ABSTRACT
Theoretical models of protoplanetary discs have shown the vertical shear instability (VSI) to be a prime candidate to explain turbulence in the dead zone of the disc. However, simulations of ...the VSI have yet to show consistent levels of key disc turbulence parameters like the stress-to-pressure ratio α. We aim to reconcile these different values by performing a parameter study on the VSI with focus on the disc density gradient p and aspect ratio h = H/R. We use full 2π 3D simulations of the disc for chosen set of both parameters. All simulations are evolved for 1000 reference orbits, at a resolution of 18 cells per h. We find that the saturated stress-to-pressure ratio in our simulations is dependent on the disc aspect ratio with a strong scaling of α∝h2.6, in contrast to the traditional α model, where viscosity scales as ν∝αh2 with a constant α. We also observe consistent formation of large scale vortices across all investigated parameters. The vortices show uniformly aspect ratios of χ ≈ 10 and radial widths of approximately 1.5H. With our findings we can reconcile the different values reported for the stress-to-pressure ratio from both isothermal and full radiation hydrodynamics models, and show long-term evolution effects of the VSI that could aide in the formation of planetesimals.
We perform simulations of the dust and gas disk evolution to investigate the observational features of a dust pileup at the dead-zone inner edge. We show that the total mass of accumulated dust ...particles is sensitive to the turbulence strength in the dead zone, dead, because of the combined effect of turbulence-induced particle fragmentation (which suppresses particle radial drift) and turbulent diffusion. For a typical critical fragmentation velocity of silicate dust particles of 1 m s−1, the stress-to-pressure ratio dead needs to be lower than 3 × 10−4 for dust trapping to operate. The obtained dust distribution is postprocessed using the radiative transfer code RADMC-3D to simulate infrared scattered-light images of the inner part of protoplanetary disks with a dust pileup. We find that a dust pileup at the dead-zone inner edge, if present, casts a shadow extending out to ∼10 au. In the shadowed region the temperature significantly drops, which in some cases yields even multiple water snow lines. We also find that even without a dust pileup at the dead-zone inner edge, the disk surface can become thermally unstable, and the excited waves can naturally produce shadows and ring-like structures in observed images. This mechanism might account for the ring-like structures seen in the scattered-light images of some disks, such as the TW Hya disk.
Aims.
In recent years, sub-millimeter (mm) observations of protoplanetary disks have revealed an incredible diversity of substructures in the dust emission. An important result was the finding that ...dust grains of mm size are embedded in very thin dusty disks. This implies that the dust mass fraction in the midplane becomes comparable to that of the gas, increasing the importance of the interaction between the two components there.
Methods.
We use numerical 2.5D simulations to study the interaction between gas and dust in fully globally stratified disks. To this end, we employ the recently developed dust grain module of the PLUTO code. Our model focuses on a typical T Tauri disk model, simulating a short patch of the disk at 10 au which includes grains of a constant Stokes number of
St
= 0.01 and
St
= 0.1, corresponding to grains with sizes of 0.9 cm and 0.9 mm, respectively, for the given disk model.
Results.
By injecting a constant pebble flux at the outer domain, the system reaches a quasi-steady state of turbulence and dust concentrations driven by the streaming instability. For our given setup, and using resolutions up to 2500 cells per scale height, we resolve the streaming instability that leads to local dust clumping and concentrations. Our results show dust density values of around 10–100 times the gas density with a steady-state pebble flux of between 3.5 × 10
−4
and 2.5 × 10
−3
M
Earth
yr
−1
for the models with
St
= 0.01 and
St
= 0.1.
Conclusions.
Grain size and pebble flux for model
St
= 0.01 compare well with dust evolution models of the first million years of disk evolution. For those grains, the scatter opacity dominates the extinction coefficient at mm wavelengths. These types of global dust and gas simulations are a promising tool for studies of the gas and dust evolution at pressure bumps in protoplanetary disks.
We present a survey of optical O i emission at 6300 toward 65 T Tauri stars at the spectral resolution of ∼7 km s−1. Past work identified a highly blueshifted velocity component (HVC) tracing ...microjets and a less blueshifted low-velocity component (LVC) attributed to winds. We focus here on the LVC kinematics to investigate links between winds, jets, accretion, and disk dispersal. We track the behavior of four types of LVC components: a broad and a narrow component ("BC" and "NC," respectively) in LVCs that are decomposed into two Gaussians which typically have an HVC, and single-Gaussian LVC profiles separated into those that have an HVC ("SCJ") and those that do not ("SC"). The LVC centroid velocities and line widths correlate with the HVC EW and accretion luminosity, suggesting that LVC/winds and HVC/jets are kinematically linked and connected to accretion. The deprojected HVC velocity correlates with accretion luminosity, showing that faster jets come with higher accretion. BC and NC kinematics correlate, and their blueshifts are maximum at ∼35°, suggesting a conical wind geometry with this semi-opening angle. Only SCs include n13-31 up to ∼3, and their properties correlate with this infrared index, showing that O i emission recedes to larger radii as the inner dust is depleted, tracing less dense/hot gas and a decrease in wind velocity. Altogether, these findings support a scenario where optically thick, accreting inner disks launch radially extended MHD disk winds that feed jets, and where inner disk winds recede to larger radii and jets disappear in concert with dust depletion.
