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Characterizing the physical properties of dust grains in a protoplanetary disk is critical to comprehending the planet formation process. Our study presents Atacama Large Millimeter/submillimeter Array (ALMA) high-resolution observations of the young protoplanetary disk around DG Tau at a 1.3 mm dust continuum. The observations, with a spatial resolution of ≈0.″04, or ≈5 au, revealed a geometrically thin and smooth disk without substantial substructures, suggesting that the disk retains the initial conditions of the planet formation. To further analyze the distributions of dust surface density, temperature, and grain size, we conducted a multiband analysis with several dust models, incorporating ALMA archival data of the 0.87 and 3.1 mm dust polarization. The results showed that the Toomre Q parameter is ≲2 at a 20 au radius, assuming a dust-to-gas mass ratio of 0.01. This implies that a higher dust-to-gas mass ratio is necessary to stabilize the disk. The grain sizes depend on the dust models, and for the DSHARP compact dust, they were found to be smaller than ∼400 μ m in the inner region (r ≲ 20 au) while exceeding larger than 3 mm in the outer part. Radiative transfer calculations show that the dust scale height is lower than at least one-third of the gas scale height. These distributions of dust enrichment, grain sizes, and weak turbulence strength may have significant implications for the formation of planetesimals through mechanisms such as streaming instability. We also discuss the CO snowline effect and collisional fragmentation in dust coagulation for the origin of the dust size distribution. [ABSTRACT FROM AUTHOR]

Compressive strength is a key to understanding the internal structure of dust aggregates in protoplanetary disks and their resultant bodies, such as comets and asteroids in the solar system. Previous work has modeled the compressive strength of highly porous dust aggregates with volume filling factors lower than 0.1. However, a comprehensive understanding of the compressive strength from low (<0.1) to high (>0.1) volume filling factors is lacking. In this paper, we investigate the compressive strength of dust aggregates by using aggregate compression simulations resolving constituent grains based on Johnson-Kendall-Roberts theory to formulate the compressive strength comprehensively. We perform a series of numerical simulations with moving periodic boundaries mimicking the compression behavior. As a result, we find that the compressive strength becomes sharply harder when the volume filling factor exceeds 0.1. We succeed in formulating the compressive strength comprehensively by taking into account the rolling motion of aggregates for low volume filling factors and the closest packing of aggregates for high volume filling factors. We also find that the dominant compression mechanisms for high volume filling factors are sliding and twisting motions, while rolling motion dominates for low volume filling factors. We confirm that our results are in good agreement with previous numerical studies. We suggest that our analytical formula is consistent with the previous experimental results if we assume the surface energy of silicate is ≃210 ± 90 mJ m−2. Now, we can apply our results to properties of small compact bodies, such as comets, asteroids, and pebbles. [ABSTRACT FROM AUTHOR]

It has been implicitly assumed that ices on grains in molecular clouds and protoplanetary disks are formed by homogeneous layers regardless of their composition or crystallinity. To verify this assumption, we observed the H2O deposition onto refractory substrates and the crystallization of amorphous ices (H2O, CO2, and CO) using an ultra-high-vacuum transmission electron microscope. In the H2O-deposition experiments, we found that three-dimensional islands of crystalline ice (Ic) were formed at temperatures above 130 K. The crystallization experiments showed that uniform thin films of amorphous CO and H2O became three-dimensional islands of polyhedral crystals; amorphous CO2, on the other hand, became a thin film of nano-crystalline CO2 covering the amorphous H2O. Our observations show that crystal morphologies strongly depend not only on the ice composition but also on the substrate. Using experimental data concerning the crystallinity of deposited ices and the crystallization timescale of amorphous ices, we illustrated the criteria for ice crystallinity in space and outlined the macroscopic morphology of icy grains in molecular clouds as follows: amorphous H2O covered the refractory grain uniformly, CO2 nano-crystals were embedded in the amorphous H2O, and a polyhedral CO crystal was attached to the amorphous H2O. Furthermore, a change in the grain morphology in a protoplanetary disk is shown. These results have important implications for the chemical evolution of molecules, nonthermal desorption, collision of icy grains, and sintering. [ABSTRACT FROM AUTHOR]

