Distribution of solids in the rings of the HD 163296 disk: a multiwavelength study^★,★★

¹ ETH Zurich, Institute for Particle Physics and Astrophysics, Wolfgang-Pauli-Str. 27, 8093 Zurich, Switzerland
e-mail: gguidi@ethz.ch
² Department of Physics and Astronomy, Rice University 6100 Main Street, MS-108, Houston, TX 77005, USA
³ ESO, Karl Schwarzschild str. 2, 85748 Garching bei München, Germany
⁴ National Radio Astronomy Observatory, PO Box O, Socorro, NM 87801, USA
⁵ Institute of Astronomy and Astrophysics, Academia Sinica, Roosevelt Rd, Taipei 10617, Republic of China
⁶ Leiden Observatory, Leiden University, PO Box 9513, 2300 RA Leiden, The Netherlands
⁷ School of Physics and Astronomy, University of Leicester, Leicester LE1 7RH, UK
⁸ Institute of Astronomy, University of Cambridge, Madingley Road, CB3 0HA Cambridge, UK
⁹ Institute for Astronomy, University of Hawaii at Manoa, Honolulu, HI 96822, USA
¹⁰ Joint ALMA Observatory, Avenida Alonso de Córdova 3107 Vitacura, Santiago, Chile
¹¹ European Southern Observatory, Alonso de Córdova 3107, Casilla 19001 Santiago de Chile, Chile
¹² Theoretical Division, Los Alamos National Laboratory, Los Alamo, NM 87545, USA
¹³ School of Physics and Astronomy, Sun Yat-sen University, 2 Daxue Road, Zhuhai 519082 Guangdong Province, PR China
¹⁴ Universitá di Padova, Dipartimento di Astronomia, Vicolo dell'Osservatorio 2, I-35122 Padova, Italy
¹⁵ Department of Physics and Astronomy, California State University Northridge, 18111 Nordhoff Street, Northridge, CA 91330, USA

Received: 24 September 2021
Accepted: 29 June 2022

Abstract

Context. Observations at millimeter wavelengths of bright protoplanetary disks have shown the ubiquitous presence of structures such as rings and spirals in the continuum emission. The derivation of the underlying properties of the emitting material is nontrivial because of the complex radiative processes involved.

Aims. In this paper we analyze new observations from the Atacama Large Millimeter/submillimeter Array (ALMA) and the Karl G. Jansky Very Large Array (VLA) at high angular resolution corresponding to 5 – 8 au to determine the dust spatial distribution and grain properties in the ringed disk of HD 163296.

Methods. We fit the spectral energy distribution as a function of the radius at five wavelengths from 0.9 to 9 mm, using a simple power law and a physical model based on an analytic description of radiative transfer that includes isothermal scattering. We considered eight dust populations and compared the models' performance using Bayesian evidence.

Results. Our analysis shows that the moderately high optical depth (τ>1) at λ ≤ 1.3 mm in the dust rings artificially lower the millimeter spectral index, which should therefore not be considered as a reliable direct proxy of the dust properties and especially the grain size. We find that the outer disk is composed of small grains on the order of 200 µm with no significant difference between rings at 66 and 100 au and the adjacent gaps, while in the innermost ~30 au, larger grains (≥mm) could be present. We show that the assumptions on the dust composition have a strong impact on the derived surface densities and grain size. In particular, increasing the porosity of the grains to 80% results in a total dust mass about five times higher with respect to grains with 25% porosity. Finally, we find that the derived opacities as a function of frequency deviate from a simple power law and that grains with a lower porosity seem to better reproduce the observations of HD 163296.

Conclusions. While we do not find evidence of differential trapping in the rings of HD 163296, our overall results are consistent with the postulated presence of giant planets affecting the dust temperature structure and surface density, and possibly originating a second-generation dust population of small grains.