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

## TL;DR

This study used ALMA to image and analyze a large debris disk around the nearby star HD 170773, revealing its structure, size, and gas content, and exploring its relation to planetary systems.

## Contribution

First detailed millimeter observations and modeling of the debris disk around HD 170773, highlighting its unusually large size and potential link to giant planets.

## Key findings

- The debris belt has a radius of about 193 au, larger than typical for similar stars.
- HD 170773's belt size exceeds the expected relation with stellar luminosity.
- Upper limits on CO and CN gas suggest cometary ice compositions similar to the Solar System.

## Abstract

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.