The low water content of the terrestrial planets in the solar system suggests that the protoplanets formed within the water snow line. Accurate prediction of the snow line location moving with time provides a clue to constraining the formation process of the planets. In this paper, we investigate the migration of the snow line in protoplanetary disks whose accretion is controlled by laminar magnetic fields, which have been proposed by various nonideal magnetohydrodynamic (MHD) simulations. We propose an empirical model of the disk temperature based on our nonideal MHD simulations, which show that the accretion heating is significantly less efficient than that in turbulent disks, and calculate the snow line location over time. We find that the snow line in magnetically accreting laminar disks moves inside the current Earth's orbit within 1 Myr after star formation, whereas the time for the conventional turbulent disk is much longer than 1 Myr. This result suggests that either the rocky protoplanets formed in such an early phase of the disk evolution, or the protoplanets moved outward to the current orbits after they formed close to the protosun. [ABSTRACT FROM AUTHOR]

Vertical shear instability (VSI) is a hydrodynamical instability that requires rapid gas cooling and has been suggested to operate in outer regions of protoplanetary disks. VSI drives turbulence with strong vertical motions, which could regulate dust growth and settling. However, dust growth and settling can regulate the VSI because dust depletion makes gas cooling inefficient in outer disk regions that are optically thin to their own thermal emission. In this study, we quantify these potentially stabilizing effects of dust evolution on the VSI based on linear analysis. We construct a model for calculating the cooling timescale, taking into account dust growth beyond micron sizes and size-dependent settling. Combining the model with linear stability analysis, we map the region where the VSI operates, which we call the VSI zone, and estimate the maximum growth rate at each radial position. We find that dust growth, as well as settling, makes the VSI zone more confined around the midplane. This causes a decrease in the growth rate because the vertical shear of the rotation velocity, which is the source of the instability, is weaker at lower altitudes. In our default disk model with 0.01 solar masses, dust growth from 10 μm to 1 mm causes a decrease in the growth rate by a factor of more than 10. The suppression of VSI-driven turbulence by dust evolution may promote further dust evolution in the outer regions and also explain the high degree of dust settling observed in the disk around HL Tau. [ABSTRACT FROM AUTHOR]

We present molecular line observations of 13CO and C18O J = 3 − 2, CN N = 3 − 2, and CS J = 7 − 6 lines in the protoplanetary disk around TW Hya at a high spatial resolution of ∼9 au (angular resolution of 0.″15), using the Atacama Large Millimeter/Submillimeter Array. A possible gas gap is found in the deprojected radial intensity profile of the integrated C18O line around a disk radius of ∼58 au, slightly beyond the location of the au-scale dust clump at ∼52 au, which resembles predictions from hydrodynamic simulations of planet–disk interaction. In addition, we construct models for the physical and chemical structure of the TW Hya disk, taking account of the dust surface density profile obtained from high spatial resolution dust continuum observations. As a result, the observed flat radial profile of the CN line intensities is reproduced due to a high dust-to-gas surface density ratio inside ∼20 au. Meanwhile, the CO isotopologue line intensities trace high temperature gas and increase rapidly inside a disk radius of ∼30 au. A model with either CO gas depletion or depletion of gas-phase oxygen elemental abundance is required to reproduce the relatively weak CO isotopologue line intensities observed in the outer disk, consistent with previous atomic and molecular line observations toward the TW Hya disk. Further observations of line emission of carbon-bearing species, such as atomic carbon and HCN, with high spatial resolution would help to better constrain the distribution of elemental carbon abundance in the disk gas. [ABSTRACT FROM AUTHOR]

