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Register a new userSignatures of an eccentric disc cavity: Dust and gas in IRS 48
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We test the hypothesis that the disc cavity in the `transition disc Oph IRS 48 is carved by an unseen binary companion. We use 3D dust-gas smoothed-particle hydrodynamics simulations to demonstrate that marginally coupled dust grains concentrate in the gas over-density that forms in in the cavity around a low binary mass ratio binary. This produces high contrast ratio dust asymmetries at the cavity edge similar to those observed in the disc around IRS 48 and other transition discs. This structure was previously assumed to be a vortex. However, we show that the observed velocity map of IRS 48 displays a peculiar asymmetry that is not predicted by the vortex hypothesis. We show the unusual kinematics are naturally explained by the non-Keplerian flow of gas in an eccentric circumbinary cavity. We further show that perturbations observed in the isovelocity curves of IRS 48 may be explained as the product of the dynamical interaction between the companion and the disc. The presence of a $sim$0.4 M$_{odot}$ companion at a $sim$10 au separation can qualitatively explain these observations. High spatial resolution line and continuum imaging should be able to confirm this hypothesis.
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We present ALMA observations of the $^{12}$CO, $^{13}$CO, C$^{18}$O J=2-1 transitions and the 1.3,mm continuum emission for the circumbinary disc around HD 142527, at an angular resolution of $approx$,0farcs3. We observe multiple spiral structures in intensity, velocity and velocity dispersion for the $^{12}$CO and $^{13}$CO gas tracers. A newly detected $^{12}$CO spiral originates from the dust horseshoe, and is rotating at super-Keplerian velocity or vertically ascending, whilst the inter-spiral gas is rotating at sub-Keplerian velocities. This new spiral possibly connects to a previously identified spiral, thus spanning > 360$^circ$. A spatial offset of ~30 au is observed between the $^{12}$CO and $^{13}$CO spirals, to which we hypothesize that the gas layers are propagating at different speeds (``surfing) due to a non-zero vertical temperature gradient. Leveraging the varying optical depths between the CO isotopologues, we reconstruct temperature and column density maps of the outer disc. Gas surface density peaks at r,$approx$,180,au, coincident with the peak of continuum emission. Here the dust grains have a Stokes number of $approx$,1, confirming radial and azimuthal trapping in the horseshoe. We measure a cavity radius at half-maximum surface density of $approx$,100,au, and a cavity eccentricity between 0.3 and 0.45.
Azimuthally asymmetric dust distributions observed with ALMA in transition disks have been interpreted as dust traps. We present VLA Ka band (34 GHz or 0.9 cm) and ALMA Cycle 2 Band 9 (680 GHz or 0.45 mm) observations at 0.2 resolution of the Oph IRS 48 disk, which suggest that larger particles could be more azimuthally concentrated than smaller dust grains, assuming an axisymmetric temperature field or optically thin 680 GHz emission. Fitting an intensity model to both data demonstrates that the azimuthal extent of the millimeter emission is 2.3 $pm0.9$ times as wide as the centimeter emission, marginally consistent with the particle trapping mechanism under the above assumptions. The 34 GHz continuum image also reveals evidence for ionized gas emission from the star. Both the morphology and the spectral index variations are consistent with an increase of large particles in the center of the trap, but uncertainties remain due to the continuum optical depth at 680 GHz. Particle trapping has been proposed in planet formation models to allow dust particles to grow beyond millimeter sizes in the outer regions of protoplanetary disks. The new observations in the Oph IRS 48 disk provide support for the dust trapping mechanism for centimeter-sized grains, although additional data is required for definitive confirmation.
The combination of high resolution and sensitivity offered by ALMA is revolutionizing our understanding of protoplanetary discs, as their bulk gas and dust distributions can be studied independently. In this paper we present resolved ALMA observations of the continuum emission ($lambda=1.3$ mm) and CO isotopologues ($^{12}$CO, $^{13}$CO, C$^{18}$O $J=2-1$) integrated intensity from the disc around the nearby ($d = 162$ pc), intermediate mass ($M_{star}=1.67,M_{odot}$) pre-main-sequence star CQ Tau. The data show an inner depression in continuum, and in both $^{13}$CO and C$^{18}$O emission. We employ a thermo-chemical model of the disc reproducing both continuum and gas radial intensity profiles, together with the disc SED. The models show that a gas inner cavity with size between 15 and 25 au is needed to reproduce the data with a density depletion factor between $sim 10^{-1}$ and $sim 10^{-3}$. The radial profile of the distinct cavity in the dust continuum is described by a Gaussian ring centered at $R_{rm dust}=53,$au and with a width of $sigma=13,$au. Three dimensional gas and dust numerical simulations of a disc with an embedded planet at a separation from the central star of $sim20,$au and with a mass of $sim 6textrm{-} 9,M_{rm Jup}$ reproduce qualitatively the gas and dust profiles of the CQ Tau disc. However, a one planet model appears not to be able to reproduce the dust Gaussian density profile predicted using the thermo-chemical modeling.
(Abridged) Transition disks are recognized by the absence of emission of small dust grains inside a radius of up to several 10s of AUs. Due to the lack of angular resolution and sensitivity, the gas content of such dust holes has not yet been determined, but is of importance to constrain the mechanism leading to the dust holes. Transition disks are thought to currently undergo the process of dispersal, setting an end to the giant planet formation process. We present new high-resolution observations with the Atacama Large Millimeter/ submillimeter Array (ALMA) of gas lines towards the transition disk Oph IRS 48 previously shown to host a large dust trap. ALMA has detected the $J=6-5$ line of $^{12}$CO and C$^{17}$O around 690 GHz (434 $mu$m) at a resolution of $sim$0.25$$ corresponding to $sim$30 AU (FWHM). The observed gas lines are used to set constraints on the gas surface density profile. New models of the physical-chemical structure of gas and dust in Oph IRS 48 are developed to reproduce the CO line emission together with the spectral energy distribution (SED) and the VLT-VISIR 18.7 $mu$m dust continuum images. Integrated intensity cuts and the total spectrum from models having different trial gas surface density profiles are compared to observations. Using the derived surface density profiles, predictions for other CO isotopologues are made, which can be tested by future ALMA observations of the object. The derived gas surface density profile points to the clearing of the cavity by one or more massive planet/companion rather than just photoevaporation or grain-growth.
We investigate the impact of a highly eccentric 10 $M_{rm oplus}$ (where $M_{rm oplus}$ is the Earth mass) planet embedded in a dusty protoplanetary disk on the dust dynamics and its observational implications. By carrying out high-resolution 2D gas and dust two-fluid hydrodynamical simulations, we find that the planets orbit can be circularized at large radii. After the planets orbit is circularized, partial gap opening and dust ring formation happen close to the planets circularization radius, which can explain the observed gaps/rings at the outer region of disks. When the disk mass and viscosity become low, we find that an eccentric planet can even open gaps and produce dust rings close to the pericenter and apocenter radii before its circularization. This offers alternative scenarios for explaining the observed dust rings and gaps in protoplanetary disks. A lower disk viscosity is favored to produce brighter rings in observations. An eccentric planet can also potentially slow down the dust radial drift in the outer region of the disk when the disk viscosity is low ($alpha lesssim2times10^{-4}$) and the circularization is faster than the dust radial drift.
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