Search

Aggregates consisting of submicron-sized cohesive dust grains are ubiquitous, and understanding the collisional behavior of dust aggregates is essential. It is known that low-speed collisions of dust aggregates result in either sticking or bouncing, and local and permanent compaction occurs near the contact area upon collision. In this study, we perform numerical simulations of collisions between two aggregates and investigate their compressive behavior. We find that the maximum compression length is proportional to the radius of aggregates and increases with the collision velocity. We also reveal that a theoretical model of contact between two elastoplastic spheres successfully reproduces the size- and velocity-dependence of the maximum compression length observed in our numerical simulations. Our findings on the plastic deformation of aggregates during collisional compression provide a clue to understanding the collisional growth process of aggregates., Comment: 22 pages, 15 figures. Accepted for publication in Granular Matter

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., Comment: version 2. Published in Nature Astronomy (2024), includes supplementary material

High-precision isotopic measurements of meteorites revealed that they are classified into non-carbonaceous (NC) and carbonaceous (CC) meteorites. One plausible scenario for achieving this grouping is the early formation of Jupiter because massive planets can create gaps that suppress the mixing of dust across the gap in protoplanetary disks. However, the efficiency of this suppression by the gaps depends on dust size and the strength of turbulent diffusion, allowing some fraction of the dust particles to leak across the Jovian gap. In this study, we investigate how isotopic ratios of NC and CC meteorites are varied by the dust leaking across the Jovian gap in the solar nebula. To do this, we constructed a model to simulate the evolution of the dust size distribution and the $^{54}$Cr-isotopic anomaly $\varepsilon^{54}$Cr in isotopically heterogeneous disks with Jupiter. Assuming that the parent bodies of NC and CC meteorites are formed in two dust-concentrated locations inside and outside Jupiter's orbit, referred to as the NC reservoir and CC reservoir, we derive the temporal variation of $\varepsilon^{54}$Cr at the NC and CC reservoir. Our results indicate that substantial contamination of CC materials occurs at the NC reservoir in the fiducial run. Nevertheless, the values of $\varepsilon^{54}$Cr at the NC reservoir and the CC reservoir in the run are still consistent with those of NC and CC meteorites formed around 2 Myrs after the formation of calcium-aluminum-rich inclusions. Moreover, this dust leakage causes a positive correlation between the $\varepsilon^{54}$Cr value of NC meteorites and the accretion ages of their parent bodies., Comment: 16 pages, 11 figures, 2 tables, Accepted for publication in PASJ

The spatial distribution of dust particles in protoplanetary disks affects dust evolution and planetesimal formation processes. The vertical shear instability (VSI) is one of the candidate hydrodynamic mechanisms that can generate turbulence in the outer disk region and affect dust diffusion. Turbulence driven by the VSI has a predominant vertical motion that can prevent dust settling. On the other hand, the dust distribution controls the spatial distribution of the gas cooling rate, thereby affecting the strength of VSI-driven turbulence. Here, we present a semi-analytic model that determines the vertical dust distribution and the strength of VSI-driven turbulence in a self-consistent manner. The model uses an empirical formula for the vertical diffusion coefficient in VSI-driven turbulence obtained from our recent hydrodynamical simulations. The formula returns the vertical diffusion coefficient as a function of the vertical profile of the cooling rate, which is determined by the vertical dust distribution. We use this model to search for an equilibrium vertical dust profile where settling balances with turbulent diffusion for a given maximum grain size. We find that if the grains are sufficiently small, there exists a stable equilibrium dust distribution where VSI-driven turbulence is sustained at a level of alpha_z ~ 10^{-3}, where alpha_z is the dimensionless vertical diffusion coefficient. However, as the maximum grain size increases, the equilibrium solution vanishes because the VSI can no longer stop the settling of the grains. This runaway settling may explain highly settled dust rings found in the outer part of some protoplanetary disks., Comment: 14 pages, 9 figures, Accepted for publication in PASJ

We perform three-dimensional global non-ideal magnetohydrodynamic simulations of a protoplanetary disk containing the inner dead-zone edge. We take into account realistic diffusion coefficients of the Ohmic resistivity and ambipolar diffusion based on detailed chemical reactions with single-size dust grains. We found that the conventional dead zone identified by the Els\"asser numbers of the Ohmic resistivity and ambipolar diffusion is divided into two regions: "the transition zone" and "the coherent zone". The coherent zone has the same properties as the conventional dead zone, and extends outside of the transition zone in the radial direction. Between the active and coherent zones, we discover the transition zone, the inner edge of which is identical to that of the conventional dead zone. The transition zone extends out over the regions where thermal ionization determines diffusion coefficients. The transition zone has completely different physical properties than the conventional dead zone, the so-called undead zone, and the zombie zone. The combination of amplification of the radial magnetic field owing to the ambipolar diffusion and a steep radial gradient of the Ohmic diffusivity causes the efficient evacuation of the net vertical magnetic flux from the transition zone within several rotations. Surface gas accretion occurs in the coherent zone but not in the transition zone. The presence of the transition zone prohibits mass and magnetic flux transport from the coherent zone to the active zone. Mass accumulation occurs at both edges of the transition zone as a result of mass supply from the active and coherent zones., Comment: 43 pages, 37 figures, PASJ open access (https://doi.org/10.1093/pasj/psae036)

Polarized (sub)millimeter emission from dust grains in circumstellar disks was initially thought to be due to grains aligned with the magnetic field. However, higher resolution multi-wavelength observations along with improved models found that this polarization is dominated by self-scattering at shorter wavelengths (e.g., 870 $\mu$m) and by grains aligned with something other than magnetic fields at longer wavelengths (e.g., 3 mm). Nevertheless, the polarization signal is expected to depend on the underlying substructure, and observations hitherto have been unable to resolve polarization in multiple rings and gaps. HL Tau, a protoplanetary disk located 147.3 $\pm$ 0.5 pc away, is the brightest Class I or Class II disk at millimeter/submillimeter wavelengths. Here we show deep, high-resolution 870 $\mu$m polarization observations of HL Tau, resolving polarization in both the rings and gaps. We find that the gaps have polarization angles with a significant azimuthal component and a higher polarization fraction than the rings. Our models show that the disk polarization is due to both scattering and emission from aligned effectively prolate grains. The intrinsic polarization of aligned dust grains is likely over 10%, which is much higher than what was expected in low resolution observations (~1%). Asymmetries and dust features are revealed in the polarization observations that are not seen in non-polarimetric observations., Comment: Published in Nature

