Resolving the terrestrial planet-forming region of HD 172555 with ALMA

I. Post-impact dust distribution

¹ School of Physics, Trinity College Dublin, the University of Dublin, College Green, Dublin 2, Ireland
² Malaghan Institute of Medical Research, Gate 7, Victoria University, Kelburn Parade, Wellington 6012, New Zealand
³ School of Physics and Astronomy, University of Exeter, Astrophysics Group, Stocker Road, Exeter EX4 4QL, UK
⁴ Space Science Institute, 4750 Walnut Street, Suite 205, Boulder, CO 80301, USA
⁵ Steward Observatory, University of Arizona, 933 N. Cherry Avenue, Tucson, AZ 85721-0065, USA
⁶ Center for Astrophysics, Harvard & Smithsonian, 60 Garden Street, Cambridge, MA 02138, USA
⁷ Institute of Astronomy, University of Cambridge, Madingley Road, Cambridge CB3 OHA, UK
⁸ Department of Physics, Astronomy, & Geosciences, Towson University, 8000 York Road, Towson, MD 21252, USA

^★ Corresponding author: This email address is being protected from spambots. You need JavaScript enabled to view it.

Received: 5 November 2025
Accepted: 2 December 2025

Abstract

Context. 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.

Aims. We reveal the distribution of millimeter (mm) grains in the giant impact debris disk of HD 172555 for the first time, using new ALMA 0.87 mm observations at ∼80 mas (2.3 au) resolution.

Methods. We modeled the interferometric visibilities to obtain basic spatial properties of the disk and compared these data to the disk’s dust and gas distributions at other wavelengths.

Results. We detected the star and dust emission from an inclined disk out to ∼9 au and down to 2.3 au (on-sky) from the central star, with no significant asymmetry in the dust distribution. The radiative transfer modeling of the visibilities indicates the disk surface density distribution of mm grains most likely peaks around ∼5 au, while the width inferred remains model-dependent at the S/N of the data. We highlighted an outward radial offset of the small grains traced by scattered light observations compared to the mm grains, which could be explained by the combined effect of gas drag and radiation pressure in the presence of large enough gas densities. Furthermore, our SED modeling implies a size distribution slope for the mm 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.