Abstract
Recent millimeter and infrared observations have shown that gap- and ring-like structures are common in both dust thermal emission and scattered light of protoplanetary disks. We investigate ...the impact of the so-called thermal wave instability (TWI) on the millimeter and infrared scattered light images of disks. We perform 1+1D simulations of the TWI and confirm that the TWI operates when the disk is optically thick enough for stellar light, i.e., small-grain-to-gas mass ratio of ≳0.0001. The midplane temperature varies as the waves propagate, and hence gap and ring structures can be seen in both millimeter and infrared emission. The millimeter substructures can be observed even if the disk is fully optically thick since it is induced by the temperature variation, while density-induced substructures would disappear in the optically thick regime. The fractional separation between TWI-induced ring and gap is Δ
r
/
r
∼ 0.2–0.4 at ∼10–50 au, which is comparable to those found by the Atacama Large Millimeter/submillimeter Array. Due to the temperature variation, snow lines of volatile species move radially and multiple snow lines are observed even for a single species. The wave propagation velocity is as fast as ∼0.6 au yr
−1
, which can be potentially detected with a multiepoch observation with a time separation of a few years.
Theoretical models of the ionization state in protoplanetary disks suggest the existence of large areas with low ionization and weak coupling between the gas and magnetic fields. In this regime ...hydrodynamical instabilities may become important. In this work we investigate the gas and dust structure and dynamics for a typical T Tauri system under the influence of the vertical shear instability (VSI). We use global 3D radiation hydrodynamics simulations covering all 360° of azimuth with embedded particles of 0.1 and 1 mm size, evolved for 400 orbits. Stellar irradiation heating is included with opacities for 0.1-10 m sized dust. Saturated VSI turbulence produces a stress-to-pressure ratio of . The value of is lowest within 30 au of the star, where thermal relaxation is slower relative to the orbital period and approaches the rate below which VSI is cut off. The rise in from 20 to 30 au causes a dip in the surface density near 35 au, leading to Rossby wave instability and the generation of a stationary, long-lived vortex spanning about 4 au in radius and 40 au in azimuth. Our results confirm previous findings that millimeter-sized grains are strongly vertically mixed by the VSI. The scale height aspect ratio for 1 mm grains is determined to be 0.037, much higher than the value H/r = 0.007 obtained from millimeter-wave observations of the HL Tau system. The measured aspect ratio is better fit by nonideal MHD models. In our VSI turbulence model, the millimeter grains drift radially inwards and many are trapped and concentrated inside the vortex. The turbulence induces a velocity dispersion of ∼12 m s−1 for the millimeter grains, indicating that grain-grain collisions could lead to fragmentation.
Context.
The transition between magnetorotational instability (MRI)-active and magnetically dead regions corresponds to a sharp change in the disk turbulence level, where pressure maxima may form, ...hence potentially trapping dust particles and explaining some of the observed disk substructures.
Aims.
We aim to provide the first building blocks toward a self-consistent approach to assess the dead zone outer edge as a viable location for dust trapping, under the framework of viscously driven accretion.
Methods.
We present a 1+1D global magnetically driven disk accretion model that captures the essence of the MRI-driven accretion, without resorting to 3D global nonideal magnetohydrodynamic (MHD) simulations. The gas dynamics is assumed to be solely controlled by the MRI and hydrodynamic instabilities. For given stellar and disk parameters, the Shakura–Sunyaev viscosity parameter,
α
, is determined self-consistently under the adopted framework from detailed considerations of the MRI with nonideal MHD effects (Ohmic resistivity and ambipolar diffusion), accounting for disk heating by stellar irradiation, nonthermal sources of ionization, and dust effects on the ionization chemistry. Additionally, the magnetic field strength is numerically constrained to maximize the MRI activity.
Results.
We demonstrate the use of our framework by investigating steady-state MRI-driven accretion in a fiducial protoplanetary disk model around a solar-type star. We find that the equilibrium solution displays no pressure maximum at the dead zone outer edge, except if a sufficient amount of dust particles has accumulated there before the disk reaches a steady-state accretion regime. Furthermore, the steady-state accretion solution describes a disk that displays a spatially extended long-lived inner disk gas reservoir (the dead zone) that accretes a few times 10
−9
M
⊙
yr
−1
. By conducting a detailed parameter study, we find that the extent to which the MRI can drive efficient accretion is primarily determined by the total disk gas mass, the representative grain size, the vertically integrated dust-to-gas mass ratio, and the stellar X-ray luminosity.
Conclusions.
A self-consistent time-dependent coupling between gas, dust, stellar evolution models, and our general framework on million-year timescales is required to fully understand the formation of dead zones and their potential to trap dust particles.