Characterizing the physical properties of dust grains in a protoplanetary disk is critical to comprehending the planet formation process. Our study presents 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 $\approx 0.04''$, or $\approx5$ 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 multi-band analysis with several dust models, incorporating ALMA archival data of the 0.87 mm and 3.1 mm dust polarization. The results showed that the Toomre $Q$ parameter is $\lesssim2$ 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 $\sim400$ $\mu$m in the inner region ($r\lesssim20$ 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., Comment: 29 pages, 21 figures, 5 tables. Accepted for publication in ApJ

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 JKR 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 $\simeq210\pm90\mathrm{\ mJ\ m^{-2}}$. Now, we can apply our results to properties of small compact bodies, such as comets, asteroids, and pebbles., Comment: 15 pages, 12 figures, accepted for publication in ApJ

Understanding the collisional behavior of dust aggregates is essential in the context of planet formation. It is known that low-velocity collisions of dust aggregates result in bouncing rather than sticking when the filling factor of colliding dust aggregates is higher than a threshold value. However, a large discrepancy between numerical and experimental results on the threshold filling factor was reported so far. In this study, we perform numerical simulations using soft-sphere discrete element methods and demonstrate that the sticking probability decreases with increasing aggregates radius. Our results suggest that the large discrepancy in the threshold filling factor may reflect the difference in the size of dust aggregates in earlier numerical simulations and laboratory experiments., Comment: 8 pages, 4 figures. Accepted for publication in ApJL

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 $\lesssim100~{\rm \mu m}$, 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 $\sim150~{\rm \mu m}$ or $\gtrsim$ few ${\rm cm}$. The accretion heating significantly increases the brightness temperature particularly when $a_{\rm max}\lesssim300~{\rm \mu m}$, and hence $a_{\rm max}$ needs to be either $\sim300~{\rm \mu m}$ 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 $300~{\rm \mu m}$-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., Comment: 19 pages, 25 figures, accepted for publication in A&A

Understanding the temperature structure of protoplanetary disks is crucial for answering the fundamental question of when and where in the disks rocky planets like our own form. However, the thermal structure of the inner few au of the disks is poorly understood not only because of lack of observational constraints but also because of the uncertainty of accretion heating processes. Here, we propose thermal tomography of the inner regions of protoplanetary disks with the ngVLA and ALMA. The proposed approach is based on the assumption that the inner disk regions are optically thick at submillimeter wavelengths but are marginally optically thin at longer millimeter wavelengths. By combining high-resolution millimeter continuum images from the ngVLA with submillimeter images at comparable resolutions from ALMA, we will be able to reconstruct the radial and vertical structure of the inner few au disk regions. We demonstrate that the thermal tomography we propose can be used to constrain the efficiency of midplane accretion heating, a process that controls the timing of snow-line migration to the rocky planet-forming region, in the few au regions of protoplanetary disks at a distance of 140 pc., Comment: 4 pages, 3 figures, ngVLA-Japan Memo Series P006 (2021), https://ngvla.nao.ac.jp/researcher/memo/

[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 $\alpha$-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., Comment: 23 pages, 13 figures, Accepted for publication in A&A

Typical accretion disks around massive protostars are hot enough for water ice to sublimate. We here propose to utilize the massive protostellar disks for investigating the collisional evolution of silicate grains with no ice mantle, which is an essential process for the formation of rocky planetesimals in protoplanetary disks around lower-mass stars. We for the first time develop a model of massive protostellar disks that includes the coagulation, fragmentation, and radial drift of dust. We show that the maximum grain size in the disks is limited by collisional fragmentation rather than by radial drift. We derive analytic formulas that produce the radial distribution of the maximum grain size and dust surface density in the steady state. Applying the analytic formulas to the massive protostellar disk of GGD27-MM1, where the grain size is constrained from a millimeter polarimetric observation, we infer that the silicate grains in this disk fragment at collision velocities above ~ 10 m/s. The inferred fragmentation threshold velocity is lower than the maximum grain collision velocity in typical protoplanetary disks around low-mass stars, implying that coagulation alone may not lead to the formation of rocky planetesimals in those disks. With future measurements of grain sizes in massive protostellar disks, our model will provide more robust constraints on the sticking property of silicate grains., Comment: 17 pages, 5 figures,accepted for publication to The Astrophysical Journal

We review the recent observations of protoplanetary disks together with relevant theoretical studies with an emphasis on the evolution of volatiles. In the last several years Atacama Large Millimeter/submillimeter Array (ALMA) provided evidence of grain growth, gas-dust decoupling, and sub-structures such as rings and gaps in the dust continuum. Molecular line observations revealed radial and vertical distributions of molecular abundances and also provided significant constraints on the gas dynamics such as turbulence. While sub-millimeter and millimeter observations mainly probe the gas and dust outside the radius of several au, ice and inner warm gas are investigated at shorter wavelengths. Gas and dust dynamics are key to connecting these observational findings. One of the emerging trends is inhomogeneous distributions of elemental abundances, most probably due to dust-gas decoupling., Comment: COMETS III, in press. Table 1 and some contents are corrected and updated from the previous version (e.g. refer to JWST observations)

Turbulence in protoplanetary disks plays an important role in dust evolution and planetesimal formation. The vertical shear instability (VSI) is one of the candidate hydrodynamic mechanisms that can generate turbulence in the outer disk regions. The VSI requires rapid gas cooling in addition to vertical shear. A linear stability analysis suggests that the VSI may not operate around the midplane where gas cooling is inefficient. In this study, we investigate the nonlinear outcome of the VSI in disks with a linearly VSI-stable midplane region. We perform two-dimensional global hydrodynamical simulations of an axisymmetric disk with vertically varying cooling times. The vertical cooling time profile determines the thicknesses of the linearly VSI-stable midplane layer and unstable layers above and below the midplane. We find that the thickness of the midplane stable layer determines the vertical structure of VSI-driven turbulence in the nonlinear saturated state. We identify two types of final saturated state: (1) T states characterized by vertical turbulent motion penetrating into the VSI-stable midplane layer and (2) pT states characterized by turbulent motion confined in the unstable layers. The pT states are realized when the midplane VSI-stable layer is thicker than two gas scale heights. We also find that the VSI-driven turbulence is largely suppressed at all heights when the VSI-unstable region lying above and below the midplane is thinner than two gas scale heights. We present empirical formulas that predict the strength of VSI-driven turbulence as a function of the thicknesses of the unstable and stable layers. These formulas will be useful for studying how VSI-driven turbulence and dust grains controlling the disk cooling efficiency evolve simultaneously., Comment: 16 pages, 13 figures, 2 tables, Accepted for publication in PASJ

