arXiv subjects

Mario Flock

Publications and source records attributed to Mario Flock.

At least 55 records · Page 3Linked to original sources

arXiv2024

UV-processing of icy pebbles in the outer parts of VSI-turbulent disks

Icy dust particles emerge in star-forming clouds and are subsequently incorporated in protoplanetary disks, where they coagulate into larger pebbles up to mm in size. In the disk midplane, ices are shielded from UV radiation, but moderate levels of disk turbulence can lift small particles to the disk surface, where they can be altered, or destroyed. Nevertheless, studies of comets and meteorites generally find that ices at least partly retained their interstellar medium (ISM) composition before being accreted onto these minor bodies. Here we model this process through hydrodynamical simulations with VSI-driven turbulence in the outer protoplanetary disk. We use the PLUTO code in a 2.5 D global accretion setup and include Lagrangian dust particles of 0.1 and 1 mm sizes. In a post-processing step, we use the RADMC3D code to generate the local UV radiation field to assess the level of ice processing of pebbles. We find that a small fraction ($\sim$17$\%$) of 100 $μ$m size particles are frequently lifted up to $Z/R=0.2$ which can result in the loss of their pristine composition as their residence time in this layer allows for effective CO and water photodissociation. The larger 1 mm size particles remain UV-shielded in the disk midplane throughout the dynamical evolution of the disk. Our results indicate that the assembly of icy bodies via the accretion of drifting mm-size icy pebbles can explain the presence of pristine ice from the ISM, even in VSI-turbulent disks. Nevertheless, particles $\leq$ 100 $μ$m experience efficient UV processing and may mix with unaltered icy pebbles, resulting in a less ISM-like composition in the midplane.

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arXiv2024

Variability of the inner dead zone edge in 2D radiation hydrodynamic simulations

The inner regions of protoplanetary disks are prone to thermal instability (TI), which can significantly impact the thermal and dynamic evolution of planet-forming regions. Observable as episodic accretion outbursts, such periodic disturbances shape the disk's vertical and radial structure. We investigate the stability of the inner disk edge around a Class II T Tauri star and analyse the consequences of TI on the inner disk's evolution in both the vertical and radial dimensions. A particular focus is laid on the emergence and destruction of solid-trapping pressure maxima. Our 2D axisymmetric radiation hydrodynamic models include the transition to the dead zone from a highly turbulent inner disk, heating by both stellar irradiation and viscous dissipation, vertical and radial radiative transport and an adaptive dust-to-gas mass ratio. The simulated time frames include both the TI- and quiescent phases. We track the TI on S-curves of thermal stability. The TI in our models can develop in disks with moderate accretion rates and results from the activation of the magnetorotational instability (MRI) in the dead zone. The TI creates an extensive MRI active region around the midplane and disrupts the stable pebble- and migration trap at the inner edge of the dead zone. Our simulations consistently show the occurrence of TI-reflares, which, together with the initial TI, produce pressure maxima in the inner disk within 1 AU, possibly providing favourable conditions for streaming instability. On a timescale of a few thousand years, TI regularly disrupts the disk's radial and vertical structure within 1 AU. While several pressure maxima are created, stable migration traps are destroyed and reinstated after the TI phase. Our models provide a foundation for more detailed investigations into phenomena such as short-term variability of accretion rates.

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arXiv2024

Dust mass in protoplanetary disks with porous dust opacities

ALMA surveys have suggested that protoplanetary disks are not massive enough to form the known exoplanet population, under the assumption that the millimeter continuum emission is optically thin. In this work, we investigate how the mass determination is influenced when the porosity of dust grains is considered in radiative transfer models. The results show that disks with porous dust opacities yield similar dust temperature, but systematically lower millimeter fluxes compared to disks incorporating compact dust grains. Moreover, we recalibrate the relation between dust temperature and stellar luminosity for a wide range of stellar parameters, and calculate the dust masses of a large sample of disks using the traditionally analytic approach. The median dust mass from our calculation is about 6 times higher than the literature result, and this is mostly driven by the different opacities of porous and compact grains. A comparison of the cumulative distribution function between disk dust masses and exoplanet masses show that the median exoplanet mass is about 2 times lower than the median dust mass, if grains are porous, and there are no exoplanetary systems with masses higher than the most massive disks. Our analysis suggests that adopting porous dust opacities may alleviate the mass budget problem for planet formation. As an example illustrating the combined effects of optical depth and porous dust opacities on the mass estimation, we conduct new IRAM/NIKA-2 observations toward the IRAS 04370+2559 disk and perform a detailed radiative transfer modeling of the spectral energy distribution. The best-fit dust mass is roughly 100 times higher than the value from the traditionally analytic calculation. Future spatially resolved observations at various wavelengths are required to better constrain the dust mass.

