Spirals and Vertical Motions in the Planet-Forming Disk around HD 100546. A multi-line study of its gas kinematics

Abstract

HD100546 represents a particularly interesting target to study dynamical planet-disk interactions as various features have been observed in both the dust and gas that provide direct and indirect evidence for ongoing planet formation. In this work, we aim to characterize the gas kinematics of five molecular CO emission lines observed with ALMA in HD 100546, to reveal deviations from Keplerian rotation as well as substructures in the peak intensity and line width. We fit the molecular intensity channels with the Discminer package to model the line profiles. Aside from fitting the full cube, we also conduct runs where the blue- and redshifted sides are modeled separately to search for possible asymmetries. Our analysis reveals prominent kinematical spiral features in all five lines on large scales of the disk and we reproduce their morphology with both a linear and logarithmic spiral. In 12CO 2-1, spirals are also seen in the peak intensity residuals, the line width residuals exhibit a prominent ring of enhanced line widths around 125-330 au. The models further show, that the emission from the redshifted side may originate from higher disk layers than that from the blueshifted side. The pitch angles of the spirals are consistent with those driven by an embedded companion inside of 50 au and they suggest a dynamical mechanism rather than gravitational instabilities. We further find indications of a companion around 90-150 au, where tentative dips are present in the radial profiles of the integrated intensity of 13CO and C18O and pressure minima are observed in the azimuthal velocities. For the first time, we also detect downward vertical flows in this region, which coincide with the observed dust gap. The asymmetry in the emission heights may be a result of infall from the disk's environment. Another explanation is provided by a warped inner disk, casting a shadow onto one side of the disk.

Figures (31)

• Figure 1: Possible morphology of the HD 100546 system. Substructures such as the dust rings have been observed, while the planet candidates have been proposed to explain the observations. The two planets suggested in the dust gap may refer to the same object for which the actual location is yet undetermined.
• Figure 2: Examples of the intensity channels of the five CO lines, shown in steps of three from the central channel and for a velocity resolution of 210 m s$^{-1}$. The inner ring of the millimeter continuum is overlaid as dashed contours. The upper and lower disk surface are annotated in some channels as well as a potential gap and velocity kinks. A localized feature is seen in some of the channels within the solid circle. The beam of the observation is indicated in the bottom left corner of the first column panels. The channel maps are masked below 3 $\sigma$.
• Figure 3: Integrated intensity (top), peak intensity (middle), and line-of-sight velocity (bottom) of the five CO lines studied here. A localized feature is seen in $^{12}$CO 3-2 around 125 au, marked by the solid circle. Some contours are overlaid and their levels are indicated in the color bars. The beam of the observation is shown in the bottom left corner of the first-row panels. The integrated intensity maps are masked at 3$\sigma$, the peak intensity maps at 5$\sigma$.
• Figure 4: Azimuthally averaged and normalized radial integrated intensity profiles for the different CO lines. A radial profile of the Band 6 millimeter continuum is included as black dashed lines. The vertical grey dashed line marks the location inside which the gas emission rapidly drops (gas cavity). Some features of the profiles are annotated.
• Figure 5: Examples of the intensity channels of $^{12}$CO 2-1 (top), shown in steps of four around the central channel, alongside the best-fit model obtained with the discminer (middle) and the corresponding residuals (bottom). The iso-velocity curves of the upper and lower surfaces are overlaid as solid and dashed lines, respectively. The emitting surface and disk axes are plotted in the background as gray contours and dotted lines, respectively. The beam of the observation is indicated in the bottom left corner of the first panel.