A growing number of debris discs have been detected around metal-polluted white dwarfs. They are thought to be originated from tidally disrupted exoplanetary bodies and responsible for metal accretion onto host WDs. To explain (1) the observationally inferred accretion rate higher than that induced by Poynting-Robertson drag, $\dot{M}_{\rm PR}$, and (2) refractory-rich photosphere composition indicating the accretion of terrestrial rocky materials, previous studies proposed runaway accretion of silicate particles due to gas drag by the increasing silicate vapor produced by the sublimation of the particles. Because re-condensation of the vapor diffused beyond the sublimation line was neglected, we revisit this problem by one-dimensional advection/diffusion simulation that consistently incorporates silicate sublimation/condensation and back-reaction to particle drift due to gas drag in the solid-rich disc. We find that the silicate vapor density in the region overlapping the solid particles follows the saturating vapor pressure and that no runaway accretion occurs if the re-condensation is included. This always limits the accretion rate from mono-compositional silicate discs to $\dot{M}_{\rm PR}$ in the equilibrium state. Alternatively, by performing additional simulations that couple the volatile gas (e.g., water vapor), we demonstrate that the volatile gas enhances the silicate accretion to $>\dot{M}_{\rm PR}$ through gas drag. The refractory-rich accretion is simultaneously reproduced when the initial volatile fraction of disc is $\lesssim 10$ wt\% because of the suppression of volatile accretion due to the efficient back-reaction of solid to gas. The discs originating from C-type asteroid analogs might be a possible clue to the high-$\dot{M}$ puzzle., Comment: 20 pages, 14 figures, accepted for publication in MNRAS

Recent spatially-resolved observations of protoplanetary disks revealed a plethora of substructures, including concentric rings and gaps, inner cavities, misalignments, spiral arms, and azimuthal asymmetries. This is the major breakthrough in studies of protoplanetary disks since Protostars and Planets VI and is reshaping the field of planet formation. However, while the capability of imaging substructures in protoplanetary disks has been steadily improving, the origin of many substructures are still largely debated. The structured distributions of gas and solids in protoplanetary disks likely reflect the outcome of physical processes at work, including the formation of planets. Yet, the diverse properties among the observed protoplanetary disk population, for example, the number and radial location of rings and gaps in the dust distribution, suggest that the controlling process may differ between disks and/or the outcome may be sensitive to stellar or disk properties. In this review, we (1) summarize the existing observations of protoplanetary disk substructures collected from the literature; (2) provide a comprehensive theoretical review of various processes proposed to explain observed protoplanetary disk substructures; (3) compare current theoretical predictions with existing observations and highlight future research directions to distinguish between different origins; and (4) discuss implications of state-of-the-art protoplanetary disk observations to protoplanetary disk and planet formation theory., Comment: Review Chapter for Protostars and Planets VII, Editors: Shu-ichiro Inutsuka, Yuri Aikawa, Takayuki Muto, Kengo Tomida, and Motohide Tamura. 43 pages, 7 figures, 4 tables, two incorrect citations are fixed from v1, electronic version of Tables 1 - 3 are available at http://ppvii.org/chapter/12/

We investigate the planetary transit detectability in the presence of stellar rotational activity from light curves for young M-dwarfs and estimate improvements of the detection at near-infrared (NIR) wavelengths. Making maps of the transit signal detection efficiency over the orbital period and planetary radius with light curves of members of four clusters, Hyades, Praesepe, Pleiades, and Upper Scorpius observed by the K2 mission, we evaluate the detectability for the rotation period and modulation semi-amplitude. We find that the detection efficiency remarkably decreases to about 20% for rapidly rotators with P_{rot} <= 1 d and the lack of planets in Pleiades is likely due to the high fraction of rapidly rotating M-dwarfs. We also evaluate the improvements of the planet detection with NIR photometry via tests using mock light curves assuming that the signal amplitude of stellar rotation decreases at NIR wavelengths. Our results suggest that NIR photometric monitoring would double relative detection efficiency for transiting planetary candidates with P_{rot} <= 1 d and find planets around M-dwarfs with approximately 100 Myr missing in the past transit surveys from the space., Comment: 21 pages, 16 figures, 3 tables; accepted for publication in The Astronomical Journal

This paper investigates the impact of dust size distribution on magnetic resistivity. In particular, we focus on its impact when the maximum dust size significantly increases from sub-micron. The first half of the paper describes our calculation method for magnetic resistivity based on the model of \citet{1987ApJ...320..803D} and shows that the method reproduces the results of a more realistic chemical reaction network calculations reasonably well. Then, we describe the results of the resistivity calculations for dust distributions with large maximum dust grains. Our results show that resistivity tends to decrease with dust growth, which is particularly true when the dust size power exponent $q$ is $q=2.5$. On the other hand, the decrease is less pronounced when the dust size power exponent $q$ is $q=3.5$, i.e., when the small dust is also responsible for the dust cross-section. Our results suggest that detailed dust coagulation and fragmentation processes play a vital role in the magnetic resistivities in protostar formation., Comment: 12 pages, 11 figures, accepted for publication in ApJ; https://iopscience.iop.org/article/10.3847/1538-4357/ac7b7b

The temperature structure of protoplanetary disks provides an important constraint on where in the disks rocky planets like our own form. Recent nonideal magnetohydrodynamical (MHD) simulations have shown that the internal Joule heating associated with magnetically driven disk accretion is inefficient at heating the disk midplane. A disk temperature model based on the MHD simulations predicts that in a disk around a solar-mass young star, the water snow line can move inside the current Earth's orbit within 1 Myr after disk formation. However, the efficiency of the internal Joule heating depends on the disk's ionization and opacity structures, both of which are governed by dust grains. In this study, we investigate these effects by combing the previous temperature model for magnetically accreting disks with a parameterized model for the grain size and vertical distribution. Grain growth enhances the gas ionization fraction and thereby allows Joule heating to occur closer to the midplane. However, growth beyond 10 $\rm \mu m$ causes a decrease in the disk opacity, leading to a lower midplane temperature. The combination of these two effects results in the midplane temperature being maximized when the grain size is in the range 10-100 $\rm \mu m$. Grain growth to millimeter sizes can also delay the snow line's migration to the 1 au orbit by up to a few Myr. We conclude that accounting for dust growth is essential for accurately modeling the snow line evolution and terrestrial planet formation in magnetically accreting protoplanetary disks., Comment: 20 pages, 15 figures, accepted for publication in The Astrophysical Journal