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arXiv2024

From traffic jams to roadblocks: The outer regions of TW Hya with ALMA Band 8

We present Atacama Large Millimeter Array (ALMA) observations of TW Hya at 0.65 mm with 0.5 arcsecond angular resolution, together with high angular resolution archival observations at 0.87 mm, 1.3 mm, 2.1 mm and 3.1 mm. We constrain the outer disk emission with both image-plane retrieval, and visibility-plane modeling with non-parametric and parametric fitting tools. Our results confirm emission in the outer disk regions of TW Hya (60 au <= R <= 110 au) at 0.65 mm, 0.87 mm and 1.3 mm. With image-plane retrieval, we resolve the new continuum gap and ring, namely D79 and B86, at 0.87 mm and 1.3 mm. With visibility-plane modeling, we also detect this substructure at 0.65 mm in the form of a quasi-constant emission at the 1-sigma level. Furthermore, it has a high spectral index of 3.7, which may indicate dust grain sizes << 1 mm. It may be a dust trap or a traffic jam, that has a flux density of 60 mJy and a mass (1.59 earth masses) that accounts for up to 2% of the dust disk at 0.65 mm. In conclusion, we confirm the existence of a faint ring in the outer regions of TW Hya at multiple millimeter wavelengths. With visibility-plane modeling, we are able to set constrains that are 3 times better than the resolution of our Band 8 observations.

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arXiv2024

Support for fragile porous dust in a gravitationally self-regulated disk around IM Lup

Protoplanetary disks, the birthplace of planets, are expected to be gravitationally unstable in their early phase of evolution. IM Lup, a well-known T-Tauri star, is surrounded by a protoplanetary disk with spiral arms likely caused by gravitational instability. The IM Lup disk has been observed using various methods, but developing a unified explanatory model is challenging. Here we present a physical model of the IM Lup disk that offers a comprehensive explanation for diverse observations spanning from near-infrared to millimeter wavelengths. Our findings underscore the importance of dust fragility in retaining the observed millimeter emission and reveal the preference for moderately porous dust to explain observed millimeter polarization. We also find that the inner disk region is likely heated by gas accretion, providing a natural explanation for bright millimeter emission within 20 au. The actively heated inner region in the model casts a 100-au-scale shadow, aligning seamlessly with the near-infrared scattered light observation. The presence of accretion heating also supports the fragile dust scenario in which accretion efficiently heat the disk midplane. Due to the fragility of dust, it is unlikely that a potential embedded planet at 100 au formed via pebble accretion in a smooth disk, pointing to local dust enhancement boosting pebble accretion or alternative pathways such as outward migration or gravitational fragmentation.

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arXiv2024

Mixing is easy: New insights for cosmochemical evolution from pre-stellar core collapse

