A multiwavelength ALMA view of gas and dust in binary protoplanetary system AS 205: Evidence of dust asymmetric distribution

We present Atacama Large Millimeter/Submillimeter Array observations of multi-wavelength dust emissions at 3.1 and 1.3 mm; along with molecular line emissions of CO(2-1), CO(3-2), $^{13}$CO(3-2), and C$^{18}$O(3-2) at spatial resolutions of 7-45 AU towards the protoplanetary system AS 205. The dust emissions exhibit two distinct components of AS 205 N and AS 205 S, separated by 1.3 arcsec. While gas kinematics within the dust disk regions are dominated by Keplerian rotation, the more extended gas emission displays complex morphology and kinematics strongly affected by the binary gravitational interaction in the outer regions. The stellar masses of AS 205 N and AS 205 S are estimated at $0.78\pm0.19$ and $1.93\pm0.86$ M$_\odot$, respectively. Azimuthal variation is observed in the spectral index distribution of both disks. In AS 205 N, the spectral index minimum in the southwest is coincident with the peaks of CO($2-1$), CO($3-2$), and $^{13}$CO($3-2$) integrated intensity and the relative position of its southern counterpart. On the other hand, the spectral index distribution in AS 205 S exhibits two prominent maxima, with the one in the northeast aligning with the peak of $^{13}$CO($3-2$), and the peak in the south coinciding with local maxima in CO($2-1$) and CO($3-2$) azimuthal profiles. These results suggest a correlation between dust grain size and/or optical depth with the gas distributions. Dust trapping along the spiral arms possibly contributes to the spectral index minima in AS 205 N; however, the observed asymmetry across both disks suggests the involvement of additional mechanisms.

💡 Research Summary

This paper presents a comprehensive ALMA study of the binary protoplanetary system AS 205, focusing on multi‑wavelength dust continuum at 3.1 mm and 1.3 mm and on molecular line emission from CO(2‑1), CO(3‑2), ¹³CO(3‑2), and C¹⁸O(3‑2). The observations achieve spatial resolutions of 7–45 AU, allowing the authors to resolve the two circumstellar disks (AS 205 N and AS 205 S) separated by 1.3″ (≈180 AU). Continuum imaging reveals that both disks are compact, with AS 205 N being brighter and displaying a pronounced azimuthal asymmetry elongated along the northeast–southwest direction, while AS 205 S shows a central component surrounded by a bright outer ring and a gap, linked by faint “bridge” emission. Gaussian and ellipse fitting give inclinations of ~17° for the northern disk and ~30° for the southern disk, consistent across both wavelengths.

A pixel‑by‑pixel spectral index (α) map, derived from the two continuum bands, shows clear azimuthal variations. In AS 205 N the spectral index reaches a minimum of α≈1.71±0.02 in the southwest, coincident with peaks in the integrated intensity of CO(2‑1), CO(3‑2), and ¹³CO(3‑2). In AS 205 S, two distinct α maxima (α≈3.5–4.0) appear: one in the northeast aligning with the ¹³CO(3‑2) peak, and another in the south matching local maxima in CO(2‑1) and CO(3‑2). The central regions of both disks have lower α (≈1.7–1.8), while the outer edges show higher values, indicating a transition from optically thick, possibly grain‑grown material near the star to optically thin, smaller‑grain dominated emission outward.

The molecular line data reveal Keplerian rotation within the inner disk regions but a complex, non‑Keplerian morphology in the extended gas. CO emission displays spiral‑like arms, tidal tails, and velocity perturbations that the authors attribute to the gravitational interaction between the two stars. Dynamical modeling yields stellar masses of 0.78 ± 0.19 M⊙ for AS 205 N and 1.93 ± 0.86 M⊙ for AS 205 S, though the large uncertainties leave open the possibility that the pair is not currently gravitationally bound, consistent with a proposed recent fly‑by scenario.

The authors discuss the implications of the observed dust–gas correlation. The coincidence of low α with CO intensity peaks in AS 205 N suggests dust trapping in pressure maxima associated with spiral arms, leading to grain growth and higher optical depth. However, the presence of two α maxima in AS 205 S, and the overall asymmetry across both disks, indicate that additional mechanisms—such as disk winds, shadowing, vortices, or remnants of a past close encounter—are likely at play. The paper emphasizes that multi‑wavelength, high‑resolution ALMA observations are essential for disentangling these processes.

In conclusion, this study provides the most detailed view to date of how binary interaction shapes the distribution of gas and dust in protoplanetary disks. The results highlight that dust grain size evolution and optical depth variations are tightly linked to the underlying gas dynamics, and that binary‑induced perturbations can generate significant azimuthal asymmetries. Future work combining even higher resolution imaging, polarization measurements, and sophisticated hydrodynamic simulations will be crucial to pinpoint the dominant dust‑trapping mechanisms and to assess the impact of such environments on planet formation.