We present a high-resolution (2.5 au) multiband analysis of the protoplanetary disk around TW Hya using ALMA long baseline data at Bands 3, 4, 6, and 7. We aim to reconstruct a high-sensitivity millimeter continuum image and revisit the spectral index distribution. The imaging is performed by combining new ALMA data at Bands 4 and 6 with available archive data. Two methods are employed to reconstruct the images; multi-frequency synthesis (MFS) and the fiducial image-oriented method, where each band is imaged separately and the frequency dependence is fitted pixel by pixel. We find that the MFS imaging with the second order of Taylor expansion can reproduce the frequency dependence of the continuum emission between Bands 3 and 7 in a manner consistent with previous studies and is a reasonable method to reconstruct the spectral index map. The image-oriented method provides a spectral index map consistent with the MFS imaging, but with a two times lower resolution. Mock observations of an intensity model were conducted to validate the images from the two methods. We find that the MFS imaging provides a high-resolution spectral index distribution with an uncertainty of $<10$~\%. Using the submillimeter spectrum reproduced from our MFS images, we directly calculated the optical depth, power-law index of the dust opacity coefficient ($\beta$), and dust temperature. The derived parameters are consistent with previous works, and the enhancement of $\beta$ within the intensity gaps is also confirmed, supporting a deficit of millimeter-sized grains within the gaps., Comment: 17pages, 12 figures, Accepted for publication in The Astrophysical Journal

Understanding the thermal structure of protoplanetary disks is crucial for modeling planet formation and interpreting disk observations. We present a new two-layer radiative transfer model for computing the thermal structure of axisymmetric irradiated disks. Unlike the standard two-layer model, our model accounts for the radial as well as vertical transfer of the starlight reprocessed at the disk surface. The model thus allows us to compute the temperature below "shadowed" surfaces receiving no direct starlight. Thanks to the assumed axisymmetry, the reprocessed starlight flux is given in one-dimensional integral form that can be computed at a low cost. Furthermore, our model evolves the midplane temperature using a time-dependent energy equation and can therefore treat thermal instabilities. We apply our global two-layer model to disks with a planetary induced gap and confirm that the model reproduces the disks' temperature profiles obtained from more computationally expensive Monte Carlo radiative transfer calculations to an accuracy of less than 20%. We also apply the model to study the long-term behavior of the thermal wave instability in irradiated disks. Being simple and computationally efficient, the global two-layer model will be suitable for studying the interplay between disks' thermal evolution and dust evolution., Comment: 22 pages, 18 figures, accepted for publication in PASJ (Publications of the Astronomical Society of Japan)

A projectile impact onto a granular target produces an ejecta curtain with the heterogeneous material distribution. Understanding how the heterogeneous pattern forms is potentially important for understanding how crater rays form. Previous studies predicted that the pattern formation is induced by inelastic collisions of ejecta particles in the early stages of crater formation and is terminated by the ejecta's expanding motion. In this study, we test this prediction based on a hyper-velocity impact experiment together with N-body simulations where the trajectories of inelastically colliding granular particles are calculated. Our laboratory experiment suggests that pattern formation is already completed on a timescale comparable to the geometrical expansion of the ejecta curtain, which is ~ 10 microseconds in our experiment. Our simulations confirm the previous prediction that the heterogeneous pattern grows through initial inelastic collisions of particle clusters and subsequent geometric expansion with no further cluster collisions. Furthermore, to better understand the two-stage evolution of the mesh pattern, we construct a simple analytical model that assumes perfect coalescence of particle clusters upon collision. The model shows that the pattern formation is completed on the timescale of the system's expansion independently of the initial conditions. The model also reproduces the final size of the clusters observed in our simulations as a function of the initial conditions. It is known that particles in the target are ejected at lower speeds with increased distance to the impact point. The difference in the ejection speed of the particles may result in the evolution of the mesh pattern into rays., Comment: 13 pages, 11 figures, Accepted for publication in The Planetary Science Journal

The inner solar system possesses a unique orbital structure in which there are no planets inside the Mercury orbit and the mass is concentrated around the Venus and Earth orbits. The origins of these features still remain unclear. We propose a novel concept that the building blocks of the inner solar system formed at the dead-zone inner edge in the early phase of the protosolar disk evolution, where the disk is effectively heated by the disk accretion. First, we compute the dust evolution in a gas disk with a dead zone and obtain the spatial distribution of rocky planetesimals. The disk is allowed to evolve both by a viscous diffusion and magnetically-driven winds. We find that the rocky planetesimals are formed in concentrations around $\sim$ 1 au with a total mass comparable to the mass of the current inner solar system in the early phase of the disk evolution within $\lesssim0.1$ Myr. Based on the planetesimal distribution and the gas disk structure, we subsequently perform \textit{N}-body simulations of protoplanets to investigate the dynamical configuration of the planetary system. We find that the protoplanets can grow into planets without significant orbital migration because of the rapid clearing of the inner disk by the magnetically-driven disk winds. Our model can explain the origins of the orbital structure of the inner solar system. Several other features such as the rocky composition can also be explained by the early formation of rocky planetesimals., Comment: 8 pages, 6 figures, accepted for publication in ApJL

[abridged] 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 non-ideal 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 $\alpha$ is determined self-consistently under the framework of viscously-driven accretion from detailed considerations of the MRI with non-ideal MHD effects (Ohmic resistivity and ambipolar diffusion), accounting for disk heating by stellar irradiation, non-thermal sources of ionization, and dust effects on the ionization chemistry. Additionally, the magnetic field strength is constrained and adopted to maximize the MRI activity. 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 have 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) accreting a few $10^{-9}\, M_{\odot}.\rm{yr}^{-1}$. By conducting a detailed parameter study, we find that the extend 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. 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., Comment: Accepted for publication in A&A

