arXiv subjects

Kiyoaki Doi

Publications and source records attributed to Kiyoaki Doi.

At least 19 recordsLinked to original sources

arXiv2026

Multi-wavelength Constraints on Dust Dynamics and Size Evolution in Protoplanetary Disk Rings. II. Observational Implications

Spatially resolved dust rings in protoplanetary disks are widely used to infer disk and dust physics from multi-wavelength continuum observations. Their interpretation, however, often neglects grain growth and the evolution of the size distribution, limiting the connection between observed ring profiles and dust-evolution parameters. Building on a physical dust-ring model that includes coagulation and fragmentation, we develop a Bayesian inference framework that jointly incorporates radiative transfer and finite angular resolution. When applied to two rings in HD 163296 and two in LkCa 15, our framework yields gas-dependent estimates of the key dust-evolution parameters such as turbulence strength $α$ and the fragmentation velocity $v_{\rm frag}$ in a self-consistent way. Most rings admit both a low-$α$, low-$v_{\rm frag}$ branch with small grains, and a higher-$α$, higher-$v_{\rm frag}$ branch with larger grains. Typical low-$α$ branches have $α\sim10^{-5}$--$10^{-4}$ and fragmentation velocities of order cm s$^{-1}$ level, whereas the higher-$α$ branches reach $α\sim10^{-3}$--$10^{-2}$ and fragmentation velocities of a few to $20$ m s$^{-1}$. The observed broad and wavelength-dependent profiles near the ring peaks can be reproduced by intrinsically narrow dust rings. This new framework offers a more direct route from multi-wavelength continuum data to the microphysics of dust growth and trapping---a connection that can be robustly tested with future high-resolution observations at longer wavelengths.

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arXiv2026

Dust characterization of the HD 163296 disk with high-resolution multi-wavelength ALMA observations

Context. Planets form through the growth and accumulation of dust grains in protoplanetary disks. Characterizing dust properties such as size, surface density, and temperature is key to understanding planet formation. Aims. We characterize the dust properties of the protoplanetary disk around HD 163296 by performing spectral energy distribution (SED) fitting of multi-wavelength high-resolution observations. Methods. We present new high-resolution ALMA Band 9 (0.45 mm) observations, which are sensitive to the temperature. We performed SED fitting at a common resolution of 0.066 arcsec using these new Band 9 observations along with archival ALMA Band 3, 4, 6, and 7 observations. We compared the fitted results with VLA observations and explored multiple dust models with different optical constants and porosities. Results. The Band 9 image shows the central disk, two rings at 0.67 and 1.00 arcsec, and outer extended emission previously seen at other wavelengths. At higher frequencies, the rings appear wider, the gaps appear shallower, and the extended emission appears brighter, which can be explained by optical-depth effects and/or size segregation. We characterized the dust properties, including temperature, surface density, and dust size, for each dust model. However, the inferred dust properties are dependent on the dust model, and the ALMA data alone do not allow us to determine which dust model is preferred. We identified the DSHARP Zubko (porous) model as the preferred model based on VLA profiles and physical and observational constraints. The outer ring temperature is lower than predicted by a passively irradiated disk model, suggesting shadowing by the inner ring. Although the surface density and dust size depend on the dust model, the preferred model indicates that the central disk, both rings, and the extended disk each contain more than a few Earth masses of dust.

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arXiv2026

Interpreting ALMA Multiwavelength Continuum Observations of PDS 70 c: An Optically Thick Dust Ring in the Circumplanetary Disk

Giant planets form small gas disks, called circumplanetary disks (CPDs), during gas accretion. The CPD of PDS 70 c has been detected by the Atacama Large Millimeter/submillimeter Array (ALMA) in (sub)millimeter continuum emission, which is interpreted as thermal emission from dust in the CPD. The resulting spectral index suggests that the disk is optically thick over a wide range of wavelengths. However, this is inconsistent with previous CPD dust models, which predict that the disk is optically thin because of radial dust drift. Here, we present a new interpretation of the multiwavelength observations: the CPD hosts an optically thick dust ring, whose existence has been discussed in the context of satellite formation. We demonstrate that a dust-ring model that incorporates gas accretion, dust evolution, and dust thermal emission, is consistent with the observations under reasonable conditions, whereas a conventional ring-less model requires more stringent conditions. We also show that the dust ring inferred from the observations potentially satisfies the conditions for exomoon formation via streaming instability and subsequent gravitational instability.