Signposts of early planet formation are ubiquitous in substructured young discs. Dense, hot and high-pressure regions formed during gravitational collapse process, integral to star formation, facilitate dynamical mixing of dust within the protostellar disc. This provides an incentive to constrain the role of gas-dust interaction and resolve zones of dust concentration during star-disc formation. We explore if thermal and dynamical conditions developed during disc formation can generate gas flows that efficiently mix and transport well-coupled gas and dust components. We simulated the collapse of dusty molecular cloud cores with the hydrodynamics code PLUTO augmented with radiation transport and self-gravity. We used a 2D axisymmetric geometry and follow the azimuthal component of velocity. Dust was treated as Lagrangian particles that are subject to drag from the gas, whose motion is computed on a Eulerian grid. We considered 1, 10 and 100 micron-sized neutral spherical dust. Importantly, the equation of state accurately includes molecular hydrogen dissociation. We focus on molecular cloud core masses of 1 and 3 Msun and explore effects of initial rotation rates and cloud core sizes. Our study underlines mechanisms for early transport of dust from inner hot disc regions via the occurrence of meridional flows and outflow. The vortical flow fosters dynamical mixing and retention of dust while thermal pressure driven outflow replenishes dust in the outer disc. Young dynamical precursors to planet-forming discs exhibit regions with complex hydrodynamical gas features and high-temperature structures. These can play a crucial role in concentrating dust for subsequent growth into protoplanets. Dust transport, especially, from sub-au scales surrounding the protostar to outer relatively cooler parts, offers an efficient pathway for thermal reprocessing during pre-stellar core collapse. [Abridged]

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arXiv2024

The inner disk rim of HD 163296: linking radiative hydrostatic models with infrared interferometry

Previous studies of the protoplanetary disk HD 163296 revealed that the morphology of its sub-au infrared emission encompasses the terminal sublimation front of dust grains, referred to as the inner rim, but also extends into the (supposedly) dust-free region within it. Here, we present a set of radiative hydrostatic simulations of the inner rim in order to assess how much the rim alone can contribute to the observed interferometric visibilities $V$, half-light radii $R_{\mathrm{hl}}$, and fractional disk fluxes $\mathcal{F}$ in the wavelength range $1.5$--$13\,μ\mathrm{m}$. In our set of models, we regulate the cooling efficiency of the disk via the boundary condition for radiation diffusion and we also modify the shape of the sublimation front. We find that when the cooling efficiency is reduced, the infrared photosphere at the rim becomes hotter, leading to an increase of $R_{\mathrm{hl}}$ sufficient to match the observations. However, the near-infrared disk flux is typically too low ($\mathcal{F}\simeq0.25$ at $1.5\,μ\mathrm{m}$), resulting in H-band visibility curves located above the observed data. We show that the match to the H-band observations up to moderate baselines can be improved when a wall-shaped rather than curved sublimation front is considered. Nevertheless, our model visibilities always exhibit a bounce at long baselines, which is not observed, confirming the need for additional emission interior to the rim. In summary, our study illustrates how the temperature structure and geometry of the inner rim needs to change in order to boost the rim's infrared emission.

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arXiv2023

Vertical shear instability in two-moment radiation-hydrodynamical simulations of irradiated protoplanetary disks I. Angular momentum transport and turbulent heating

We studied the linear and nonlinear evolution of the Vertical Shear Instability (VSI) in axisymmetric models of protoplanetary disks, focusing on the transport of angular momentum, the produced temperature perturbations, and the applicability of local stability conditions. We modeled the gas-dust mixture via high-resolution two-moment (M1) radiation-hydrodynamical simulations including stellar irradiation with frequency-dependent opacities. We found that, given sufficient depletion of small grains (with a dust-to-gas mass ratio of $10\%$ of our nominal value of $10^{-3}$ for $<0.25$ $μ$m grains), the VSI can operate in surface disk layers while being inactive close to the midplane, resulting in a suppression of the VSI body modes. The VSI reduces the initial vertical shear in bands of approximately uniform specific angular momentum, whose formation is likely favored by the enforced axisymmetry. Similarities with Reynolds stresses and angular momentum distributions in 3D simulations suggest that the VSI-induced angular momentum mixing in the radial direction may be predominantly axisymmetric. The stability regions in our models are well explained by local stability criteria, while the employment of global criteria is still justifiable up to a few scale heights above the midplane, at least as long as VSI modes are radially optically thin. Turbulent heating produces only marginal temperature increases of at most $0.1\%$ and $0.01\%$ in the nominal and dust-depleted models, respectively, peaking at a few (approximately three) scale heights above the midplane. We conclude that it is unlikely that the VSI can, in general, lead to any significant temperature increase since that would either require it to efficiently operate in largely optically thick disk regions or to produce larger levels of turbulence than predicted by models of passive irradiated disks.

