The Circumbinary Disk of HD 34700A: I. CO gas kinematics indicate spirals, infall, and vortex motions

TL;DR

This paper presents high-resolution ALMA Band 6 observations of the circumbinary disk around the quadruple system HD 34700A, focusing on the CBD around the spectroscopic binary HD 34700AaAb. By modeling CO kinematics with the discminer package and performing channel-by-channel fits, the authors map non-Keplerian motions that correlate with infrared spiral structures and identify an inner gas cavity alongside a bright dust crescent atop a 138 au ring. They detect potential infalling gas streams and a tentative anticyclonic vortex near the continuum crescent, along with evidence for disk warp in the outer regions. The results imply a highly perturbed disk in the upper layers, likely shaped by late-stage infall and possibly an embedded companion, with implications for disk evolution and planet formation in circumbinary environments; deeper observations are recommended to confirm the vortex and trace infall pathways.

Abstract

We present the first high-resolution ($\sim$ 0.14") Atacama Large Millimeter/submillimeter Array (ALMA) Band 6 dust continuum and CO molecular line emission observations of the quadruple system HD 34700. In particular, HD 34700AaAb is a spectroscopic binary ($M_{\rm{bin}}=4\,M_\odot$) surrounded by two low-mass companions at large separations. Its circumbinary disk is highly substructured, featuring numerous spiral arms and a large cavity observed in infrared (IR) scattered light. We analyzed the CO line channel and intensity moment maps. By fitting a Keplerian model to the line channel emission, we identified the residual motions and conducted a line spectra analysis. We resolved an asymmetric continuum crescent on top of a dust ring at 0.39" (138 au), colocated with the IR ring. The CO molecule line emissions trace a smaller cavity in gas, whose edge aligns with the inner rim of the ring detected in H$α$ emission at 0.20" (65 au). The $^{12}$CO line emission and kinematics trace highly non-Keplerian motions ($\sim0.1Δ\upsilon_{\rm kep}$), and these CO spiral features align well with the spiral structures in scattered light. The $^{12}$CO line spectra analysis reveals a streamer above the southeastern disk plane, likely falling onto the disk. The $^{13}$CO and C$^{18}$O kinematics largely follow the disk's underlying Keplerian rotation, while $^{13}$CO exhibits tentative signs of anticyclonic vortex flows at the continuum crescent location. Our multimolecular line study suggests that the circumbinary disk of HD 34700A is highly perturbed in its upper layers, possibly warped and influenced by infalling material. While late-stage infall may account for the IR spirals and the formation of the vortex through Rossby wave instability, an embedded massive companion within the cavity may also contribute to these features.

Figures (20)

• Figure 1: Multi-wavelengths observations of . (a) SPHERE/IRDIS IR scattered light observations and (b) ZIMPOL H$\alpha$ observations presented in Columba_ea_2024, with the gray circles representing the coronograph ($r=92.5\,$mas $\approx32\,$au). (c) ALMA 225.3 GHz continuum observations. (d), (e), and (f) show the moment 0 maps for the , , and fiducial cubes, while (g), (h), and (i) show the peak intensity maps for the same cubes, respectively. The brightness temperature was computed using the Rayleigh-Jeans approximation. The white contour in the peak intensity maps encloses 5$\sigma$ of the root-mean-square (RMS) noise of each line. The location of the spectroscopic binary is highlighted with the star. The beam size is shown in the lower left corner for the ALMA observations.
• Figure 2: Brightness temperature radial profiles of CO isotopologs and dust continuum using the high-resolution cubes $0\farcs14\times0\farcs11$ (49 x 39 au) cubes. The latter was multiplied by a factor of 10 to enhance visibility. The colored shaded region shows the standard deviation within each annulus. For the asymmetric continuum emission this traces the pronounced azimuthal variations, rather than an error. The gray-shaded area shows the extent of the H$\alpha$ ring, and the dashed-dotted line shows the peak of the IR ring Columba_ea_2024uyama2020.
• Figure 3: Line emission features of . Left: Moment 0 map using the high-resolution cube. Right: Residuals from a moment 0 map subtracted with a high-pass filtered moment map, limited to disk emission above 5 $\sigma$. The black and white contours in both maps highlight the IR emission, identical to Fig. \ref{['fig:overview']} (a).
• Figure 4: Selected channel maps with highlighted non-Keplerian features. White contours enclose 5$\sigma$. The beam size of the high-resolution cube employed is shown in the lower left corner and channel id in the upper right corner.
• Figure 5: position-velocity diagram for fiducial cube. Spectra are extracted along all four angles of the major (red and blue at $\phi=$0°) and minor (cyan and purple at $\phi=$90°) disk axes, as highlighted in the Gaussian centroid moment map (left panel). The dashed lines in the PV diagrams (colors corresponding to axes on the left) highlight the disk's Keplerian rotation. The gray shaded areas mask one major beam size in radius from the center. In the $\phi=0$ panel, the enclosing black dashed lines correspond to 20% deviations from Keplerian rotation. The location of the IR ring ($R=175\,$au) is plotted in gray dotted lines in both PV diagrams.