Abstract
High-angular resolution observations at submillimeter/millimeter wavelengths of disks surrounding young stars have shown that their morphology is made of azimuthally symmetric or ...point-symmetric substructures, in some cases with spiral arms or localized spur- or crescent-shaped features. The majority of theoretical studies with the aim of interpreting the observational results have focused on disk models with planets under the assumption that the disk substructures are due to disk–planet interaction. However, so far, only in very few cases have exoplanets been detected in these systems. Furthermore, some substructures are expected to appear
before
planets form, as they are necessary to drive the concentration of small solids which can lead to the formation of planetesimals. In this work we present observational predictions from high-resolution 3D radiative hydrodynamical models that follow the evolution of gas and solids in a prototoplanetary disk. We focus on substructures in the distribution of millimeter-sized and smaller solid particles produced by the vertical shear instability. We show that their characteristics are compatible with some of the shallow gaps detected in recent observations at sub-mm/mm wavelengths and present predictions for future observations with better sensitivity and angular resolution with ALMA and a Next Generation Very Large Array.
Context.
Dynamical and turbulent motions of gas in a protoplanetary disk are crucial for their evolution and are thought to affect planet formation. Recent (sub-)millimeter observations show evidence ...of weak turbulence in the disk’s outer regions. However, the detailed physical mechanism of turbulence in these outer regions remains uncertain. The vertical shear instability (VSI) is a promising candidate mechanism to produce turbulence in the outer parts of the disk.
Aims.
Our objective is to study the observability of the gas velocity structure produced by the VSI via CO kinematics with the Atacama Large Millimetre/submillimetre Array (ALMA).
Methods.
We performed global 3D hydrodynamical simulations of an inviscid and locally isothermal VSI-unstable disk. We post-processed the simulation results with radiative transfer calculations and produced synthetic predictions of CO rotational emission lines. Next, we computed the line of sight velocity map and its deviations from a sub-Keplerian equilibrium solution. We explored the detectability of the VSI by identifying kinematic signatures using realistic simulated observations using the CASA package.
Results.
Our 3D hydrodynamical simulations of the VSI show the steady state dynamics of the gas in great detail. From the velocity structure, we infer a turbulent stress value of
α
rϕ
= 1.4 × 10
−4
. On large scales, we observe clear velocity deviations of the order of 50 m s
−1
as axisymmetric rings with radially interspersed signs. By comparing synthetic observations at different inclinations we find optimal conditions at
i
≲ 20° to trace for the kinematic structures of the VSI. We found that current diagnostics to constrain gas turbulence from nonthermal broadening of the molecular line emission are not applicable to anisotropic VSI turbulence.
Conclusions.
We conclude that the detection of kinematic signatures produced by the VSI is possible with ALMA’s current capabilities. Observations including an extended antenna configuration are required to resolve the structure (beam sizes below ~10 au). The highest spectral resolution available is needed (~0.05 km s
−1
with ALMA Band 6) for a robust detection. The characterization of the large-scale velocity perturbations is required to constrain the turbulence level produced by the VSI from gas observations.
Context. The increasing number of newly detected exoplanets at short orbital periods raises questions about their formation and migration histories. Planet formation and migration depend heavily on ...the structure and dynamics of protoplanetary disks. A particular puzzle that requires explanation arises from one of the key results of the Kepler mission, namely the increase in the planetary occurrence rate with orbital period up to 10 days for F, G, K and M stars. Aims. We investigate the conditions for planet formation and migration near the dust sublimation front in protostellar disks around young Sun-like stars. We are especially interested in determining the positions where the drift of pebbles would be stopped, and where the migration of Earth-like planets and super-Earths would be halted. Methods. For this analysis we use iterative 2D radiation hydrostatic disk models which include irradiation by the star, and dust sublimation and deposition depending on the local temperature and vapor pressure. Results. Our results show the temperature and density structure of a gas and dust disk around a young Sun-like star. We perform a parameter study by varying the magnetized turbulence onset temperature, the accretion stress, the dust mass fraction, and the mass accretion rate. Our models feature a gas-only inner disk, a silicate sublimation front and dust rim starting at around 0.08 au, an ionization transition zone with a corresponding density jump, and a pressure maximum which acts as a pebble trap at around 0.12 au. Migration torque maps show Earth- and super-Earth-mass planets halt in our model disks at orbital periods ranging from 10 to 22 days. Conclusions. Such periods are in good agreement with both the inferred location of the innermost planets in multiplanetary systems, and the break in planet occurrence rates from the Kepler sample at 10 days. In particular, models with small grains depleted produce a trap located at a 10-day orbital period, while models with a higher abundance of small grains present a trap at around a 17-day orbital period. The snow line lies at 1.6 au, near where the occurrence rate of the giant planets peaks. We conclude that the dust sublimation zone is crucial for forming close-in planets, especially when considering tightly packed super-Earth systems.
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