Ringed Structures of the HD 163296 Protoplanetary Disk Revealed by ALMA

We present Atacama Large Millimeter and Submillimeter Array observations of the protoplanetary disk around the Herbig Ae star HD 163296 that trace the spatial distribution of millimeter-sized particles and cold molecular gas on spatial scales as small as 25 astronomical units (A.U.). The image of the disk recorded in the 1.3 mm continuum emission reveals three dark concentric rings that indicate the presence of dust depleted gaps at about 60, 100, and 160 A.U. from the central star. The maps of the ${}^{12}\mathrm{CO}$, ${}^{13}\mathrm{CO}$, and $\mathrm{C}{}^{18}\mathrm{O}$ $J=2-1$ emission do not show such structures but reveal a change in the slope of the radial intensity profile across the positions of the dark rings in the continuum image. By comparing the observations with theoretical models for the disk emission, we find that the density of CO molecules is reduced inside the middle and outer dust gaps. However, in the inner ring there is no evidence of CO depletion. From the measurements of the dust and gas densities, we deduce that the gas-to-dust ratio varies across the disk and, in particular, it increases by at least a factor 5 within the inner dust gap compared to adjacent regions of the disk. The depletion of both dust and gas suggests that the middle and outer rings could be due to the gravitational torque exerted by two Saturn-mass planets orbiting at 100 and 160 A.U. from the star. On the other hand, the inner dust gap could result from dust accumulation at the edge of a magnetorotational instability dead zone, or from dust opacity variations at the edge of the CO frost line. Observations of the dust emission at higher angular resolution and of molecules that probe dense gas are required to establish more precisely the origins of the dark rings observed in the HD 163296 disk.

Figure 1

ALMA images of the HD 163296 disk emission recorded in 1.3 mm dust continuum (top left), ${}^{12}\mathrm{CO}$ (top middle), ${}^{13}\mathrm{CO}$ (bottom left), and $\mathrm{C}{}^{18}\mathrm{O}$ (bottom middle) $J=2-1$ line emission. The angular resolution of the observations, ${0.22}^{\prime \prime }\times {0.14}^{\prime \prime }$, is indicated by the filled white ellipse in the continuum image. The dashed ellipses in the CO maps indicate the locations of the dark rings seeing in the continuum map. Azimuthally averaged profiles, normalized to the peak intensities, are shown in the right panels. They are calculated by averaging on elliptical annuli with a position angle of 132°, an eccentricity of 0.7, and a width equal to $1/4$ of the angular resolution of the observations. The error bars are calculated by dividing the root mean square noise of the observations (see Table I in the Supplemental Material [26]) by the square root of the number of independent beams in each annulus. The vertical lines indicate the position of the dark ($D$) and bright ($B$) rings observed in the continuum map. The horizontal bar in the top right panel indicates the angular resolution of the observations.

Figure 2

Constraints on the dust and gas surface density (left and middle panels, respectively), and on the gas-to-dust ratio (right panel), derived by comparing ALMA observations of the HD163296’s disk with synthetic models for the disk emission. The red solid curve indicates the prediction for a disk model perturbed by three planets with masses of 0.1, 0.3, and 0.3$M_J$ orbiting at 62, 105, and 160 A.U. from the central star.

Figure 3

Temperature of the HD 163296 disk inferred from the observations of the CO emission. The white solid lines correspond to temperatures of 18, 23, and 28 K, and indicate the range of condensation temperatures for CO molecules. The vertical dashed line indicates the location of the dust gaps. The green and yellow curves indicate the pressure scale height of gas ($h_{\mathrm{gas}}$) and of large dust grains ($h_{\mathrm{dust}}$), respectively.