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arXiv2026

Direct Retrieval of Protoplanetary Disk Dust Properties using Auto-differentiable Gaussian Processes and Its Application to the HD 169142 Disk

Retrieving dust properties in protoplanetary disks, including the surface density distribution, temperature, and grain size distribution, is a fundamental task in observational studies of planet formation. While multi-wavelength analysis of the spectral energy distribution (SED) using interferometers such as the Atacama Large Millimeter/submillimeter Array (ALMA) is a powerful diagnostic tool, traditional methods are often hindered by strong biases arising from a limited imaging beamsize. In this paper, we present a new retrieval framework for dust disk properties designed to overcome this challenge. We assume that the underlying physical structures are expressed as sample paths from Gaussian processes, compute the radial intensity distributions at observed wavelengths, and produce one-dimensional visibility models. The models are compared with the observed data and the posterior distributions are sampled via the Markov-Chain Monte-Carlo method. The whole procedure is implemented in JAX, which enables end-to-end auto-differentiation and significantly accelerates the inference. We validate our methodology using mock datasets, and find that the results are not strongly biased and better reproduce the input profiles. We also demonstrate its capabilities through an application to ALMA Band 3, 6, and 9 observations of the HD 169142 disk, revealing a new complex structure. Our developed code is publicly available as a Python module, FRAP (Flexible Radial Analysis of Protoplanetary disks). This framework provides a next-generation infrastructure for disk SED modeling, enabling high-precision studies of the physical environments in which planets form.

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arXiv2026

An archival summary: 15 years of ALMA observations on disks and planet formation

The Atacama Large (sub-)millimeter Array (ALMA) has been in scientific operations for almost 15 years. We celebrate this achievement by providing a summary of the ``Disks and planet formation'' scientific category, with an emphasis on the disks located in the nearby star-forming regions. As of the beginning of February 2026, ALMA had observed 3933 independent coordinates, which we analyzed by their location in the sky, frequency coverage, exposure time, spectral line coverage, and angular resolution. We encourage the community to explore new scientific questions that are made possible through the archival datasets.

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arXiv2026

Multi-wavelength ALMA Imaging of HD 34282: Dust-trapping Signatures of a Vortex Candidate

Azimuthal arcs in millimeter continuum emission from protoplanetary disks are often attributed to dust-trapping vortices, but definitive observational confirmation of vortices remains lacking. We present sub-0.1" resolution ALMA continuum observations of the HD 34282 disk at 0.9, 1.3, 2.1, and 3.1 mm. These observations resolve a bright azimuthal arc superposed on a compact double-gap, triple-ring morphology, most clearly at shorter wavelengths, and enable us to probe the physical origin of the arc. It exhibits a lower spectral index than the surrounding rings, consistent with enhanced grain growth and/or higher dust surface density of a dust-trapping vortex. Its azimuthal width decreases with increasing wavelength, consistent with tighter confinement of larger grains, or lower optical depths at longer wavelengths. These observations probe dust with Stokes numbers St < 0.03. Vortex models predict negligible peak shifts in this regime, consistent with the 1.3 to 3.1 mm data. At 0.9 mm, however, the arc peak is offset by 15 +/- 4 degree in the direction of disk rotation relative to longer wavelengths, and the near-side ring emission is locally dimmer compared to the far-side, likely reflecting optical-depth or temperature effects. These observations are consistent with azimuthal dust trapping, potentially associated with a vortex-induced pressure maximum.

