Resolving the terrestrial planet-forming region of HD 172555 with ALMA: I. Post-impact dust distribution

TL;DR

This study uses ALMA observations to map the distribution of millimeter grains in the debris disk of HD 172555, revealing insights into post-impact dust dynamics and grain size distribution in a young planetary system.

Contribution

First direct imaging and modeling of millimeter dust distribution in HD 172555's impact debris disk, linking dust dynamics to planetary collision processes.

Findings

Disk extends to ~9 au with no significant asymmetry.

Millimeter grains peak around ~5 au, indicating dust concentration.

Small grains are offset outward, likely due to gas drag and radiation pressure.

Abstract

Giant impacts between planetary embryos are a natural step in the terrestrial planet formation process and are expected to create disks of warm debris in the terrestrial regions of their stars. Understanding the gas and dust debris produced in giant impacts is vital for comprehending and constraining models of planetary collisions. We reveal the distribution of millimeter grains in the giant impact debris disk of HD 172555 for the first time, using new ALMA 0.87 mm observations at $\sim$80 mas (2.3 au) resolution. We modeled the interferometric visibilities to obtain basic spatial properties of the disk, and compared it to the disk's dust and gas distributions at other wavelengths. We detect the star and dust emission from an inclined disk out to $\sim$9 au and down to 2.3 au (on-sky) from the central star, with no significant asymmetry in the dust distribution. Radiative transfer modeling of the visibilities indicates the disk surface density distribution of millimeter grains most likely peaks around $\sim$5 au, while the width inferred remains model-dependent at the S/N of the data. We highlight an outward radial offset of the small grains traced by scattered light observations compared to the millimeter grains, which could be explained by the combined effect of gas drag and radiation pressure in the presence of large enough gas densities. Furthermore, SED modeling implies a size distribution slope for the millimeter grains consistent with the expectation of collisional evolution and flatter than inferred for the micron-sized grains, implying a break in the grain size distribution and confirming an overabundance of small grains.