[Paper Review] Evidence for a massive dust-trapping vortex connected to spirals

P. Cazzoletti, E. F. van Dishoeck+7 more|arXiv (Cornell University)|Sep 11, 2018

Astrophysics and Star Formation Studies79 references35 citations

TL;DR

This study presents high-resolution ALMA observations of the HD 135344B protoplanetary disk, identifying a massive, asymmetric dust vortex at millimeter wavelengths. The vortex, confirmed by wavelength-dependent azimuthal shifts and enhanced large grains, likely acts as a planetesimal factory and may drive the observed spiral arms in scattered light, potentially eliminating the need for an outer planet to explain the morphology.

ABSTRACT

Context. Spiral arms, rings and large scale asymmetries are structures observed in high resolution observations of protoplanetary disks, and it appears that some of the disks showing spiral arms in scattered light also show asymmetries in millimeter-sized dust. HD 135344B is one such disk. Planets are invoked as the origin of these structures, but no planet has been observed so far and upper limits are becoming more stringent with time. Aims. We want to investigate the nature of the asymmetric structure in the HD 135344B disk in order to understand the origin of the spirals and of the asymmetry seen in this disk. Ultimately, we aim to understand whether or not one or more planets are needed to explain such structures. Methods. We present new ALMA sub-0.1′′ resolution observations at optically thin wavelengths (λ = 2.8 and 1.9 mm) of the HD 135344B disk. The high spatial resolution allows us to unambiguously characterize the mm-dust morphology of the disk. The low optical depth of continuum emission probes the bulk of the dust content of the vortex. Moreover, we have combined the new observations with archival data at shorter wavelengths to perform a multi-wavelength analysis and to obtain information about the dust distribution and properties inside the observed asymmetry. Results. We resolve the asymmetric disk into a symmetric ring + asymmetric crescent, and observe that (1) the spectral index strongly decreases at the centre of the vortex, consistent with the presence of large grains; (2) for the first time, an azimuthal shift of the peak of the vortex with wavelength is observed; (3) the azimuthal width of the vortex decreases at longer wavelengths, as expected for dust traps. These features allow confirming the nature of the asymmetry as a vortex. Finally, under the assumption of optically thin emission, a lower limit to the total mass of the vortex is 0.3 M Jupiter . Considering the uncertainties involved in this estimate, it is possible that the actual mass of the vortex is higher and possibly within the required values (~4 M Jupiter ) to launch spiral arms similar to those observed in scattered light. If this is the case, then explaining the morphology does not require an outer planet.

Motivation & Objective

• Investigate the origin of spiral arms and dust asymmetries in the HD 135344B protoplanetary disk.
• Determine whether the observed asymmetry is due to a vortex or a planet-induced structure.
• Assess whether the vortex alone can explain the spiral morphology seen in scattered light.
• Estimate the dust mass and physical properties of the vortex to evaluate its role in planet formation.
• Break degeneracies in disk structure interpretation using multi-wavelength continuum analysis.

Proposed method

• Conduct new ALMA Band 3 (2.8 mm) and Band 4 (1.9 mm) observations with sub-0.1′′ resolution to resolve dust morphology.
• Combine new data with archival ALMA Band 7 (343 GHz) and Band 9 data for multi-wavelength analysis.
• Model the uv-plane data using a symmetric inner ring plus an outer asymmetric, co-radial crescent (vortex) component.
• Fit the data with a double Gaussian in radial and azimuthal directions to quantify wavelength-dependent morphology.
• Calculate the spectral index α across the disk to infer grain size distribution and optical depth.
• Use the optically thin assumption to derive a lower limit on the vortex mass, assuming a gas-to-dust ratio of 10.

Experimental results

Research questions

• RQ1 Is the asymmetric mm-dust emission in HD 135344B best explained by a vortex or a planet-induced structure?
• RQ2 Can the observed wavelength-dependent shift in the peak emission position be explained by dust trapping in a vortex?
• RQ3 What is the mass of the dust vortex, and is it sufficient to drive spiral density waves in the disk?
• RQ4 How does the azimuthal width of the emission change with wavelength, and what does this imply about grain size and trapping?
• RQ5 Can the vortex alone explain the spiral arms observed in scattered light, eliminating the need for an outer planet?

Key findings

• The asymmetric structure is best modeled as a symmetric inner ring and an outer, co-radial crescent, confirming the presence of a dust-trapping vortex.
• The spectral index α decreases to approximately 2 within the vortex, indicating the presence of large, cm-sized grains consistent with dust trapping.
• For the first time, a monotonic azimuthal shift in the peak emission position is observed across wavelengths, inconsistent with Keplerian rotation and consistent with size-dependent trapping in a vortex.
• Emission at longer wavelengths (2.8 mm) is more azimuthally concentrated than at shorter wavelengths (1.9 mm), confirming predictions of dust trapping models where larger grains are less coupled to gas.
• The vortex has a lower mass limit of 0.3 M_Jupiter under the optically thin assumption, but could be as high as ~1.7 M_Jupiter if dust emission is marginally optically thin, approaching the mass needed to drive spiral arms.
• Approximately half of the disk’s total dust mass is trapped in the vortex, making it a prime site for planetesimal formation, potentially explaining the observed asymmetries without requiring an outer planet.