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arXiv2023

Vertical shear instability in two-moment radiation-hydrodynamical simulations of irradiated protoplanetary disks II. Secondary instabilities and stability regions

The vertical shear instability (VSI) is a hydrodynamical instability likely to produce turbulence in the dead zones of protoplanetary disks. Various aspects of this instability remain to be understood, including the disk regions where it can operate and the physical phenomena leading to its saturation. In this work, we studied the growth and evolution of secondary instabilities parasitic to the VSI, examining their relation with its saturation in axisymmetric radiation-hydrodynamical simulations of protoplanetary disks. We also constructed stability maps for our disk models, considering temperature stratifications enforced by stellar irradiation and radiative cooling and incorporating the effects of dust-gas collisions and molecular line emission. We found that the flow pattern produced by the interplay of the axisymmetric VSI modes and the baroclinic torque forms bands of nearly uniform specific angular momentum. In the high-shear regions in between these bands, the Kelvin-Helmholtz instability (KHI) is triggered. The significant transfer of kinetic energy to small-scale eddies produced by the KHI and possibly even the baroclinic acceleration of eddies limit the maximum energy of the VSI modes, likely leading to the saturation of the VSI. A third instability mechanism, consisting of an amplification of eddies by baroclinic torques, forms meridional vortices with Mach numbers up to $\sim 0.4$. Our stability analysis suggests that protoplanetary disks can be VSI-unstable in surface layers up to tens of au for reasonably high gas emissivities, even in regions where the midplane is stable. This picture is consistent with current observations of disks showing thin midplane millimeter-sized dust layers while appearing vertically extended in optical and near-infrared wavelengths.

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arXiv2023

Kinematic signatures of planet-disk interactions in VSI-turbulent protoplanetary disks

Context. Planets are thought to form inside weakly ionized regions of protoplanetary disks, where turbulence creates ideal conditions for solid growth. However, the nature of this turbulence is still uncertain. In this zone, vertical shear instability (VSI) can operate, inducing a low level of gas turbulence and large-scale motions. Resolving kinematic signatures of VSI may reveal the origin of turbulence in planet-forming disks. However, an exploration of kinematic signatures of the interplay between VSI and forming planets is needed for a correct interpretation of radio interferometric observations. Robust detection of VSI would lead to a deeper understanding of the impact of gas turbulence on planet formation. Aims. The goal of this study is to explore the effect of VSI on the disk substructures triggered by an embedded massive planet. We focus on the impact of this interplay on CO kinematic observations with ALMA. Methods. We conduct global 3D hydrodynamical simulations of VSI-unstable disks with and without embedded massive planets, exploring Saturn- and Jupiter-mass cases. We study the effect of planets on the VSI gas dynamics, comparing with viscous disks. Post-processing the simulations with a radiative transfer code, we examine the kinematic signatures expected in CO molecular line emission, varying disk inclination. Further, we simulate ALMA high-resolution observations to test the observability of VSI and planetary signatures. Results. The embedded planet dampens the VSI along a radial region, most effective at the disk midplane. For the Saturn case, the VSI modes are distorted by the planet's spirals producing mixed kinematic signatures. For the Jupiter case, the planet's influence dominates the disk gas kinematics. Conclusions. The presence of massive embedded planets can weaken the VSI large-scale gas flows, limiting its observability in CO kinematic observations.