It has been implicitly assumed that ices on grains in molecular clouds and proto planetary 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 proto-planetary disk is shown. These results have important implications for the chemical evolution of molecules, non-thermal desorption, collision of icy grains, and sintering., Comment: 60 pages, 18 figures, 3 tables, published in ApJ

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 constrain 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 in turbulent disks, and calculate the snow line location over time. We find that the snow line in the 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., Comment: 17 pages, 12 figures, accepted for publication in ApJ

The 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. The VSI drives turbulence with strong vertical motions, which could regulate the 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 this potentially stabilizing effects of dust evolution on the VSI based on the 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 the 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 altitude. In our default disk model with 0.01 solar masses, dust growth from 10 micron 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 a high degree of dust settling observed in the disk around HL Tau., Comment: 15 pages, 12 figures, Accepted for publication in ApJ

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 towards 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., Comment: 22 pages, 7 figures, Accepted for publication in ApJ

The largest moon of Neptune, Triton, possess a cold and hazy atmosphere. Since the discovery of 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 C$_2$H$_4$ ice. The haze particles can grow into fractal aggregates with mass-equivalent sphere sizes of $\sim0.1$--$1~{\rm {\mu}m}$ and fractal dimension of $D_{\rm f} = 1.8$--$2.2$. The ice-free hazes cannot simultaneously explain both UV and visible observations of Voyager 2, while including the condensation of C$_2$H$_4$ ices provides two better solutions. For ice aggregates, the required total haze mass flux is $\sim2\times{10}^{-15}~{\rm g~{cm}^{-2}~s^{-1}}$. For the icy sphere scenario, the column integrated C$_2$H$_4$ production rate is $\sim8\times{10}^{-15}~{\rm g~{cm}^{-2}~s^{-1}}$, and the ice-free mass flux of $\sim6\times{10}^{-17}~{\rm g~{cm}^{-2}~s^{-1}}$. The UV occultation observations at short wavelength $<0.15~{\rm {\mu}m}$ 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., Comment: 29 pages, 14 figures, accepted for publication in ApJ, several typos had been amended in the proof of correction

Around the snow line, icy pebbles and silicate dust may locally pile-up and form icy and rocky planetesimals via streaming instability and/or gravitational instability. We perform 1D diffusion-advection simulations that include the back-reaction to radial drift and diffusion of icy pebbles and silicate dust, ice sublimation, release of silicate dust, and their recycling through recondensation and sticking onto pebbles outside the snow line. We use a realistic description of the scale height of silicate dust obtained from Ida et al. and that of pebbles including the effects of a Kelvin-Helmholtz instability. We study the dependence of solid pile-up on distinct effective viscous parameters for turbulent diffusions in the radial and vertical directions ($\alpha_{\rm Dr}$ and $\alpha_{\rm Dz}$) and for the gas accretion to the star ($\alpha_{\rm acc}$) as well as that on the pebble-to-gas mass flux ($F_{\rm p/g}$). We derive the sublimation width of drifting icy pebbles which is a critical parameter to characterize the pile-up of silicate dust and pebbles around the snow line. We identify a parameter space (in the $F_{\rm p/g}-\alpha_{\rm acc}-\alpha_{\rm Dz}(=\alpha_{\rm Dr})$ space) where pebbles no longer drift inward to reach the snow line due to the back-reaction that slows down radial velocity of pebbles. We show that the pile-up of solids around the snow line occurs in a broader range of parameters for $\alpha_{\rm acc}=10^{-3}$ than for $\alpha_{\rm acc}=10^{-2}$. Above a critical $F_{\rm p/g}$ value, the runaway pile-up of silicate dust inside the snow line is favored for $\alpha_{\rm Dr}/\alpha_{\rm acc} \ll 1$, while that of pebbles outside the snow line is favored for $\alpha_{\rm Dr}/\alpha_{\rm acc} \sim 1$. Our results imply that a distinct evolutionary path could produce a diversity of outcomes in terms of planetesimal formation around the snow line., Comment: 18 pages, 6 Figures, 3 Figures in Appendix, accepted for publication in Astronomy & Astrophysics (A&A)

Ring structures are observed by (sub-)millimeter dust continuum emission in various circumstellar disks from early stages of Class 0 and I to late stage of Class II young stellar objects (YSOs). In this paper, we study one of the possible scenarios of such ring formation in early stage, which is coagulation of dust aggregates. 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. With radiative transfer calculations based on the dust coagulation model, we find that the growth front can be observed as a ring structure because dust surface density is sharply changed at this position. Furthermore, we confirm that the observed ring positions in the YSOs with an age of $\lesssim1$ Myr are consistent with the growth front. The growth front could be important to create the ring structure in particular for early stage of the disk evolution such as Class 0 and I sources., Comment: 15 pages, 12 figures, 1 table, accepted for publication in ApJ

Context: The formation of rocky planetesimals is a long-standing problem in planet formation theory. One of the possibilities is that it results from gravitational instability as a result of pile-up of small silicate dust particles released from sublimating icy pebbles that pass the snow line. Aims: We want to understand and quantify the role of the water snow line for the formation of rock-rich and ice-rich planetesimals. In this paper, we focus on the formation of rock-rich planetesimals. A companion paper examines the combined formation of both rock-rich and ice-rich planetesimals. Methods: We develop a new Monte Carlo code to calculate the radial evolution of silicate particles in a turbulent accretion disk, accounting for the back-reaction (i.e., inertia) of the particles on their radial drift velocity and diffusion. Results depend in particular on the particle injection width (determined from the radial sublimation width of icy pebbles), the pebble scale height and the pebble mass flux through the disk. The scale height evolution of the silicate particles, which is the most important factor for the runaway pile-up, is automatically calculated in this Lagrange method. Results: From the numerical results, we derive semi-analytical relations for the scale height of the silicate dust particles and the particles-to-gas density ratio at the midplane, as functions of a pebble-to-gas mass flux ratio and the $\alpha$ parameters for disk gas accretion and vertical/radial diffusion. We find that the runaway pile-up of the silicate particles (formation of rocky planetesimals) occurs if the pebble-to-gas mass flux ratio is $> [(\alpha_{Dz}/\alpha_{acc})/3 \times 10^{-2}]^{1/2}$ where $\alpha_{Dz}$ and $\alpha_{acc}$ are the $\alpha$ parameters for vertical turbulent diffusion and disk gas accretion., Comment: 15 pages, accepted for publication in Astronomy & Astrophysics

