ALMA 873 µm polarization observations of the PDS 70 disk

¹ Department of Physics, National Sun Yat-Sen University, No. 70, Lien-Hai Road, Kaohsiung City 80424, Taiwan, ROC
² Center of Astronomy and Gravitation, National Taiwan Normal University, Taipei 116, Taiwan
³ Department of Astronomical Science, School of Physical Sciences, Graduate University for Advanced Studies (SOKENDAI), 2-21-1 Osawa, Mitaka, Tokyo 181-8588, Japan
⁴ National Astronomical Observatory of Japan, 2-21-1 Osawa, Mitaka, Tokyo 181-8588, Japan
⁵ Departamento de Astronomía, Universidad de Chile, Casilla 36-D, Santiago, Chile
⁶ Data Observatory Foundation, to Data Observatory Foundation, Eliodoro Yańẽz 2990, Providencia, Santiago, Chile
⁷ Millennium Nucleus on Young Exoplanets and Their Moons (YEMS), Santiago, Chile
⁸ Kavli Institute for Astronomy and Astrophysics, Peking University, 5 Yiheyuan Road, Haidian District, Beijing 100871, PR China
⁹ Institute of Astronomy and Astrophysics, Academia Sinica, 11F of Astronomy-Mathematics Building, AS/NTU No.1, Sec. 4, Roosevelt Rd, Taipei 10617, Taiwan, ROC
¹⁰ Departamento de Física, Universidad de Santiago de Chile, Av. Víctor Jara 3493, Santiago, Chile
¹¹ Millennium Nucleus on Young Exoplanets and their Moons (YEMS), Chile
¹² Center for Interdisciplinary Research in Astrophysics Space Exploration (CIRAS), Universidad de Santiago, Chile

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

Received: 17 December 2025
Accepted: 3 February 2026

Abstract

At a 112.4 pc distance, the PDS 70 protoplanetary disk is a rare case that has been confirmed to host two accreting planets. This makes it the most important laboratory for studying dust growth in the context of planet formation. Here we present the first deep full polarization observations at a wavelength of 873 µm. We detected ∼1–2.5% linear polarization over the bulk of the ~55–100 AU (sub)millimeter ring. The polarization position angles align preferentially with the projected minor axis of the disk. The standard interpretation is that the observed polarization is caused by dust self-scattering, with a maximum dust grain size of ~100 µm. On ≳10 AU scales, which can be resolved by the presented 873–3075 µm observations, the ring is marginally optically thick at the 873 µm wavelength. Using Monte Carlo radiative transfer simulations, we found that an azimuthally asymmetric, marginally optically thick ring with a maximum dust grain size of ∼87 µm can reproduce the observed 873 µm polarization position angles and percentages. This study indicates that the coagulation of ice-coated dust in the protoplanetary disk may be limited by fragmentation or bouncing.