Substructures in protoplanetary disks (PPDs), whose ubiquity was unveiled by recent Atacama Large Millimeter/submillimeter Array observations, are widely discussed regarding their possible origins. We carry out global 2D magnetohydrodynamic (MHD) simulations in axisymmetry, coupled with self-consistent ray-tracing radiative transfer, thermochemistry, and nonideal MHD diffusivities. The abundance profiles of grains are also calculated based on the global dust evolution calculation, including sintering effects. We found that dust size plays a crucial role in the ring formation around the snow lines of PPDs through the accretion process. Disk ionization structures and thus tensorial conductivities depend on the size of grains. When grains are significantly larger than polycyclic aromatic hydrocarbons (PAHs), the nonideal MHD conductivities change dramatically across each snow line of major volatiles, leading to a sudden change in the accretion process across the snow lines and the subsequent formation of gaseous rings/gaps there. Specific layout of magnetic fields can suppress wind launching in certain regions by canceling out different stress components. On the other hand, the variations of conductivities are a lot less with only PAH-sized grains in disks and then these disks retain smoother radial density profiles across snow lines. [ABSTRACT FROM AUTHOR]

The largest moon of Neptune, Triton, possesses a cold and hazy atmosphere. Since the discovery of the near-surface haze layer during the Voyager fly in 1989, the haze formation mechanism has not been investigated in detail. Here we provide the first haze microphysical model on Triton. Our model solves the evolution of both size and porosity distributions of haze particles in a self-consistent manner. We simulated the formation of sphere and aggregate hazes with and without condensation of the C2H4 ice. The haze particles can grow into fractal aggregates with mass-equivalent sphere sizes of ∼0.1–1 μm and fractal dimensions of Df = 1.8–2.2. The ice-free hazes cannot simultaneously explain both UV and visible observations of Voyager 2, while including the condensation of C2H4 ices provides two better solutions. For ice aggregates, the required total haze mass flux is ∼2 × 10−15 g cm−2 s−1. For the icy sphere scenario, the column-integrated C2H4 production rate is ∼8 × 10−15 g cm−2 s−1, and the ice-free mass flux is ∼6 × 10−17 g cm−2 s−1. The UV occultation observations at short wavelengths, <0.15 μm, may slightly favor the icy aggregates. Observations of the haze optical depth and the degree of forward scattering in UV and visible should be able to distinguish whether Triton's hazes are icy spheres or ice aggregates in future Triton missions. [ABSTRACT FROM AUTHOR]

Ring structures are observed through (sub)millimeter dust continuum emission in various circumstellar disks from the early stages of class 0 and I to the late stage of class II young stellar objects (YSOs). In this paper, we study one of the possible scenarios for such ring formation, which is the coagulation of dust aggregates in the early stage. The dust grains grow in an inside-out manner because the growth timescale is roughly proportional to the orbital period. The boundary of the dust evolution can be regarded as the growth front, where the growth time is comparable to the disk age. Using radiative transfer calculations based on the dust coagulation model, we find that the growth front can be observed as a ring structure because the dust surface density changes sharply at this position. Furthermore, we confirm that the observed ring positions in YSOs with an age of ≲1 Myr are consistent with the growth front. The growth front could be important in creating the ring structure in particular for the early stage of disk evolution, such as class 0 and I sources. [ABSTRACT FROM AUTHOR]

Disintegrating planets are ultrashort-period exoplanets that appear to have a comet-like dust tail. They are commonly interpreted as low-mass planets whose solid surface is evaporating, and whose tails are made of recondensing minerals. Transmission spectroscopy of the dust tails could thus allow us to directly probe the elementary compositions of these planets. Previous work already investigated the feasibility of such observations using the James Webb Space Telescope (JWST) mid-infrared instrument. In this study, we explore if one can obtain a strong constrain on the tail composition by adding spectroscopy at longer wavelengths using the Space Infrared Telescope for Cosmology and Astrophysics (SPICA) mid-infrared instrument. We use a simple model for the spatial distribution of the dust tails and produce their synthetic transmission spectra assuming various dust compositions. We find that combined infrared spectra from JWST and SPICA will allow us to diagnose various components of the dust tails. JWST will be able to detect silicate and carbide absorption features with a feature-to-noise ratio of ≳3 in the tail transmission spectrum of a disintegrating planet located within 100 pc from the Earth, with a transit depth deeper than 0.5%. SPICA can distinguish between Fe- and Mg-bearing crystalline silicates for planets at ≲100 pc with a transit depth of ≳2%. Transit searches with current and future space telescopes (e.g., TESS and PLATO) will provide ideal targets for such spectroscopic observations. [ABSTRACT FROM AUTHOR]