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arXiv2023

Probing the Temperature Structure of the Inner Region of a Protoplanetary Disk

Midplane heating induced by disk accretion plays a key role in determining the disk temperature particularly at the inner disk midplane where planets form. However, the efficiency of accretion heating has been not well constrained by observations. We construct two-dimensional models of the Class II disk around CW Tau, taking into account the midplane heating. The models are compared with the ALMA dust continuum observations at Bands 4, 6, 7 and 8, with an angular resolution of 0.1 arcsec. The observed brightness temperatures are almost wavelength-indenpendent at $\lesssim$10 au. We find that if the maximum dust size $a_{\rm max}$ is $\lesssim100~{\rm μm}$, the brightness temperatures predicted by the model exceed the observed values, regardless of the efficiency of accretion heating. The low observed brightness temperatures can be explained if millimeter scattering reduces the intensity. If the disk is passive, $a_{\rm max}$ needs to be either $\sim150~{\rm μm}$ or $\gtrsim$ few ${\rm cm}$. The accretion heating significantly increases the brightness temperature particularly when $a_{\rm max}\lesssim300~{\rm μm}$, and hence $a_{\rm max}$ needs to be either $\sim300~{\rm μm}$ or $\gtrsim$ few ${\rm cm}$. The midplane temperature is expected to be $\sim$1.5-3 times higher than the observed brightness temperatures, depending on the models. The dust settling effectively increases the temperature of the dust responsible for the millimeter emission in the active disk, which makes the model with $300~{\rm μm}$-sized dust overpredicts the brightness temperatures when strong turbulence is absent. Porous dust (porosity of 0.9) makes the accretion heating more efficient so that some sort of reduction in accretion heating is required. Future longer wavelength and higher angular resolution observations will help us constrain the heating mechanisms of the inner protoplanetary disks.

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arXiv2023

Meridional Circulation driven by Planetary Spiral Wakes in Radiative and Magnetized Protoplanetary Discs

We study a Jupiter-mass planet formation for the first time in radiative magneto-hydrodynamics (MHD) simulations and compare it with pure hydrodynamical simulations, as well as to different isothermal configurations. We found that the meridional circulation is the same in every setup. The planetary spiral wakes drive a vertical stirring inside the protoplanetary disc and the encounter with these shock fronts also helps in delivering gas vertically onto the Hill-sphere. The accretion dynamics are unchanged: the planet accretes vertically, and there is outflow in the midplane regions inside the Hill-sphere. We determined the effective $α$-viscosity generated in the disc by the various angular momentum loss mechanisms, which showed that magnetic fields produce high turbulence in the ideal MHD limit, that grows from $α\sim 10^{-2.5}$ up to $\sim 10^{-1.5}$ after the planet spirals develop. In the HD simulations, the planetary spirals contribute to $α\sim 10^{-3}$, making this a very important angular momentum transport mechanism. Due to the various $α$ values in the different setups, the gap opening is different in each case. In the radiative MHD setups, the high turbulent viscosity prevents gap opening, leading to a higher Hill mass, and no clear dust trapping regions. While the Hill accretion rate is $10^{-6} \rm{M_{Jup}/yr}$ in all setups, the accretion variability is orders of magnitude higher in radiative runs than in isothermal ones. Finally, with higher-resolution runs, the magneto-rotational instability started to be resolved, changing the effective viscosity and increasing the heating in the disc.

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arXiv2023

The impact of dust evolution on the dead zone outer edge in magnetized protoplanetary disks

[Abridged] Aims. We provide an important step toward a better understanding of the magnetorotational instability (MRI)-dust coevolution in protoplanetary disks by presenting a proof of concept that dust evolution ultimately plays a crucial role in the MRI activity. Methods. First, we study how a fixed power-law dust size distribution with varying parameters impacts the MRI activity, especially the steady-state MRI-driven accretion, by employing and improving our previous 1+1D MRI-driven turbulence model. Second, we relax the steady-state accretion assumption in this disk accretion model, and partially couple it to a dust evolution model in order to investigate how the evolution of dust (dynamics and grain growth processes combined) and MRI-driven accretion are intertwined on million-year timescales. Results. Dust coagulation and settling lead to a higher gas ionization degree in the protoplanetary disk, resulting in stronger MRI-driven turbulence as well as a more compact dead zone. On the other hand, fragmentation has an opposite effect because it replenishes the disk in small dust particles. Since the dust content of the disk decreases over million years of evolution due to radial drift, the MRI-driven turbulence overall becomes stronger and the dead zone more compact until the disk dust-gas mixture eventually behaves as a grain-free plasma. Furthermore, our results show that dust evolution alone does not lead to a complete reactivation of the dead zone. Conclusions. The MRI activity evolution (hence the temporal evolution of the MRI-induced $α$-parameter) is controlled by dust evolution and occurs on a timescale of local dust growth, as long as there is enough dust particles in the disk to dominate the recombination process for the ionization chemistry. Once it is no longer the case, it is expected to be controlled by gas evolution and occurs on a viscous evolution timescale.

