  Debris disks are extrasolar analogs to our own Kuiper Belt and 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
exoplanets within the system. We used ALMA to obtain 1.3 mm observations of the
debris disk around the nearby F5V star HD 170773. We image the face-on ring and
determine its fundamental parameters by forward-modeling the interferometric
visibilities through a Markov Chain Monte Carlo approach. Using a symmetric
Gaussian surface density profile, we find a 71 $\pm$ 4 au wide belt with a
radius of 193$^{+2}_{-3}$ au, a relatively large radius compared to most other
millimeter-resolved belts around 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 belt radius - stellar
luminosity relation. Two of these systems are known to host directly imaged
giant planets, which may point to a connection between large belts and 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 exocomets that constitute
this belt have CO and HCN ice mass fractions of <77% and <3%, respectively,
consistent with Solar System comets and other exocometary belts.
