Ringed Structures of the HD 163296 Protoplanetary Disk Revealed by ALMA

Abstract

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 CO, 13 CO, and C 18 O J ¼ 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.

Spiral Arms in Disks: Planets or Gravitational Instability?

“…More graphically, high-resolution submillimeter continuum images of disks made with ALMA commonly show stunning rings and gaps (ALMA Partnership et al 2015;Andrews et al 2016;Cieza et al 2016Cieza et al , 2017Isella et al 2016;Loomis et al 2017;Dipierro et al 2018;Fedele et al 2018;Huang et al 2018), structures that signal that disk solids have grown beyond centimeter sizes and possibly into planetary-mass objects (e.g., Dipierro et al 2015;Dong et al 2015cDong et al , 2017a). These results exclaim that disks are highly evolved and call into question disk mass estimates made under the usual assumption of simple, unevolved, optically thin disks.…”

Searching for Baby Planets in a Star’s Dusty Rings

“…(1) and (2), and the procedure described in the Supplemental Material [26], we find that the dust emission measured at angular radii less than 0.3 00 and lager than 1.8 00 is consistent with a dust surface density characterized by γ ¼ 0.1, r c ¼ 90 A.U., Σ c ¼ 0.42 g cm −2 . The low value of γ implies that the dust density within about 90 A.U.…”

“…We ascribe the 13 CO or C 18 O central minima to an oversubtraction of the continuum emission, and not to a physical decrement of the gas density or temperature. This point is discussed in more detail in the Supplemental Material [26]. Third, the molecular emission is much more extended and decreases with the orbital radius more slowly than the continuum.…”

“…The acquisition and calibration of the data are described in the Supplemental Material [26]. Also displayed are the radial intensity profiles across the disk calculated by averaging the flux density over elliptical annuli characterized by a position angle of 132°and an eccentricity of 0.7 to correct for the disk inclination and orientation [22,25].…”

“…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.…”