughout the disk, (2) the radial variation of dust temperature is dictated by the energy balance of individual grains in the stellar radiation field, and (3) the vertical scale height of the disk follows a power-law radial variation. The results confirm the ring-like morphology, but also show that the geometric center is displaced from the star by about 8 AU and that the ring has an apocentric enhancement of approximately 14% in integrated column density. If we interpret the displacement in terms of elliptical orbital motion due to gravitational perturbation by an unseen planet, then the implied forced eccentricity is ~ 0.06; dynamical modeling then predicts an apocentric density enhancement consistent with that inferred from the tau map.