ALMA 870 $μ$m continuum observations of HD 100546. Evidence of a giant planet on a wide orbit (2106.07757v1)

Abstract: This paper reports on a new analysis of archival ALMA $870\,\mu$m dust continuum observations. Along with the previously observed bright inner ring ($r \sim 20-40\,$au), two addition substructures are evident in the new continuum image: a wide dust gap, $r \sim 40-150\,$au, and a faint outer ring ranging from $r \sim 150\,$au to $r \sim 250\,$au and whose presence was formerly postulated in low-angular-resolution ALMA cycle 0 observations but never before observed. Notably, the dust emission of the outer ring is not homogeneous, and it shows two prominent azimuthal asymmetries that resemble an eccentric ring with eccentricity $e = 0.07 $. The characteristic double-ring dust structure of HD 100546 is likely produced by the interaction of the disk with multiple giant protoplanets. This paper includes new smoothed-particle-hydrodynamic simulations with two giant protoplanets, one inside of the inner dust cavity and one in the dust gap. The simulations qualitatively reproduce the observations, and the final masses and orbital distances of the two planets in the simulations are 3.1 $M_{J}$ at 15 au and 8.5 $M_{J}$ at 110 au, respectively. The massive outer protoplanet substantially perturbs the disk surface density distribution and gas dynamics, producing multiple spiral arms both inward and outward of its orbit. This can explain the observed perturbed gas dynamics inward of 100 au as revealed by ALMA observations of CO. Finally, the reduced dust surface density in the $\sim 40-150\,$au dust gap can nicely clarify the origin of the previously detected H$_2$O gas and ice emission.

• The paper demonstrates that a double-ring disk structure, with an inner ring at 20–40 au and an outer ring at 150–250 au, supports the influence of giant planets.
• The paper employs smoothed-particle-hydrodynamic simulations to reproduce the disk morphology, identifying protoplanets of 3.1 M_Jup at 15 au and 8.5 M_Jup at 110 au.
• The paper shows that the dust gap, correlating with detected H2O signatures, facilitates UV-induced photo-desorption, illuminating key astrochemical processes in the disk.

ALMA 870 μm Continuum Observations of HD 100546: Evidence of a Giant Planet on a Wide Orbit

The paper presented provides a detailed analysis of the protoplanetary disk around the pre-main-sequence star HD 100546, conducted using archival Atacama Large Millimeter/submillimeter Array (ALMA) 870 μm continuum data. This investigation aims to elucidate the complex structural features of the disk and associates specific substructures with the presence of giant planets.

Key Findings

1. Disk Structure: The observations reveal a distinct double-ring structure in the disk. The inner ring lies between approximately 20 to 40 astronomical units (au) from the star. This region is followed by a wide dust gap extending from 40 to 150 au, and then by an outer ring, which was previously undetected and is now resolved between 150 and 250 au.
2. Substructures and Dynamics: The outer ring exhibits two significant azimuthal asymmetries, positing an eccentric ring with an eccentricity of 0.07. This structural complexity suggests dynamic interactions possibly induced by giant protoplanets within the disk.
3. Protoplanetary Presence: By deploying smoothed-particle-hydrodynamic simulations, featuring two giant protoplanets—one within the inner cavity and another in the dust gap—the paper successfully reproduces the observed disk configuration. Within the simulations, the planets culminate at masses of approximately 3.1 M_Jup at 15 au and 8.5 M_Jup at 110 au, respectively, with the outer planet notably affecting the disk by instigating multiple spiral arms.

Theoretical and Practical Implications

• Giant Planet Formation: The uncovered dynamics and resultant gap formations align with existing theories positing that giant planets significantly perturb disk structures, leading to visible gaps and asymmetries in their host disks. This research suggests that the outer protoplanet, particularly, may have formed farther outward before migrating inward, supporting theories that favor planet formation at significant distances from the central star via gravitational instabilities rather than core accretion.
• Astrochemical Consequences: The characterized dust gap coincides spatially with previously detected H2O gas and ice signatures. This suggests that the reduced dust density within the gap facilitates enhanced UV penetration, which can induce photo-desorption of ice, thus explicating the presence and distribution of water vapor in this region.

Speculations on Future Developments

The findings underscore the utility of high-resolution ALMA imaging in identifying and understanding planetary influences on protoplanetary disk morphology. Given the potential to apply similar methodologies to other young stellar objects, future research could further explore the viability of planet-induced disk features as proxies for the presence of planets, thereby advancing our understanding of planet formation and migration mechanisms.

Furthermore, extending similar observational techniques to additional molecular tracers at different wavelengths, particularly CO isotopologs, could illuminate the complex interactions between gas dynamics and embedded planetesimals, offering a more nuanced understanding of early planetary development.

In summary, the research provides critical empirical evidence connecting observed disk substructures with embedded planets, contributing to the evolving narrative of planet formation in circumstellar environments.