Large-scale magnetic fields are key ingredients of magnetically driven disk accretion. We study how large-scale poloidal fields evolve in accretion disks, with the primary aim of quantifying the viability of magnetic accretion mechanisms in protoplanetary disks. We employ a kinematic mean-field model for poloidal field transport and focus on steady states where inward advection of a field balances with outward diffusion due to effective resistivities. We analytically derive the steady-state radial distribution of poloidal fields in highly conducting accretion disks. The analytic solution reveals an upper limit on the strength of large-scale vertical fields attainable in steady states. Any excess poloidal field will diffuse away within a finite time, and we demonstrate this with time-dependent numerical calculations of the mean-field equations. We apply this upper limit to large-scale vertical fields threading protoplanetary disks. We find that the maximum attainable strength is about 0.1 G at 1 AU, and about 1 mG at 10 AU from the central star. When combined with recent magnetic accretion models, the maximum field strength translates into the maximum steady-state accretion rate of ∼10–7M☼ yr–1, in agreement with observations. We also find that the maximum field strength is ∼1 kG at the surface of the central star provided that the disk extends down to the stellar surface. This implies that any excess stellar poloidal field of strength ≳ kG can be transported to the surrounding disk. This might in part resolve the magnetic flux problem in star formation. [ABSTRACT FROM AUTHOR]

We examine whether the magnetorotational instability (MRI) can serve as a mechanism of angular momentum transport in circumplanetary disks. For the MRI to operate the ionization degree must be sufficiently high and the magnetic pressure must be sufficiently lower than the gas pressure. We calculate the spatial distribution of the ionization degree and search for the MRI-active region where the two criteria are met. We find that there can be thin active layers at the disk surface depending on the model parameters, however, we find hardly any region which can sustain well-developed MRI turbulence; when the magnetic field is enhanced by MRI turbulence at the disk surface layer, a magnetically dominated atmosphere encroaches on a lower altitude and a region of well-developed MRI turbulence becomes smaller. We conclude that if there are no angular momentum transfer mechanisms other than MRI in gravitationally stable circumplanetary disks, gas is likely to pile up until disks become gravitationally unstable, and massive disks may survive for a long time. [ABSTRACT FROM AUTHOR]

We develop a fast and accurate calculation method for ionization degrees in protoplanetary and circumplanetary disks including dust grains. We apply our method to calculate the ionization degree of circumplanetary disks. It is important to understand the structure and evolution of protoplanetary/circumplanetary disks since they are thought to be the sites of planet/satellite formation. The turbulence that causes gas accretion is supposed to be driven by magnetorotational instability (MRI) that occurs only when the ionization degree is high enough for magnetic field to be coupled to gas. We calculate the ionization degrees in circumplanetary disks to estimate the sizes of MRI-inactive regions. We properly include the effect of dust grains because they efficiently capture charged particles and make ionization degree lower. Inclusion of dust grains complicates the reaction equations and requires expensive computation. In order to accelerate the calculation of ionization reactions, we develop a semianalytic method based on the charge distribution model proposed previously. This method enables us to study the ionization state of disks for a wide range of model parameters. For a previous model of circum-Jovian disk, we find that an MRl-inactive region covers almost all regions even without dust grains. This suggests that the gas accretion rates in circumplanetary disks are much smaller than previously thought. [ABSTRACT FROM AUTHOR]

Turbulence driven by magnetorotational instability (MRI) crucially affects the evolution of solid bodies in protoplanetary disks. On the other hand, small dust particles stabilize MRI by capturing ionized gas particles needed for the coupling of the gas and magnetic fields. To provide an empirical basis for modeling the coevolution of dust and MRI, we perform three-dimensional, ohmic-resistive MHD simulations of a vertically stratified shearing box with an MRI-inactive "dead zone" of various sizes and with a net vertical magnetic flux of various strengths. We find that the vertical structure of turbulence is well characterized by the vertical magnetic flux and three critical heights derived from the linear analysis of MRI in a stratified disk. In particular, the turbulent structure depends on the resistivity profile only through the critical heights and is insensitive to the details of the resistivity profile. We discover scaling relations between the amplitudes of various turbulent quantities (velocity dispersion, density fluctuation, vertical diffusion coefficient, and outflow mass flux) and vertically integrated accretion stresses. We also obtain empirical formulae for the integrated accretion stresses as a function of the vertical magnetic flux and the critical heights. These empirical relations allow us to predict the vertical turbulent structure of a protoplanetary disk for a given strength of the magnetic flux and a given resistivity profile. [ABSTRACT FROM AUTHOR]