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arXiv2023

Forbidden emission lines in protostellar outflows and jets with MUSE

Forbidden emission lines in protoplanetary disks are a key diagnostic in studies of the evolution of the disk and the host star. We report spatially resolved emission lines, [OI] 6300, 6363, [NII] 6548, 6583, H$α$, and [SII] 6716, 6730 Angstrom that are believed to be associated with jets and magnetically driven winds in the inner disks. Observations were carried out with the optical integral field spectrograph of the Multi Unit Spectroscopic Explorer (MUSE), at the Very Large Telescope (VLT). With a resolution of 0.025 X 0.025 arcsec$^{2}$, we aim to derive the position angle of the outflow/jet (PA$_{outflow/jet}$) that is connected with the inner disk. The forbidden emission lines analyzed here have their origin at the inner parts of the protoplanetary disk. From the maximum intensity emission along the outflow/jet in DL Tau, CI Tau, DS Tau, IP Tau, and IM Lup, we were able to reliably measure the PA$_{outflow/jet}$ for most of the identified lines. We found that our estimates agree with PA$_{dust}$ for most of the disks. These estimates depend on the signal-to-noise level and the collimation of the outflow (jet). The outflows/jets in CIDA 9, GO Tau, and GW Lup are too compact for a PA$_{outflow/jet}$ to be estimated. Based on our kinematics analysis, we confirm that DL Tau and CI Tau host a strong outflow/jet with line-of-sight velocities much greater than 100 km s$^{-1}$, whereas DS Tau, IP Tau, and IM Lup velocities are lower and their structures encompass low-velocity components to be more associated with winds. Our estimates for the mass-loss rate, $\dot{M}_{loss}$, range between (1.1-6.5)X10$^{-7}$-10$^{-8}$ $M_{\odot}$ yr$^{-1}$ for the disk-outflow/jet systems analyzed here. The outflow/jet systems analyzed here are aligned within around 1 degree between the inner and outer disk. Further observations are needed to confirm a potential misalignment in IM Lup.

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arXiv2022

On the underestimation of dust mass in protoplanetary disks: Effects of disk structure and dust properties

The total amount of dust grains in protoplanetary disks is one of the key properties that characterize the potential for planet formation. With (sub-)millimeter flux measurements, literature studies usually derive the dust mass using an analytic form under the assumption of optically thin emission, which may lead to substantial underestimation. In this work, we conduct a parameter study with the goal of investigating the effects of disk structure and dust properties on the underestimation through self-consistent radiative transfer models. Different dust models, scattering modes and approaches for dust settling are considered and compared. The influences of disk substructures, such as rings and crescents, on the mass derivation are investigated as well. The results indicate that the traditional analytic method can underestimate the mass by a factor of a few to hundreds, depending on the optical depth along the line of sight set mainly by the true dust mass, disk size and inclination. As an application, we perform a detailed radiative transfer modeling of the spectral energy distribution of DoAr 33, one of the observed DSHARP disks. When the DSHARP dust opacities are adopted, the most probable dust mass returned from the Bayesian analysis is roughly 7 times higher than the value given by the analytic calculation. Our study demonstrates that estimating disk dust masses from radiative transfer modeling is one solution for alleviating the problem of insufficient mass for planet formation raised in the ALMA era.

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arXiv2022

Trapping (sub-)Neptunes similar to TOI-216b at the inner disk rim: Implications for the disk viscosity and the Neptunian desert