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arXiv2026

A Protoplanet Candidate in the PDS 66 Disk Indicated by Silicon Sulfide Isotopologues

Despite observational progress in planet formation, the stage in which planetesimals grow into planets remains poorly understood. During this phase, protoplanets may develop gaseous envelopes that are warmer than the surrounding disk gas, potentially providing observable signatures through molecules otherwise depleted in cold regions. In this Letter, we report the detection of the silicon sulfide isotopologues ${\rm ^{28}SiS}\ J{=}16{-}15$ and ${\rm ^{30}SiS}\ J{=}18{-}17$ in the protoplanetary disk around PDS 66 (MP Mus) at a significance of ${\sim}5{-}6σ$, using the Atacama Large Millimeter/submillimeter Array. These constitute the second and first detections of $\rm ^{28}SiS$ and $\rm ^{30}SiS$ in a protoplanetary disk, respectively. The emission appears as a compact source at $r \simeq 60$ au in the southwestern region of the disk, unresolved with a ${\sim}0.\!\!^{\prime\prime}5$ beam, and shows a velocity consistent with Keplerian rotation, suggesting a protoplanetary origin. By modeling the line fluxes, we constrain the emitting radius to ${\sim}0.5{-}4$ au and estimate an SiS mass of $10^{22}{-}10^{23}$ g, corresponding to at least ${\sim}10\%$ of the silicon contained in local dust grains. Because complete sublimation of a substantial fraction of dust grains by local processes is difficult to achieve, this result instead implies an accumulation of silicon from a larger region. We propose that a circumplanetary envelope surrounding a low-mass protoplanet, where pebble accretion and subsequent sublimation of grains may enhance gaseus silicon abundance with respect to observable dust grains around it, can account for the observed characteristics.

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arXiv2026

ALMA 873 $μ$m Polarization Observations of the PDS~70 Disk

At a 112.4 pc distance, the PDS70 protoplanetary disk is a rare case that has been confirmed to host two accreting planets. This makes it the most important laboratory for studying dust growth in the context of planet formation. Here we present the first deep, full polarization observations at 873 $μ$m wavelength. We detected $\sim$1%-2.5% linear polarization over the bulk of the $\sim$55-100 AU (sub)millimeter ring. The polarization position angles align preferentially with the projected minor axis of the disk. The standard interpretation is that the observed polarization is caused by dust self-scattering, with a maximum dust grain size of $\sim$100 $μ$m. On $\gtrsim$10 AU scales, which can be resolved by the presented 873-3075 $μ$m observations, the ring is marginally optical thick at 873 $μ$m wavelength. Using Monte Carlo radiative transfer simulations, we found that an azimuthally asymmetric, marginally optically thick ring with a maximum dust grain size of $\sim$87 $μ$m can reproduce the observed 873 $μ$m polarization position angles and percentages. This study indicates that the coagulation of ice-coated dust in the protoplanetary disk may be limited by fragmentation or bouncing.

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arXiv2025

Winding Motion of Spirals in a Gravitationally Unstable Protoplanetary Disk

The discovery of wide-orbit giant exoplanets has posed a challenge to our conventional understanding of planet formation by coagulation of dust grains and planetesimals, and subsequent accretion of protoplanetary disk gas. As an alternative mechanism, the direct in-situ formation of planets or planetary cores by gravitational instability (GI) in protoplanetary disks has been proposed. However, observational evidence for GI in regions where wide-orbit planets are formed is still lacking. Theoretical studies predict that GI induces spiral arms moving at the local Keplerian speed in a disk. Here, with multiple high angular resolution observations over a seven-year time baseline using the Atacama Large Millimeter/submillimeter Array (ALMA), we report the evidence for spiral arms following the Keplerian rotation in the dust continuum disk around the young star IM Lup. This demonstrates that GI can operate in wide-orbit planet-formation regions, establishing it as a plausible formation mechanism for such planets.

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arXiv2025

Puffed-up Inner Rings and Razor-thin Outer Rings in Structured Protoplanetary Disks

The vertical distribution of pebbles in protoplanetary disks is a fundamental property influencing planet formation, from dust aggregation to the assembly of planetary cores. In the outer region of protoplanetary disks, the intensity of the optically thin but geometrically thick dust ring decreases along the minor axis due to reduced line-of-sight optical depth. Multi-ring disks thus provide an excellent opportunity to study the radial variation of the vertical properties of dust. We investigate the vertical dust distribution in 6 protoplanetary disks with resolved double rings, using high-resolution ALMA Band 6 continuum observations. By modeling the azimuthal intensity variations in these rings, we constrain the dust scale heights for each ring. Our results reveal a dichotomy: inner rings exhibit puffed-up dust layers with heights comparable to the gas scale height, while outer rings are significantly more settled, with dust scale heights less than 20\% of the gas scale height. This suggests a radial dependence in dust settling efficiency within the disks, potentially driven by localized planetary interactions or the global radial dependence of the Vertical Shear Instability (VSI). We discuss the implications of these findings for dust trapping, planet formation, and protoplanetary disk evolution. Our work highlights the importance of vertical dust distribution in understanding the early stages of planet formation and suggests that outer ($>80$~au), settled rings are preferred sites for planet formation over inner ($<80$~au), turbulent rings.

