Spiral excitation in protoplanetary disks through gap-edge illumination: Distinctive kinematic signatures in CO isotopologues

TL;DR

This work investigates how shadow-induced spirals in protoplanetary disks differ kinematically from spirals generated by exterior multi-Jupiter companions. Using two PLUTO simulations (one with full 3-temperature radiative transfer for shadow-driven spirals and one with beta-cooling for planet-driven spirals) and RADMC3D post-processing, the authors produce synthetic CO isotopologue datacubes and H-band images to analyze moment maps and Fourier modes. They find that shadow-driven spirals produce strong, two-armed, vertical-motion signatures—especially in $^{12}$CO near face-on orientations—and exhibit a predominance of even Fourier modes ($m=2$), while planet-driven cases show weaker, more localized kinematics and mixed-mode content. These distinctive signatures offer a practical observational diagnostic to distinguish the underlying mechanism and guide future direct-imaging campaigns, with broader implications for interpreting disk substructure across multi-wavelength tracers.

Abstract

High-resolution, near-infrared observations have revealed prominent, two-armed spirals in a multitude of systems, such as MWC~758, SAO~206462, and V1247~Ori. Alongside the classical theory of disk-companion interaction, shadow-based driving has come into vogue as a potential explanation for such large-scale substructures. How might these two mechanisms be distinguished from one another in observations? To investigate this question, we ran a pair of hydrodynamical simulations with \texttt{PLUTO}. One, with full radiation hydrodynamics and gas-grain collision, was designed to develop shadow-driven spirals at the outer gap edge of a sub-thermal, Saturn-mass planet. The other, with parametrized $β$-cooling, was set up to capture the more standard view of spiral wave excitation by a super-thermal, multi-Jupiter-mass, exterior planetary companion. Post-processing of these simulations with the Monte Carlo radiative transfer (MCRT) code \texttt{RADMC3D} revealed that strong vertical velocities in the shadow-driven case create a prominent two-armed feature in the moment-1 CO maps, particularly when the disk is viewed face-on in optically thicker isotopologues; such a feature is not seen in the standard planet-driven case. Conversely, the presence or absence of such signatures in two-armed spiral systems would distinguish those potentially driven by exterior, multi-Jupiter-mass companions, and thus help identify promising targets for future direct-imaging campaigns.

Figures (15)

• Figure 1: Plots of the vertical $\tau_z = 1$ emission surfaces for the two transitions ($J = 2-1, J = 3-2$) of each of the three isotopologues ($^{12}$CO,$^{13}$CO,C$^{18}$O) in the initial condition for our fiducial protoplanetary disk. Black solid lines indicate simulation boundaries (in this case, $r_{\rm out} = 100 {\rm \ au}$), while dashed grey lines indicate wave-damping regions in the hydrodynamical simulations. The sharp transition in the $^{13}$CO $J = 3-2$$\tau_z = 1$ surface is caused by freezeout of the immediately overlying material at large radii.
• Figure 2: Kinematic moment-1 (line-of-sight velocity) maps for the shadow-driven spirals, above, and the planet-driven spirals, below, in the $J=2-1$ transition of the $^{12}$CO isotopologue; in each case, the moment-1 map of the initial condition is subtracted from that at simulation end. From left to right, we plot disk inclinations of 0$^\circ$, 30$^\circ$, and 60$^\circ$. The FWHM of the fiducial ALMA beam ($0.19" \times 0.17"$) is indicated by a grey hatched ellipse. The spiral morphology is much more prominent in the shadow-driven case than in the spiral-driven case.
• Figure 3: Kinematic moment-1 (line-of-sight velocity) maps at simulation end, with moment-1 from the initial condition subtracted off, as in Figure \ref{['fig:moment_1_0']}. In contrast to the plots in Figure \ref{['fig:moment_1_0']}, we fix the disk inclination to 0$^\circ$ (face-on), and plot, from left to right, the maps corresponding to the $J=2-1$ transitions of the $^{12}$CO , $^{13}$CO , and C$^{18}$O isotopologues, which trace the upper, intermediate, and midplane layers of the disk respectively. In C$^{18}$O, the two-armed signature of the shadow-driven spiral is virtually absent, but for the standard planet-driven case the inner Lindlbad spirals remain visible.
• Figure 4: Modeled near-infrared (NIR) image in the H-band ($\lambda_H = 1.62 \ \mu$m of both the shadow-driven (above) and planet-driven (below) spiral cases. In each panel, the location of the planet is identified with a white dot, and an Archimedian spiral fit in grey dashed lines. The beam is indicated with a grey hatched ellipse, while the effect of a coronagraph is mimicked by the black circle 20 au (0.2") in radius. The standard planetary Lindblad spirals are weakened in the disk atmosphere---traced by NIR scattering off of the entrained dust grains---by wave refraction at the temperature transition corresponding to the disk's $\tau_r$ = 1 surface.
• Figure 5: Plot of $^{12}$CO $J = 2-1$ lines, for the planet-driven spiral models (above) and the shadow-driven models (below), at a position $R = 60$ au and $\phi = 0$ with respect to the $x$-axes of Figure \ref{['fig:scattered_light']} (and, equivalently, of the qualitative maps in Figure \ref{['fig:moment_2_maps']}). In the initial condition, line broadening results exclusively from thermal motion, and is largely unaffected by beam convolution. In evolved disks, however, the line profile is shifted and broadened by vertical motions within spiral arms, with beam convolution over regions of differing velocity dampening the overall directional shift, while enhancing broadening.