[Abridged] The occurrence rate of observed sub-Neptunes has a break at 0.1 au, which is often attributed to a migration trap at the inner rim of protoplanetary disks where a positive co-rotation torque prevents inward migration. We argue that conditions in inner disk regions are such that sub-Neptunes are likely to open gaps, lose the support of the co-rotation torque as their co-rotation regions become depleted, and the trapping efficiency then becomes uncertain. We study what it takes to trap such gap-opening planets at the inner disk rim. We performed 2D locally isothermal and non-isothermal hydrodynamic simulations of planet migration. A viscosity transition was introduced in the disk to (i) create a density drop and (ii) mimic the viscosity increase as the planet migrated from a dead zone towards a region with active magneto-rotational instability (MRI). We chose TOI-216b as a Neptune-like upper-limit test case, but we also explored different planetary masses, both on fixed and evolving orbits. For planet-to-star mass ratios $q\simeq(4$-$8)\times10^{-5}$, the density drop at the disk rim becomes reshaped due to a gap opening and is often replaced with a small density bump centered on the planet's corotation. Trapping is possible only if the bump retains enough gas mass and if the co-rotation region becomes azimuthally asymmetric, with an island of librating streamlines that accumulate a gas overdensity ahead of the planet. The overdensity exerts a positive torque that can counteract the negative torque of spiral arms. In our model, efficient trapping depends on the $α$ viscosity and its contrast across the viscosity transition. In order to trap TOI-216b, $α_{\mathrm{DZ}}=10^{-3}$ in the dead zone requires $α_{\mathrm{MRI}}\gtrsim5\times10^{-2}$ in the MRI-active zone. If $α_{\mathrm{DZ}}=5\times10^{-4}$, $α_{\mathrm{MRI}}\gtrsim7.5\times10^{-2}$ is needed.

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arXiv2022

Millimeter Gap Contrast as a Probe for Turbulence Level in Protoplanetary Disks

Turbulent motions are believed to regulate angular momentum transport and influence dust evolution in protoplanetary disks. Measuring the strength of turbulence is challenging through gas line observations because of the requirement for high spatial and spectral resolution data, and an exquisite determination of the temperature. In this work, taking the well-known HD 163296 disk as an example, we investigated the contrast of gaps identified in high angular resolution continuum images as a probe for the level of turbulence. With self-consistent radiative transfer models, we simultaneously analyzed the radial brightness profiles along the disk major and minor axes, and the azimuthal brightness profiles of the B67 and B100 rings. By fitting all the gap contrasts measured from these profiles, we constrained the gas-to-dust scale height ratio $Λ$ to be $3.0_{-0.8}^{+0.3}$, $1.2_{-0.1}^{+0.1}$ and ${\ge}\,6.5$ for the D48, B67 and B100 regions, respectively. The varying gas-to-dust scale height ratios indicate that the degree of dust settling changes with radius. The inferred values for $Λ$ translate into a turbulence level of $α_{\rm turb}\,{<}\,3\times10^{-3}$ in the D48 and B100 regions, which is consistent with previous upper limits set by gas line observations. However, turbulent motions in the B67 ring are strong with $α_{\rm turb}\,{\sim}1.2\,{\times}\,10^{-2}$. Due to the degeneracy between $Λ$ and the depth of dust surface density drops, the turbulence strength in the D86 gap region is not constrained.

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arXiv2022

Modeling the arc and ring structures in the HD 143006 disk

Rings and asymmetries in protoplanetary disks are considered as signposts of ongoing planet formation. In this work, we conduct three-dimensional radiative transfer simulations to model the intriguing disk around HD 143006 that has three dust rings and a bright arc. A complex geometric configuration, with a misaligned inner disk, is assumed to account for the asymmetric structures. The two-dimensional surface density is constructed by iteratively fitting the ALMA data. We find that the dust temperature displays a notable discontinuity at the boundary of the misalignment. The ring masses range from 0.6 to 16Mearth that are systematically lower than those inferred in the younger HL Tau disk. The arc occupies nearly 20% of the total dust mass. Such a high mass fraction of dust grains concentrated in a local region is consistent with the mechanism of dust trapping into vortices. Assuming a gas-to-dust mass ratio of 30 that is constant throughout the disk, the dense and cold arc is close to the threshold of being gravitationally unstable, with the Toomre parameter Q~1.3. Nevertheless, our estimate of Q relies on the assumption for the unknown gas-to-dust mass ratio. Adopting a lower gas-to-dust mass ratio would increase the inferred Q value. Follow-up high resolution observations of dust and gas lines are needed to clarify the origin of the substructures.

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