A Circumplanetary Disk Around PDS70c

Abstract: PDS70 is a unique system in which two protoplanets, PDS70b and c, have been discovered within the dust-depleted cavity of their disk, at $\sim$22 and 34au respectively, by direct imaging at infrared wavelengths. Subsequent detection of the planets in the H$\alpha$ line indicates that they are still accreting material through circumplanetary disks. In this Letter, we present new Atacama Large Millimeter/submillimeter Array (ALMA) observations of the dust continuum emission at 855$\mu$m at high angular resolution ($\sim$20mas, 2.3au) that aim to resolve the circumplanetary disks and constrain their dust masses. Our observations confirm the presence of a compact source of emission co-located with PDS70c, spatially separated from the circumstellar disk and less extended than $\sim$1.2au in radius, a value close to the expected truncation radius of the cicumplanetary disk at a third of the Hill radius. The emission around PDS70c has a peak intensity of $\sim$86$\pm$16 $\mu \mathrm{Jy}~\mathrm{beam}^{-1}$ which corresponds to a dust mass of $\sim$0.031M${\oplus}$ or $\sim$0.007M${\oplus}$, assuming that it is only constituted of 1 $\mu$m or 1 mm sized grains, respectively. We also detect extended, low surface brightness continuum emission within the cavity near PDS70b. We observe an optically thin inner disk within 18au of the star with an emission that could result from small micron-sized grains transported from the outer disk through the orbits of b and c. In addition, we find that the outer disk resolves into a narrow and bright ring with a faint inner shoulder.

• The paper reports the detection of a compact circumplanetary disk (CPD) around PDS 70 c using ALMA’s high-resolution imaging.
• It identifies an 86 µJy emission peak and estimates a dust mass of 0.031 M⊕, aligning with predictions of CPD truncation at one-third of the Hill radius.
• The findings imply active planetary accretion, providing empirical evidence that informs models of satellite formation and disk dynamics.

Observations of a Circumplanetary Disk Around PDS 70 c

In "A Circumplanetary Disk Around PDS 70 c", Myriam Benisty et al. explore groundbreaking observations made possible by the Atacama Large Millimeter/submillimeter Array (ALMA). These observations provide evidence for a circumplanetary disk (CPD) around the protoplanet PDS 70 c. This detection is corroborated by millimeter and sub-millimeter imaging using ALMA, which allows unprecedented spatial resolution of this young planetary system, composed of the star and two known protoplanets, PDS 70 b and PDS 70 c.

Key Observational Insights

1. Detection of CPD: ALMA observed a compact source of emission associated with PDS 70 c, identified as a CPD. The emission shows a peak intensity near 86 µJy, translating to a dust mass of approximately 0.031 M⊕, if composed entirely of 1 µm grains. The CPD's estimated extent aligns closely with theoretical predictions of the CPD’s truncation at around a third of PDS 70 c's Hill radius.
2. Disk Structures: The protoplanetary disk of PDS 70 presents complex structures with a notable dust-depleted cavity. Within this cavity, the circumstellar disk is observed to host significant asymmetries, visible as an inner disk reaching roughly 18 au. Further, a bright disk around PDS 70 and a secondary disk of smaller radius exhibit morphological intricacies such as rings and potential spiral arms.
3. Planetary Interaction: The circumplanetary material confirms active accretion, as shown by the Hα line, highlighting the role of CPDs in planetary mass growth and satellite formation.

Theoretical Implications

• Satellite Formation: This CPD detection provides vital empirical data for theoretical models that simulate the formation and evolution conditions of moons around giant planets.
• Planetary Growth: CPDs contribute to gaseous accretion onto planets, influencing their ultimate mass and dynamical characteristics. The ALMA observations suggest that while large grains are trapped in the outer disk, smaller grains continue to accrete onto the planet via the CPD.
• Disk Dynamics: The observed structures in the protoplanetary disk, including gaps and rings, may also provide insights into planet-disk interactions and the migration process implicated in the PDS 70 architecture.

Future Prospects

Further observation campaigns, especially those focused on complementary wavelength regimes, will refine our understanding of CPDs. Higher resolution studies might elucidate other physical processes at play within these circumstellar environments, potentially discovering new features like inner disk truncations or additional planetary bodies. Moreover, refining CPD observations in systems like PDS 70 will consolidate our theoretical understanding of planetary accretion mechanics and satellite formation, critical for cultivating a more comprehensive narrative of solar system evolution.