Disintegrating planets are ultra-short-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 tail is made of recondensing minerals. Transmission spectroscopy of the dust tails could thus allow us to directly probe the elementary compositions of the planets. Previous work already investigated the feasibility of such observations using the 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 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 $\gtrsim$ 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 $\lesssim$ 100 pc with a transit depth of $\gtrsim$ 2%. Transit searches with current and future space telescopes (e.g., $TESS$ and PLATO) will provide ideal targets for such spectroscopic observations., Comment: 14 pages, 10 figures, accepted for publication in The Astrophysical Journal

Substructures in PPDs, whose ubiquity was unveiled by recent ALMA observations, are widely discussed regarding their possible origins. We carry out global full magnetohydrodynamic (MHD) simulations in axisymmetry, coupled with self-consistent ray-tracing radiative transfer, thermochemistry, and non-ideal 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 protoplanetary disks (PPDs) through the accretion process. Disk ionization structures and thus tensorial conductivities depend on the size of grains.When grains are significantly larger than PAHs, the non-ideal MHD conductivities change dramatically across each snow line of major volatiles, leading to a sudden change of the accretion process across the snow lines and the subsequent formation of gaseous rings/gaps there. 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., Comment: 9 pages, 6 figures, submitted to AAS journals

It is generally accepted that the four major (Galilean) satellites formed out of the gas disk that accompanied Jupiter's formation. However, understanding the specifics of the formation process is challenging as both small particles (pebbles) as well as the satellites are subject to fast migration processes. Here, we hypothesize a new scenario for the origin of the Galilean system, based on the capture of several planetesimal seeds and subsequent slow accretion of pebbles. To halt migration, we invoke an inner disk truncation radius, and other parameters are tuned for the model to match physical, dynamical, compositional, and structural constraints. In our scenario it is natural that Ganymede's mass is determined by pebble isolation. Our slow-pebble-accretion scenario then reproduces the following characteristics: (1) the mass of all the Galilean satellites; (2) the orbits of Io, Europa, and Ganymede captured in mutual 2:1 mean motion resonances; (3) the ice mass fractions of all the Galilean satellites; (4) the unique ice-rock partially differentiated Callisto and the complete differentiation of the other satellites. Our scenario is unique to simultaneously reproduce these disparate properties., Comment: 25 pages, 11 figures, 6 tables, accepted for publication in ApJ

Transmission spectrum surveys have suggested the ubiquity of high-altitude clouds in exoplanetary atmospheres. Theoretical studies have investigated the formation processes of the high-altitude clouds; however, cloud particles have been commonly approximated as compact spheres, which is not always true for solid mineral particles that likely constitute exoplanetary clouds. Here, we investigate how the porosity of cloud particles evolve in exoplanetary atmospheres and influence the cloud vertical profiles. We first construct a porosity evolution model that takes into account the fractal aggregation and the compression of cloud particle aggregates. Using a cloud microphysical model coupled with the porosity model, we demonstrate that the particle internal density can significantly decrease during the cloud formation. As a result, fluffy-aggregate clouds ascend to altitude much higher than that for compact-sphere clouds assumed so far. We also examine how the fluffy-aggregate clouds affect transmission spectra. We find that the clouds largely obscure the molecular features and produce a spectral slope originated by the scattering properties of aggregates. Finally, we compare the synthetic spectra with the observations of GJ1214 b and find that its flat spectrum could be explained if the atmospheric metallicity is sufficiently high ($\ge100\times$ solar) and the monomer size is sufficiently small ($r_{\rm mon}<1~{\rm {\mu}m}$). The high-metallicity atmosphere may offer the clues to explore the gas accretion processes onto past GJ1214b., Comment: 23 pages, 12 figures, accepted for publication in ApJ. v3: Figures 5-9 have been corrected

Dust coagulation in a protoplanetary disk is the first step of planetesimal formation. However, a pathway from dust aggregates to planetesimals remains unclear. Both numerical simulations and laboratory experiments have suggested the importance of dust structure in planetesimal formation, but it is not well constrained by observations. We study how dust structure and porosity alters polarimetric images at millimeter wavelength by performing 3D radiative transfer simulations. Aggregates with different porosity and fractal dimension are considered. As a result, we find that dust aggregates with lower porosity and/or higher fractal dimension are favorable to explain observed millimeter-wave scattering polarization of disks. Aggregates with extremely high porosity fail to explain the observations. In addition, we also show that particles with moderate porosity show weak wavelength dependence of scattering polarization, indicating that multi-wavelength polarimetry is useful to constrain dust porosity. Finally, we discuss implications for dust evolution and planetesimal formation in disks., Comment: 17 pages, 10 Figures, 1 Table, accepted for publication in ApJ

We report the detection of an excess in dust continuum emission at 233~GHz (1.3~mm in wavelength) in the protoplanetary disk around TW~Hya revealed through high-sensitivity observations at $\sim$3~au resolution with the Atacama Large Millimeter/submillimeter Array (ALMA). The sensitivity of the 233~GHz image has been improved by a factor of 3 with regard to that of our previous cycle 3 observations. The overall structure is mostly axisymmetric, and there are apparent gaps at 25 and 41 au as previously reported. The most remarkable new finding is a few au-scale excess emission in the south-west part of the protoplanetary disk. The excess emission is located at 52 au from the disk center and is 1.5 times brighter than the surrounding protoplanetary disk at a significance of 12$\sigma$. We performed a visibility fitting to the extracted emission after subtracting the axisymmetric protoplanetary disk emission and found that the inferred size and the total flux density of the excess emission are 4.4$\times$1.0~au and 250~$\mu$Jy, respectively. The dust mass of the excess emission corresponds to 0.03~$M_\oplus$ if a dust temperature of 18~K is assumed. Since the excess emission can also be marginally identified in the Band 7 image at almost the same position, the feature is unlikely to be a background source. The excess emission can be explained by a dust clump accumulated in a small elongated vortex or a massive circumplanetary disk around a Neptune mass forming-planet., Comment: 9pages, 5 figures, Accepted for publication in The Astrophysical Journal Letters

