Multi-frequency observations of PDS70c: Radio emission mechanisms in the circumplanetary environment

¹ Departamento de Astronomía, Universidad de Chile, Casilla 36-D, Santiago, Chile
² Data Observatory Foundation, Eliodoro Yáñez 2990, Providencia, Santiago, Chile
³ Department of Physics, National Sun Yat-Sen University, No. 70, Lien-Hai Road, Kaohsiung City 80424, Taiwan, ROC
⁴ Center of Astronomy and Gravitation, National Taiwan Normal University, Taipei 116, Taiwan
⁵ School of Physics and Astronomy, Sun Yat-sen University, Guangdong 519082, PR China
⁶ University of Santiago of Chile (USACH), Faculty of Engineering, Computer Engineering Department, Chile
⁷ Departamento de Física, Universidad de Santiago de Chile, Av. Víctor Jara 3493, Santiago, Chile
⁸ Millennium Nucleus on Young Exoplanets and their Moons (YEMS), Chile
⁹ Center for Interdisciplinary Research in Astrophysics and Space Exploration (CIRAS), Universidad de Santiago de Chile, Chile
¹⁰ Charles University, Faculty of Math and Physics, Astronomical Institute, V Holešovičkách 747/2, 180 00 Prague 8, Czech Republic
¹¹ Division of Space Research & Planetary Sciences, Physics Institute, University of Bern, Gesellschaftsstr. 6, 3012 Bern, Switzerland
¹² Max-Planck-Institut für Astronomie, Königstuhl 17, 69117 Heidelberg, Germany
¹³ Fakultät für Physik, Universität Duisburg-Essen, Lotharstraße 1, 47057 Duisburg, Germany
¹⁴ University Observatory, Faculty of Physics, Ludwig-Maximilians Universität München, Scheinerstr. 1, 81679 Munich, Germany
¹⁵ Exzellenzcluster “Origins”, Boltzmannstr. 2, 85748 Garching, Germany
¹⁶ Institute for Computational Science, University of Zürich, Winterthurerstrasse 190, 8057 Zürich, Switzerland
¹⁷ ETH Zürich, Department of Physics, Wolfgang-Pauli-Strasse 27, 8093, Zürich, Switzerland

^★ Corresponding author: oriana.dominguez@ug.uchile.cl

Received: 12 March 2025
Accepted: 25 July 2025

Abstract

Aims. PDS 70c is a source of Hα emission and variable sub-millimetre signal. Knowledge of the emission mechanisms may enable observations of accretion rates and physical conditions in the circumplanetary environment.

Methods. We report ALMA observations of PDS 70 at 145 GHz (Band 4), 343.5 GHz (Band 7), and 671 GHz (Band 9) and compare them with archival data at 97.5 GHz (Band 3). The derived radio spectral energy distribution (SED) of PDS 70c is coeval within two months, and is interpreted in terms of analytic models of dusty and viscous discs (i.e. circumplanetary discs, CPDs). In a novel approach, we include the free-free continuum from H I, metals (e.g. K I) and H⁻.

Results. New detections in Bands 3 (tentative at 2.6 σ), 4 (5 σ), and 7 (re-detected at 9 σ) are consistent with optically thick thermal emission from PDS 70c (spectral index α = 2 ± 0.2). However, a non-detection in Band 9 breaks this trend, with a flux density falling below an optically thick extrapolation at 2.6 σ. A viscous dusty disc is inconsistent with the data, even with the inclusion of ionised jets. Interestingly, the central temperatures in such CPD models are high enough to ionise H I, with huge emission measures and an optically thick spectrum that marginally accounts for the radio SED (within 3 σ of Band 9). Since there is no room for steeper components (with α > 2), the dust-to-gas ratio is lower than 10⁻⁵. By contrast, uniform-slab models suggest much lower emission measures to account for the Band 9 drop, with ionisation fractions of ∼10⁻⁷ and an outer radius of ∼0.1 au. Such conditions are recovered if the CPD interacts with a planetary magnetic field, leading to a radially variable viscosity, α(R) ≲ 1, and central temperatures of ∼10³ K that regulate metal ionisation. However, the H⁻ opacity still results in an optically thick SED, overshooting Band 9. We find that the optically thin turnover at ≳600 GHz is only recovered if a thin shocked layer is present at the CPD surface, as is suggested by simulations. A photospheric shock or accretion funnels are ruled out as radio emission sources because their small solid angles require T ∼10⁶ K, which are unrealistic temperatures in planetary shock accretion.

Conclusions. The SED of PDS 70c collected here is optically thick up to Band 7 but probably (2.6 σ) turns over towards Band 9. An optically thick spectrum can be explained by atomic plasma radiation from a magnetised disc, where the radio opacity stems from metal and H⁻ free-free. If so, PDS 70c is depleted of sub-millimetre-emitting dust by a factor of at least 1000. However, the turnover can only be accounted for by H I free-free from an accretion shock at the surface of a CPD.