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arXiv2025

Multi-wavelength Constraints on Dust Dynamics and Size Evolution in Protoplanetary Disk Rings. I. Method

Observations with the Atacama Large Millimeter/submillimeter Array (ALMA) and the Jansky Very Large Array (JVLA) have revealed many dust rings in protoplanetary disks, often interpreted as dust traps at gas pressure bumps. Previous studies have typically modeled these rings by assuming a single dust species in drift-diffusion equilibrium, neglecting dust size evolution resulting from coagulation and fragmentation. In this work, we perform numerical simulations that incorporate both dust-gas dynamics (drift and diffusion) and dust size evolution. Our results show that the radial distributions of different dust species (up to the fragmentation limit) are nearly identical in the dust ring, as dust growth dominates over drift and diffusion (e.g., with a typical dust-to-gas ratio of $ε\sim 10^{-2}$). Building on this finding, we develop a comprehensive, self-consistent analytical theory that describes the dust ring structure while explicitly accounting for size evolution effects. Our model provides a unified framework for interpreting multi-wavelength observations by linking the physical dust distribution to the observed ring properties, thus laying the foundation for future observational modeling.

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arXiv2025

Leaky dust trap in the PDS 70 disk revealed by ALMA Band 9 observations

We present new observations of the PDS 70 disc obtained with the Atacama Large Millimeter/sub-millimeter Array (ALMA) in Band 9 (671 GHz) at 0.242$^{\prime\prime}$ resolution, which provide valuable insights into the spatial distribution of sub-millimetre grains in the disc. The data reveal a ring-like morphology, with a radial peak located between those previously observed at infrared wavelengths and longer millimetre observations. Additionally, we detect a tentative outer shoulder in Band 9 that is not observed at longer wavelengths. These findings suggest that small grains ($\sim 100 μ$m) traced by Band 9 may be escaping from the pressure bump both radially inwards and outwards, or may be tracing different disc layers than those probed at longer wavelengths. A multi-wavelength analysis of the disc at millimetre wavelengths and the best fit to the spectral energy distribution shows the presence of centimetre grains around the ring location, where the dust surface density also peaks, compatible with dust trap models. The grain size in the disc cavity is not well constrained but is consistent with grains as small as 10 $μ$m, supporting the hypothesis that small dust grain filters through the cavity. We use dust evolution models to demonstrate that a turbulent viscosity of $α\gtrsim 10^{-3}$ allows small grains to filter through the disc gap, while $α\lesssim 5 \times 10^{-3}$ is required to retain large grains in the pressure bump. The Band 9 observations of PDS 70 validate theoretical models and confirm the presence of pebble flux through the disc gap.

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arXiv2025

Inner disc and circumplanetary material in the PDS 70 system

The two giant protoplanets directly imaged in the dust-depleted cavity of PDS 70 offer a unique opportunity to study ongoing planet formation. Both planets have been detected in infrared thermal emission and in H$α$, indicating active accretion. We calibrate and analyse archival ALMA Band 6 and 7 observations of PDS 70 from 2019, 2021, and 2023 to search for circumplanetary material and assess its motion. Using 2D visibility modelling of the high-resolution (~0.11"x0.08" in Band 6; ~0.05"x0.05" in Band 7) dust continuum from the outer disc, we subtract the model and image the cavity at multiple epochs. We re-detect compact dust emission around PDS 70 c in all datasets with >$3.8σ$ significance, and tentatively detect emission near PDS 70 b at ~$3σ$ in Band 6, with peak fluxes of $59\pm17μ$Jy/beam and $46\pm14μ$Jy/beam. The relative astrometry of the compact emission around PDS 70 c is consistent with the expected position of the planet between 2019-2023. We find a peak flux difference up to $64\pm34μ$Jy/beam at 1$σ$, but Bayesian analysis indicates no significant variability. We detect no flux variability in the inner disc. The inferred dust mass near PDS 70 c and in the inner disc ranges from $0.008$-$0.063 M_\oplus$ and $0.04$-$0.31 M_\oplus$, respectively, consistent with prior estimates. Finally, we measure Band 6-7 spectral indices of $2.5\pm1.2$ (PDS 70 c) and $3.2\pm0.5$ (inner disc), suggesting that the inner disc emission is dominated by optically thin dust.

