ALMA unveils rings and gaps in the protoplanetary system HD 169142: signatures of two giant protoplanets (1702.02844v1)

Abstract: The protoplanetary system HD 169142 is one of the few cases where a potential candidate protoplanet has been recently detected via direct imaging. To study the interaction between the protoplanet and the disk itself observations of the gas and dust surface density structure are needed. This paper reports new ALMA observations of the dust continuum at 1.3\,mm, $^{12}$CO, $^{13}$CO and C$^{18}$O $J=2-1$ emission from the system HD 169142 at angular resolution of $\sim 0".18 - 0".28$ ($\sim 20\,$au$ - 33\,$au). The dust continuum emission reveals a double-ring structure with an inner ring between $0".17-0".28$ ($\sim 20 - 35\,$au) and an outer ring between $0".48-0".64$ ($\sim 56 - 83\,$au). The size and position of the inner ring is in good agreement with previous polarimetric observations in the near-infrared and is consistent with dust trapping by a massive planet. No dust emission is detected inside the inner dust cavity ($R \lesssim 20\,$au) or within the dust gap ($\sim 35 - 56\,$au). In contrast, the channel maps of the $J=2-1$ line of the three CO isotopologues reveal the presence of gas inside the dust cavity and dust gap. The gaseous disk is also much larger than the compact dust emission extending to $\sim 1'.5$ ($\sim 180\,$au) in radius. This difference and the sharp drop of the continuum emission at large radii point to radial drift of large dust grains ($>$ \micron-size). Using the thermo-chemical disk code \textsc{dali}, the continuum and the CO isotopologues emission are modelled to quantitatively measure the gas and dust surface densities. The resulting gas surface density is reduced by a factor of $\sim 30-40$ inward of the dust gap. The gas and dust distribution hint at the presence of multiple planets shaping the disk structure via dynamical clearing (dust cavity and gap) and dust trapping (double ring dust distribution).

Analyzing Protoplanetary Disk Structures in HD 169142 with ALMA

The protoplanetary system HD 169142 presents an intriguing case for the paper of planet formation using the ALMA array. This research leverages high-resolution observations of the HD 169142 system to examine the interaction between a candidate protoplanet and the gas and dust components of its surrounding disk. Specifically, the paper utilizes ALMA observations of dust continuum emissions at 1.3 mm and CO isotopologues to elucidate the disk's structural characteristics.

Observational Insights

The ALMA data reveal a double-ring structure in the dust distribution with significant gaps and cavities, indicating regions of depleted dust density. The inner dust ring lies between approximately 20 and 35 astronomical units (au) from the central star, while the outer ring is located between 56 and 83 au. Notably, the observations record no dust emission inside these gaps down to the noise level, suggesting the physical absence of millimeter-sized particles in these regions.

Conversely, the channel maps of $^{12}$CO, $^{13}$CO, and C$^{18}$O emission lines indicate the presence of gas within the dust cavities and gaps. This highlights a persistent gas-phase disk component that extends to about 180 au, significantly beyond the regions where dust emissions are detected. The differential size of the gas and dust disks suggests the existence of radial drift of larger dust grains.

Computational Modeling

The paper employs the thermo-chemical disk modeling tool dali to simulate the gas and dust surface densities, incorporating the radiative transfer and chemical processes at play in the disk environment. A key finding from model fitting is a substantial reduction—by a factor of 30 to 40—in gas surface density inward of the dust gap. This is consistent with processes such as dynamical clearing by embedded planets or radial drift influencing the dust distribution.

Theoretical Implications

The gap structures observed in HD 169142's disk could result from several possible mechanisms. However, the sharp demarcation of dust rings and gaps is strongly indicative of planet-disk interactions. The modeling suggests the presence of one or possibly multiple giant planets leading to the observed aerodynamic dust trapping in pressure bumps, while allowing gas to penetrate further inward, resulting in the observed 'gap' and 'ring' structures.

These findings are in line with theoretical predictions of planet formation, suggesting that massive planetary bodies within the disk are likely the agents responsible for the material separation. The absence of significant azimuthal asymmetries, often associated with vortices generated by planet-induced gas motion, also implies that forming planets may be relatively low mass and lack sufficient energy to generate large vortices.

Conclusions and Future Directions

The paper of HD 169142 via ALMA's high-resolution capabilities has underscored the potential for inferring the characteristics of embedded planets through their signatures in disk structures. The data interaction model highlights the complexity of feedback processes between newly forming planets and their natal material environment.

Looking forward, further ALMA observations focusing on the distribution and kinematics of different molecular species and dust size populations could refine our understanding of planet formation processes. Continued observation and modeling efforts will also support the broader paper of how young planetary systems evolve and potentially align theoretical predictions with observed disk characteristics. These insights contribute significantly to our understanding of protoplanetary disk dynamics and planet formation.