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

TL;DR

This study uses high-resolution ALMA observations across 3.1 mm and 1.3 mm continuum and multiple CO isotopologue lines to dissect the binary protoplanetary system AS 205, resolving two disks around AS 205 N and S and revealing an extended, non-Keplerian gas component linked to binary interactions. Through spectral-index mapping and PV-diagram fitting, the authors connect dust grain growth and optical-depth effects to azimuthal dust asymmetries, with N showing a southwest α minimum aligned to CO peaks and the S disk’s outer ring showing a notable wavelength-dependent azimuthal offset. They derive stellar masses from PV analysis, obtaining $M_N=0.78\pm0.19\,M_{\odot}$ and $M_S=1.93\pm0.86\,M_{\odot}$, totaling $M_{N+S}=2.62\pm1.05\,M_{\odot}$, and discuss whether the system is gravitationally bound or on a hyperbolic fly-by trajectory, concluding that the dynamical state remains uncertain. Overall, the work demonstrates how binary gravity shapes dust and gas distributions in planet-forming disks and informs models of planet formation in multiple-star environments.

Abstract

We present Atacama Large Millimeter/Submillimeter Array observations of multi-wavelength dust emissions at 3.1\,mm and 1.3\,mm; along with molecular line emissions of CO(2--1), CO(3--2), \mbox{$^{13}$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 $0.78\pm0.19$\,M$_\odot$ 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 \ass~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.

Figures (11)

• Figure 1: From left to right: Brightness temperature distributions of dust continuum emission observed at $\lambda$ = 3.1 mm and 1.3 mm; and spectral index derived from the data. The middle panels show the maps of both components, while the top and bottom panels zoom in to AS 205 N and AS 205 S components, respectively. The brightness temperature is calculated from the intensity using the Planck function. The beam is shown in the lower left corner with a color bar on the top of each map. The contour levels present in each map are [2.9, 4.6, 10.8, 16.6, 25.5, 39.4, 60.6, 93.3] K (top left), [4.7, 6.9, 10.2, 15.0, 22.0, 32.4, 47.8, 70.3, 103.6] K (top middle), [2.5, 4.0, 5.3, 9.0, 18.9, 38.0] K (bottom left), and [9.0, 11.0, 21.0, 31.0, 41.0] K (bottom middle). The black crosses in the spectral index maps mark the peak dust emissions of AS 205 N and AS 205 S.
• Figure 2: From left to right: Integrated intensity maps (upper panels) and peak brightness temperature maps (lower panels) of $\rm{CO}(2-1)$, $\rm{CO}(3-2)$, $^{13}\rm{CO}(3-2)$, and C$^{18}$O(3--2). The contour levels for the integrated intensity maps are [1, 3, 5, 7, 15, 20, 25, 30] $\times \sigma_{^{12}\mathrm{CO}(2-1)}$; [1, 5, 9, 14, 19, 25, 31, 37, 43, 49, 69, 89] $\times \sigma_{^{12}\mathrm{CO}(3-2)}$; [0.5, 1, 3, 5, 7, 9, 11, 13, 15] $\times \sigma_{^{13}\mathrm{CO}(3-2)}$; and [1, 2, 3] $\times \sigma_{\mathrm{C}^{18}\mathrm{O}(3-2)}$, respectively. The contour levels in the peak brightness temperature maps are [5, 10, 15, 20, 25, 30, 40]; [5, 15, 25, 35, 45, 55, 65, 75, 85, 95, 105]; [6, 8, 10, 15, 20, 25, 30, 35, 40]; [5, 7, 9, 11] times of its corresponding RMS noise level, respectively. The beam size and color scale are displayed in the lower left corner and at the top of each panel. In the left panels, A and B denote the spiral-like structures A and B, as mentioned in Kurtovic_2018.
• Figure 3: From left to right: Mean velocity maps of $\rm{CO}(2-1)$, $\rm{CO}(3-2)$, $^{13}\rm{CO}(3-2)$, and C$^{18}$O(3--2) emission of the AS 205 system. The top and bottom panels show the zoom-in images toward the center region of AS 205 N and AS 205 S, respectively. The black contours show the 1.3 mm dust emission at the levels of [5, 25, 300]$\sigma$. In each panel, the beam are shown in the lower left corner.
• Figure 4: From left to right: Position-Velocity diagram of four molecular lines $\rm{CO}(2-1)$, $\rm{CO}(3-2)$, $^{13}\rm{CO}(3-2)$, and C$^{18}$O(3--2), cut along the major axis of the observed dust disks AS 205 N (PA=$99^\circ$, upper panels) and AS 205 S (PA=$110^\circ$, lower panels). The extracted points along the edge and ridge are presented in a pink triangle and a red circle, respectively. The orange curves show the fitted results whose parameters are presented in Table \ref{['tab:pv_fit_results']}. Label A indicates the positions in the diagram of spiral-like structure A, which was shown in Figure \ref{['fig:CO_emissions']} and originally found in Kurtovic_2018.
• Figure 5: Top: Azimuthal distribution of the spectral index of AS 205 N within the radial distance of $0.2-0.4"$ (left) and the outer ring of AS 205 S, which is defined at $0.2-0.4"$ (right). Bottom: Azimuthal distribution of the normalized integrated intensity for CO($2-1$), CO($3-2$), and $^{13}$CO($3-2$), within the radial distance of $0.8"$ in AS 205 N (left) and $0.6"$ in AS 205 S (right). The azimuthal angle $\psi$ is measured eastward from the north direction.