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arXiv2024

Dust Scattering Albedo at Millimeter-Wavelengths in the TW Hya Disk

Planetary bodies are formed by coagulation of solid dust grains in protoplanetary disks. Therefore, it is crucial to constrain the physical and chemical properties of the dust grains. In this study, we measure the dust albedo at mm-wavelength, which depends on dust properties at the disk midplane. Since the albedo and dust temperature are generally degenerate in observed thermal dust emission, it is challenging to determine them simultaneously. We propose to break this degeneracy by using multiple optically-thin molecular lines as a dust-albedo independent thermometer. In practice, we employ pressure-broadened CO line wings that provide an exceptionally high signal-to-noise ratio as an optically thin line. We model the CO $J=2-1$ and $3-2$ spectra observed by the Atacama Large Millimeter/sub-millimeter Array (ALMA) at the inner region ($r<6\ {\rm au}$) of the TW Hya disk and successfully derived the midplane temperature. Combining multi-band continuum observations, we constrain the albedo spectrum at $0.9-3$ mm for the first time without assuming a dust opacity model. The albedo at these wavelengths is high, $\sim0.5-0.8$, and broadly consistent with the Ricci et al. (2010), DIANA, and DSHARP dust models. Even without assuming dust composition, we estimate the maximum grain size to be $\sim 340\ μm$, the power law index of the grain size distribution to be $>-4.1$, and porosity to be $<0.96$. The derived dust size may suggest efficient fragmentation with the threshold velocity of $\sim 0.08\ {\rm m\ s^{-1}}$. We also note that the absolute flux uncertainty of $\sim10\%$ ($1σ$) is measured and used in the analysis, which is approximately twice the usually assumed value.

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arXiv2024

Asymmetric dust accumulation of the PDS 70 disk revealed by ALMA Band 3 observations

The PDS 70 system, hosting two planets within its disk, is an ideal target for examining the effect of planets on dust accumulation, growth, and ongoing planet formation. Here, we present high-resolution ($0.''07 = 8 \ \mathrm{au}$) dust continuum observations of the PDS 70 disk in ALMA Band 3 (3.0 mm). While previous Band 7 observations showed a dust ring with slight asymmetry, our Band 3 observations reveal a more prominent asymmetric peak in the northwest direction, where the intensity is 2.5 times higher than in other directions and the spectral index is at the local minimum with $α_{\mathrm{SED}} \sim 2.2$. This indicates that a substantial amount of dust is accumulated both radially and azimuthally in the peak. We also detect point-source emission around the stellar position in the Band 3 image, which is likely to be free-free emission. We constrain the eccentricity of the outer ring to be $e<0.04$ from the position of the central star and the outer ring. From the comparison with numerical simulations, we constrain the mass of PDS 70c to be less than 4.9 Jupiter masses if the gas turbulence strength $α_{\mathrm{turb}} = 10^{-3}$. Then, we discuss the formation mechanism of the disk structures and further planet formation scenarios in the disk.

