The REASONS Survey: Resolved Millimeter Observations of a Large Debris Disk Around the Nearby F Star HD 170773

Debris disksare extrasolar analogs to our own Kuiper Beltand they are detected around at least 17% of nearby Sun-like stars. The morphology and dynamics of a disk encode information about its history, as well as that of any exoplanetswithin the system. We used ALMA to obtain 1.3 mm observations of the debris diskaround the nearby F5V star HD 170773. We image the face-on ring and determine its fundamental parameters by forward-modeling the interferometric visibilitiesthrough a Markov Chain Monte Carlo approach. Using a symmetric Gaussian surfacedensity profile, we find a 71 $\pm$ 4 au wide beltwith a radius of 193$^{+2}_{-3}$ au, a relatively large radius compared to most other millimeter-resolved beltsaround late A / early F type stars. This makes HD 170773 part of a group of four disks around A and F stars with radii larger than expected from the recently reported planetesimal beltradius - stellar luminosity relation. Two of these systems are known to host directly imaged giant planets, which may point to a connection between large beltsand the presence of long-period giant planets. We also set upper limits on the presence of CO and CN gas in the system, which imply that the exocometsthat constitute this belthave CO and HCN ice mass fractions of <77% and <3%, respectively, consistent with Solar System comets and other exocometary belts.