An icy Kuiper-Belt around the young solar-type star HD 181327

TL;DR

This study characterizes the dust and gas properties of the circumstellar belt around the young star HD 181327, revealing a Kuiper Belt-like structure with icy, porous grains and limited gas, indicative of a post-planet formation stage.

Contribution

The paper provides detailed modeling of dust composition, geometry, and mass in HD 181327's belt, and constrains the gas content using multi-wavelength observations and radiative transfer analysis.

Findings

Dust consists of micron-sized porous silicates and carbonaceous material.

The dust mass is approximately 0.05 Earth masses.

Gas mass is constrained to be less than 17 Earth masses if PAHs are abundant.

Abstract

HD 181327 is a young F5/F6V star belonging to the Beta Pictoris moving group (12 Myr). It harbors an optically thin belt of circumstellar material at 90 AU. We aim to study the dust properties in the belt in details, and to constrain the gas-to-dust ratio. We obtained far-IR observations with the Herschel/PACS instrument, and 3.2 mm observations with the ATCA array. The geometry of the belt is constrained with newly reduced HST/NICMOS images that break the degeneracy between the disk geometry and the dust properties. We use the radiative transfer code GRaTer to compute a large grid of models, and we identify the grain models that best reproduce the Spectral Energy Distribution through a Bayesian analysis. We attempt to detect the [OI] and [CII] lines with PACS spectroscopy, providing observables to our photochemical code ProDiMo. The HST observations confirm that the dust is confined in a narrow belt. The continuum is detected in the far-IR with PACS and the disk is resolved with both PACS and ATCA. A medium integration of the gas spectral lines only provides upper limits on the line fluxes. We show that the HD 181327 dust disk consists of micron-sized grains of porous amorphous silicates and carbonaceous material surrounded by an important layer of ice, for a total dust mass of 0.05 M\oplus (up to 1 mm). We discuss evidences that the grains are fluffy aggregates. The upper limits on the gas lines do not provide unambiguous constraints: only if the PAH abundance is high, the gas mass must be lower than 17 M\oplus. Despite the weak constraints on the gas disk, the age of HD 181327 and the properties of the dust disk suggest that it has passed the stage of gaseous planets formation. The dust reveals a population of icy planetesimals, similar to the primitive Kuiper Belt, that may be a source for the future delivery of water and volatiles onto forming terrestrial planets.