The magnetohydrodynamics (MHD) of protoplanetary disks are strongly subject to the non-ideal MHD effects arising from the low ionization fraction of the disk gas. A strong electric field induced by gas motions can heat ionized gas particles and can thereby affect the ionization balance in the disks. Our previous studies revealed that in dusty protoplanetary disks, the Ohmic conductivity decreases with increasing electric field strength until the electrical breakdown of the disk gas occurs. In this study, we extend our previous work to more general cases where both electric and magnetic fields affect the motion of plasma particles, allowing us to study the impacts of plasma heating on all non-ideal MHD effects: Ohmic, Hall, and ambipolar diffusion. We find that the upper limit on the electric current we previously derived applies even in the presence of magnetic fields. Although the Hall and ambipolar resistivities can either increase or decrease with electric field strength depending on the abundance of charged dust grains, the Ohmic resistivity always increases with electric field strength. An order-of-magnitude estimate suggests that a large-scale electric current generated by gas motions in the inner part of protoplanetary disks could exceed the upper limit. This implies that MHD motions of the inner disk, such as the motion driven by the Hall-shear instability, could either get suppressed or trigger electrical breakdown (lightning discharge). This may have important implications for gas accretion and chondrule formation in the inner part of protoplanetary disks., Comment: 16 pages, 5 figures, accepted for publication in ApJ

The poor stickiness of silicate dust grains is a major obstacle to the formation of rocky planetesimals. In this study, we examine the possibility that silicate grains with an organic mantle, which we call Organic-Mantled Grains (OMGs), form planetesimals through direct coagulation. Organic mantles are commonly found in interplanetary dust particles, and laboratory experiments show that they are softer than silicates, in particular in warm environments. This, combined with the theory of particle adhesion, implies that OMGs are stickier than bare silicate grains. Because organic mantles can survive up to 400 K, silicate grains inside the water snow line in protoplanetary disks can in principle hold such mantles. We construct a simple grain adhesion model to estimate the threshold collision velocity below which aggregates of OMGs can grow. The model shows that aggregates of 0.1 \micron-sized OMGs can overcome the fragmentation barrier in protoplanetary disks if the mantles are as thick as those in interplanetary dust particles and if the temperature is above $\sim$ 200 K. We use this adhesion model to simulate the global evolution of OMG aggregates in the inner part of a protoplanetary disk, demonstrating that OMG aggregates can indeed grow into planetesimals under favorable conditions. Because organic matter is unstable at excessively high temperatures, rocky planetesimal formation by the direct sticking of OMGs is expected to occur in a disk annulus corresponding to the temperature range $ \sim$ 200--400 K. The organic-rich planetesimals may grow into carbon-poor rocky planetesimals by accreting a large amount of carbon-poor chondrules., Comment: 13 pages, 6 figures, accepted for publication in ApJ

Recent high resolution observations unveil ring structures in circumstellar disks. The origin of these rings has been widely investigated under various theoretical scenarios. In this work we perform global 3D non-ideal MHD simulations including effects from both Ohmic resistivity and ambipolar diffusion (AD) to model the HL Tau disk. The non-ideal MHD diffusion profiles are calculated based on the global dust evolution calculation including sintering effects. Disk ionization structure changes dramatically across the snow line due to the change of dust size distribution close to snow line of major volatiles. We find that accretion is mainly driven by disk wind. Gaps and rings can be quickly produced from different accretion rates across snow line. Furthermore, ambipolar diffusion (AD) leads to highly preferential accretion at midplane, followed by magnetic reconnection. This results a local zone of decretion that drains of mass in the field reconnection area, which leaves a gap and an adjacent ring just outside it. Overall, under the favorable condition, both snow lines and non-ideal MHD effects can lead to gaseous gaps and rings in protoplanetary disks., Comment: 13 pages, 10 figures, published on ApJ

Recent (sub)millimeter polarimetric observations toward the young star HL Tau have successfully detected polarization emission from its circumstellar disk. The polarization pattern observed at 0.87 mm is uniform and parallel to the disk's minor axis, consistent with the self-scattering of thermal emission by dust particles whose maximum radius is $\approx 100~\mu m$. However, this maximum size is considerably smaller than anticipated from dust evolution models that assume a high sticking efficiency for icy particles. Here, we propose that the unexpectedly small particle size can be explained if CO$_2$ ice covers the particles in the outer region of the HL Tau disk. CO$_2$ ice is one of the most major interstellar ices, and laboratory experiments show that it is poorly sticky. Based on dust evolution models accounting for CO$_2$ ice mantles as well as aggregate sintering and post-processing radiative transfer, we simulate the polarimetric observation of HL Tau at 0.87 mm. We find that the models with CO$_2$ ice mantles better match the observation. These models also predict that only particles lying between the H$_2$O and CO$_2$ snow lines can grow to millimeter to centimeter sizes, and that their rapid inward drift results in a local dust gap similar to the 10 au gap of the HL Tau disk. We also suggest that the millimeter spectral index for the outer part of the HL Tau disk is largely controlled by the optical thickness of this region and does not necessarily indicate dust growth to millimeter sizes., Comment: 15 pages, 6 figures, accepted for publication in ApJ

Dust grains emit intrinsic polarized emission if they are elongated and aligned in the same direction. The direction of the grain alignment is determined by external forces, such as magnetic fields, radiation, and gas flow against the dust grains. In this letter, we apply the concept of the grain alignment by gas flow, which is called mechanical alignment, to the situation of a protoplanetary disk. We assume that grains have a certain helicity, which results in the alignment with the minor axis parallel to the grain velocity against the ambient disk gas and discuss the morphology of polarization vectors in a protoplanetary disk. We find that the direction of the polarization vectors depends on the Stokes number, which denotes how well grains are coupled to the gas. If the Stokes number is less than unity, orientation of polarization is in the azimuthal direction since the dust velocity against the gas is in the radial direction. If the Stokes number is as large as unity, the polarization vectors show a leading spiral pattern since the radial and azimuthal components of the gas velocity against the dust grains are comparable. This suggests that if the observed polarization vectors show a leading spiral pattern, it would indicate that Stokes number of dust grains is around unity, which is presumably radially drifting., Comment: 7 pages, 5 figures, accepted for publication in The Astrophysical Journal Letters