The ionization state of the gas plays a key role in the magnetohydrodynamics (MHD) of protoplanetary disks. However, the ionization state can depend on the gas dynamics, because electric fields induced by MHD turbulence can heat up plasmas and thereby affect the ionization balance. To study this nonlinear feedback, we construct an ionization model that includes plasma heating by electric fields and impact ionization by heated electrons, as well as charging of dust grains. We show that when plasma sticking onto grains is the dominant recombination process, the electron abundance in the gas decreases with increasing electric field strength. This is a natural consequence of electron-grain collisions whose frequency increases with the electron's random velocity. The decreasing electron abundance may lead to a self-regulation of MHD turbulence. In some cases, not only the electron abundance but also the electric current decreases with increasing field strength in a certain field range. The resulting N-shaped current-field relation violates the fundamental assumption of the non-relativistic MHD that the electric field is uniquely determined by the current density. At even higher field strengths, impact ionization causes an abrupt increase of the electric current as expected by previous studies. We find that this discharge current is multi-valued (i.e., the current-field relation is S-shaped) under some circumstances, and that the intermediate branch is unstable. The N/S-shaped current-field relations may yield hysteresis in the evolution of MHD turbulence in some parts of protoplanetary disks. [ABSTRACT FROM AUTHOR]

We study the time evolution of a large-scale magnetic flux threading an accretion disk. The induction equation of the mean poloidal field is solved under the standard viscous disk model. Magnetic flux evolution is controlled by two timescales: one is the timescale of the inward advection of the magnetic flux, τadv. This is induced by the dragging of the flux by the accreting gas. The other is the outward diffusion timescale of the magnetic flux τdif. We consider diffusion due to the Ohmic resistivity. These timescales can be significantly different from the disk viscous timescale τdisk. The behaviors of the magnetic flux evolution are quite different depending on the magnitude relationship of the timescales τadv, τdif, and τdisk. The most interesting phenomena occur when τadv ≪ τdif, τdisk. In such a case, the magnetic flux distribution approaches a quasi-steady profile much faster than the viscous evolution of the gas disk, and the magnetic flux has also been tightly bundled to the inner part of the disk. In the inner part, although the poloidal magnetic field becomes much stronger than the interstellar magnetic field, the field strength is limited to the maximum value that is analytically given by our previous work. We also find a condition for the initial large magnetic flux, which is a fossil of the magnetic field dragging during the early phase of star formation that survives for a duration in which significant gas disk evolution proceeds. [ABSTRACT FROM AUTHOR]

A correction to the article "Electric charging of dust aggregates and its effect on dust coagulation in protoplanetary disks" by Satoshi Okuzumi published in the previous issue is presented.

Geometric cross sections of dust aggregates determine their coupling with disk gas, which governs their motions in protoplanetary disks. Collisional outcomes also depend on geometric cross sections of initial aggregates. In a previous paper, we performed three-dimensional N-body simulations of sequential collisions of aggregates composed of a number of sub-micron-sized icy particles and examined radii of gyration (and bulk densities) of the obtained aggregates. We showed that collisional compression of aggregates is not efficient and that aggregates remain fluffy. In the present study, we examine geometric cross sections of the aggregates. Their cross sections decrease due to compression as well as to their gyration radii. It is found that a relation between the cross section and the gyration radius proposed by Okuzumi et al. is valid for the compressed aggregates. We also refine the compression model proposed in our previous paper. The refined model enables us to calculate the evolution of both gyration radii and cross sections of growing aggregates and reproduces well our numerical results of sequential aggregate collisions. The refined model can describe non-equal-mass collisions as well as equal-mass cases. Although we do not take into account oblique collisions in the present study, oblique collisions would further hinder compression of aggregates. [ABSTRACT FROM AUTHOR]