ALMA chemical survey of disk-outflow sources in Taurus (ALMA-DOT) VII: the layered molecular outflow from HL Tau and its relationship with the ringed disk

TL;DR

This work analyzes the HL Tau disk-outflow system with ALMA-DOT CO (2-1) data at ~0.28" resolution to map the outer wind and relate it to the disk’s ring structure. Through tomographic reconstruction of channel maps and PV diagrams, the authors identify nested rotating wind shells whose footpoints align with dust rings at 58, 72, and 86 au, consistent with magnetized disk winds launching over a broad disk region. The wind parameters imply a magnetic lever arm λ in the range ~4–5 and a launching region extending to ~90 au, suggesting that magnetic effects can drive angular-momentum transport and influence disk evolution and planet formation, possibly without invoking planets to explain rings. The results support non-ideal MHD scenarios where magnetic instabilities organize rings and a layered wind, while highlighting the need for extended modeling and observation of the outer disk wind.

Abstract

The ringed disk around HL Tau stands out as the iconic signature of planet formation, but the origin of the substructures is still debated. The HL Tau system also drives a powerful bipolar wind, and we analyze its outermost component traced by CO emission, to determine the relationship of the flow with the disk and its substructures. We use ALMA observations of the ${}^{12}$CO (2-1) line at 1.3 mm, with 0.2 km/s and ~ 0.28" resolution, conducted within the ALMA-DOT project. The channel maps and position-velocity diagrams show a rich structure of concatenated bubble- and arc-shaped features, whose size and distance from the source increase with velocity. The superposition of the features generates the apparent conical shape. The tomographic reconstruction of the morphology and kinematics of the red-shifted lobe suggests the presence of distinct nested shells having higher velocity and steeper velocity gradient for shells closer to the axis, rotating in the same sense of the disk. Such configuration can be justified by different classes of models. In this paper we compare the derived wind parameters with the predictions of magnetohydrodynamic (MHD) disk winds. Under this hypothesis, the launch radii of the three outermost shells are found to be at about the position of three adjacent dust rings in the disk at 58, 72 and 86 au. The wind may be capable of removing angular momentum from the outer disk, and we derive a magnetic lever arm of $λ\sim 4 - 5$, higher than that commonly adopted for MHD winds from these regions. Interpretations are discussed. The arrangement of the wind in nested shells with brighter emission rooted at the location of ring substructures could support the results of non-ideal MHD simulations according to which magnetic instabilities can generate the disk ring-gap system with a connected layered wind, alternatively to the action of yet undetected protoplanets.

Figures (24)

• Figure 1: Left: Moment 0 maps of the CO (2$-1)$ emission integrated over the velocity interval V$_{\rm LSR} = (-10, +3)$ km s$^{-1}\,$, highlighting the morphology of the blueshifted outflow lobe. The red contours at the center of the panel corresponds to the disk continuum emission at 1.3 mm drawn at [10, 200]$\sigma_{\rm c}$, with $\sigma_{\rm c}$=1.2$\times$10$^{-4}$ Jy beam$^{-1}$. The green square in the center of the contours marks the position of the continuum peak. The white dashed line at PA=48$^{\circ}$ indicates the orientation of the disk minor axis, and the white solid curves outline the spatial limits of the main wind components described in the text. Right: Same as left panel, but with velocity integration range V$_{\rm LSR} = (+8, +30)$ km s$^{-1}\,$, illustrating the structure of the redshifted lobe. The beam is drawn in the bottom left corner. We note the difference in brightness between the two lobes: the blueshifted lobe is about ten times fainter.
• Figure 2: Moment 1 map of the CO (2$-1)$ emission over the velocity interval V$_{\rm LSR} = (-10, +32)$ km s$^{-1}\,$. The solid curves, disk contours, dashed line, and central square are as in Fig.\ref{['fig:moment0maps']}. The map was produced using a 4$\sigma$ clipping.
• Figure 3: Channel maps at selected V$_{\rm LSR}$ velocities of the redshifted emission in the CO (2-1) line in the SW outflow lobe. The star symbol and the contour indicate the position of the source and the disk emission at 1.3 mm drawn at 10$\sigma_{\rm c}$, respectively. The white solid line lying at PA=228$^{\circ}$ coincides with the direction of the disk minor axis. The magenta and black dots in the lower right corner of the leftmost panel report the beam size in the continuum and line observations, respectively.
• Figure 4: Zoomed-in images of channel maps at selected V$_{\rm LSR}$ velocities, illustrating the system of arcs and bubble-like substructures visible in the redshifted outflow. The CO emission contours are drawn at [3, 6, 12, 24, 48]$\sigma_\mathrm{CO}$. The two white contours of the 1.3 continuum at [30, 700]$\sigma_\mathrm{c}$ indicate the disk position. The green line oriented at PA=228$\degr$ coincides with the direction of the disk minor axis, and defines the reference axis of the offset $y$ from the source in the plane of the sky, with positive offsets toward SW. The other green lines mark the $\pm38\degr$ average opening angle of the apparent conical cavity at 12.6 km s$^{-1}\,$, to illustrate the narrowing of the flow with increasing velocity. In the vicinity of the source, a number of arcs are seen to combine to form closed curves. Farther from the source only arcs opened toward the SW are visible. The superposed white ellipses are examples of visual fits to the identified features, labeled in families sharing common properties.
• Figure 5: Longitudinal position-velocity diagram (PV$_\parallel$) formed along the axis (PA=228$^{\circ}$) with the source in $y=0$ and positive offsets toward the SW. For the redshifted lobe, and for each branch, the connected crosses correspond to the intersections of the ellipse traces in the channel maps with the axis, each family with a different color. For the A features, the subscript $0$ ($1$) refers to the intersections of the side of the ellipses closer to (farther from) the source. The horizontal dotted lines mark the V$_{\rm LSR}$ of the channel maps in Fig. \ref{['fig:ellipses2']}; the vertical dotted lines are the offsets $y$ of the transverse PV diagrams shown in \ref{['fig:PVperp-selec']}. The solid cyan line is at V$_{\rm sys}$. The symbols in color indicate corresponding emission points in the diagrams of \ref{['fig:PVperp-selec']}.