We study scattered light properties of protoplanetary discs at near-infrared wavelengths for various dust size and structure by performing radiative transfer simulations. We show that different dust structures might be probed by measuring disk polarisation fraction as long as the dust radius is larger than the wavelength. When the radius is larger than the wavelength, disc scattered light will be highly polarised for highly porous dust aggregates, whereas more compact dust structure tends to show low polarisation fraction. Next, roles of monomer radius and fractal dimension for scattered light colours are studied. We find that, outside the Rayleigh regime, as fractal dimension or monomer radius increases, colours of the effective albedo at near-infrared wavelengths vary from blue to red. Our results imply that discs showing grey or slightly blue colours and high polarisation fraction in near-infrared wavelengths might be explained by the presence of large porous aggregates containing sub-microns sized monomers., Comment: Accepted for publication in MNRAS, 18 pages, 19 figures

The gas temperature in protoplanetary disks (PPDs) is determined by a combination of irradiation heating and accretion heating, with the latter conventionally attributed to turbulent dissipation. However, recent studies have suggested that the inner disk (a few AU) is largely laminar, with accretion primarily driven by magnetized disk winds, as a result of nonideal magnetohydrodynamic (MHD) effects from weakly ionized gas, suggesting an alternative heating mechanism by Joule dissipation. We perform local stratified MHD simulations including all three nonideal MHD effects (ohmic, Hall, and ambipolar diffusion) and investigate the role of Joule heating and the resulting disk vertical temperature profiles. We find that in the inner disk, as ohmic and ambipolar diffusion strongly suppress electrical current around the midplane, Joule heating primarily occurs at several scale heights above the midplane, making the midplane temperature much lower than that with the conventional viscous heating model. Including the Hall effect, Joule heating is enhanced/reduced when the magnetic fields threading the disks are aligned/anti-aligned with the disk rotation, but it is overall ineffective. Our results further suggest that the midplane temperature in the inner PPDs is almost entirely determined by irradiation heating, unless viscous heating can trigger thermal ionization in the disk innermost region to self-sustain magnetorotational instability turbulence., Comment: 23 pages, 11 figures; matches the published version

The Earth's ocean mass is only 2.3 x 10^{-4} of the whole planet mass. Even including water in the interior, it would be at most 10^{-3}-10^{-2}. Ancient Mars may have had a similar or slightly smaller water fraction. It is important to clarify the water delivery mechanism to rocky planets in habitable zones in exoplanetary systems, as well as that to the Earth and Mars. Here, we consider water delivery to planets by icy pebbles after the snowline inwardly passes the planetary orbits and derive the water mass fraction (f_{water}) of the final planet as a function of disk parameters and discuss the parameters that reproduce f_{water} comparable to that inferred for the Earth and ancient Mars. We calculate the growth of icy pebbles and their radial drift with a 1D model, and accretion of icy pebbles onto planets, by simultaneously solving the snowline migration and the disk dissipation, to evaluate f_{water} of the planets. We find that f_{water} is regulated by the total mass (M_{res}) of icy dust materials preserved in the outer disk regions at the timing (t = t_{snow}) of the snowline passage of the planetary orbit. Because M_{res} decays rapidly after the pebble formation front reaches the disk outer edge (at t = t_{pff}), f_{water} is sensitive to the ratio t_{snow}/t_{pff}, which is determined by the disk parameters. We find t_{snow}/t_{pff} < 10 or > 10 is important. Deriving an analytical formula for f_{water} that reproduces the numerical results, we find that f_{water} of a rocky planet near 1 au is ~ 10^{-4}-10^{-2}, in the disks with initial disk size ~ 30-50 au and the initial disk mass accretion rate ~ (10^{-8}-10^{-7}) M_sun/y. Because these disks may be median or slightly compact/massive disks, the water fraction of rocky planets in habitable zones may be often similar to that of the Earth, if the icy pebble accretion is responsible for the water delivery., Comment: published in A&A in 2019; a corrected version from the published version; in the previous replacement, some figures were not shown. This is the version with complete figures

The Hall effect plays a significant role in star formation because it induces rotation in the infalling envelope, which in turn affects the formation and evolution of the circumstellar disk. The importance of the Hall effect varies with the Hall coefficient, and this coefficient is determined by the fractional abundances of charged species. These abundance values are primarily based on the size and quantity of dust grains as well as the cosmic ray intensity, which respectively absorb and create charged species. Thus, the Hall coefficient varies with both the properties of dust grains and the cosmic ray rate (or ionization source). In this study, we explore the dependence of the Hall coefficient on the grain size and cosmic ray ionization rate using a simplified chemical network model. Following this, using an analytic model, we estimate the typical size of a circumstellar disk induced solely by the Hall effect. The results show that the disk grows during the main accretion phase to a size of ${\sim}$ 3 - 100 au, with the actual size depending on the parameters. These findings suggest that the Hall effect greatly affects circumstellar disk formation, especially in the case that the dust grains have a typical size of ${\sim}$ 0.025 ${\mu}$m - 0.075 ${\mu}$m., Comment: Accepted for publication in MNRAS. 39 pages, 13 figures

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, $\alpha_{\rm dead}$, 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 $1~{\rm m~s^{-1}}$, the stress to pressure ratio $\alpha_{\rm dead}$ needs to be lower than $3 \times 10^{-4}$ 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 $\sim 10~{\rm au}$. 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., Comment: 16 pages, 13 figures, accepted for publication in ApJ

When a planet forms a deep gap in a protoplanetary disk, dust grains cannot pass through the gap. As a consequence, the density of the dust grains can increase up to the same level of the density of the gas at the outer edge. The feedback on the gas from the drifting dust grains is not negligible, in such a dusty region. We carried out two-dimensional two-fluid (gas and dust) hydrodynamic simulations. We found that when the radial flow of the dust grains across the gap is halted, a broad ring of the dust grains can be formed because of the dust feedback and the diffusion of the dust grains. The minimum mass of the planet to form the broad dust ring is consistent with the pebble-isolation mass, in the parameter range of our simulations. The broad ring of the dust grains is good environment for the formation of the protoplanetary solid core. If the ring is formed in the disk around the sun-like star at $\sim 2\ \mbox{AU}$, a massive solid core ($\sim 50M_{\oplus}$) can be formed within the ring, which may be connected to the formation of Hot Jupiters holding a massive solid core such as HD 149026b. In the disk of the dwarf star, a number of Earth-sized planets can be formed within the dust ring around $\sim 0.5\ \mbox{AU}$, which potentially explain the planet system made of multiple Earth-sized planets around the dwarf star such as TRAPPIST-1., Comment: Accepted for publication in ApJ, 18 pages, 17 figures