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arXiv2024

First JVLA Radio Observation on PDS70

PDS~70 is a protoplanetary system that hosts two actively accreting gas giants, namely PDS~70b and PDS~70c. The system has a $\sim$60--100 au dusty ring that has been resolved by ALMA, along with circumplanetary disks around the two gas giants. Here we report the first JVLA Q (40--48 GHz), Ka (29--37 GHz), K (18--26 GHz), and X (8--12 GHz) bands continuum observations, and the complementary ALMA Bands 3 ($\sim$98 GHz) and 4 ($\sim$145 GHz) observations towards PDS~70. The dusty ring appears azimuthally asymmetric in our ALMA images. We obtained firm detections at Ka and K bands without spatially resolving the source; we obtained a marginal detection at Q band, and no detection at X band. The spectral indices ($α$) are 5$\pm$1 at 33--44 GHz and 0.6$\pm$0.2 at 22--33 GHz. At 10--22 GHz, the conservative lower limit of $α$ is 1.7. The 33--44 GHz flux density is likely dominated by the optically thin thermal emission of grown dust with $\gtrsim$1 mm maximum grain sizes, which may be associated with the azimuthally asymmetric substructure induced by planet-disk interaction. Since PDS~70 was not detected at X band, we found it hard to explain the low spectral index at 22--33 GHz only with free-free emission. Hence, we attribute the dominant emission at 22--33 GHz to the emission of spinning nanometer-sized dust particles, while free-free emission may partly contribute to emission at this frequency range. In some protoplanetary disks, the emission of spinning nanometer-sized dust particles may resemble the 20--50 GHz excess in the spectra of millimeter-sized dust. The finding of strong continuum emission of spinning nanometer-sized particles can complicate the procedure of constraining the properties of grown dust. Future high-resolution, multi-frequency JVLA/ngVLA and SKA observations may shed light on this issue.

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arXiv2024

Forming localized dust concentrations in a dust ring: DM Tau case study

The previous, high angular resolution 225 GHz ($\sim$1.3 mm) continuum observations on the transitional disk DM Tau have resolved an outer ring at 20-120 au radii that is weakly azimuthally asymmetric. We aimed to examine dust growth and filtration in the outer ring. We performed the $\sim$0$''$.06 ($\sim$8.7 au) resolution Karl G. Jansky Very Large Array (JVLA) 40-48 GHz ($\sim$7 mm; Q band) continuum observations and the complementary observations at lower frequencies. In addition, we analyzed the archival JVLA observations that were taken since 2010. Intriguingly, the Q band image resolved the azimuthally highly asymmetric, knotty dust emission sources close to the inner edge of the outer ring. Fitting the 8-700 GHz spectral energy distribution (SED) with two dust components indicates that the maximum grain size in these knotty dust emission sources is likely $\gtrsim$300 $μ$m while it is $\lesssim$50 $μ$m in the rest of the ring. These results may be explained by trapping of inward migrating grown dust close to the ring inner edge. The exact mechanism for developing the azimuthal asymmetry has not yet been identified, which may be due to planet-disk interaction that might also be responsible for the creation of the dust cavity and pressure bump, or the fluid instabilities and vortex formation due to shear motions. Finally, we remark that the asymmetries in DM Tau are hard to diagnose from the $\gtrsim$225 GHz observations owing to a high optical depth at the ring. In other words, the apparent symmetric or asymmetric morphology of the transitional disks may be related to the optical depths of those disks at the observing frequency.

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arXiv2023

Constraints on the dust size distributions in the HD 163296 disk from the difference of the apparent dust ring widths between two ALMA Bands

The dust size in protoplanetary disks is a crucial parameter for understanding planet formation, while the observational constraints on dust size distribution have large uncertainties. In this study, we present a new method to constrain the dust size distribution from the dust spatial distribution, utilizing the fact that larger dust grains are more spatially localized. We analyze the ALMA Band 6 (1.25 mm) and Band 4 (2.14 mm) high-resolution images and constrain the dust size distribution in the two rings of the HD 163296 disk. We find that the outer ring at 100 au appears narrower at the longer wavelengths, while the inner ring at 67 au appears to have similar widths across the two wavelengths. We model dust rings trapped at gas pressure maxima, where the dust grains follow a power-law size distribution, and the dust grains of a specific size follow a Gaussian spatial distribution with the width depending on the grain size. By comparing the observations with the models, we constrain the maximum dust size $a_{\mathrm{max}}$ and the exponent of the dust size distribution $p$. We constrain that $0.9 \ \mathrm{mm} < a_{\mathrm{max}} < 5 \ \mathrm{mm}$ and $p < 3.3$ in the inner ring, and $a_{\mathrm{max}} > 3 \times 10^1 \ \mathrm{mm}$ and $3.4 < p < 3.7$ in the outer ring. The larger maximum dust size in the outer ring implies a spatial dependency in dust growth, potentially influencing the formation location of the planetesimals. We further discuss the turbulence strength $α$ derived from the constrained dust spatial distribution, assuming equilibrium between turbulent diffusion and accumulation of dust grains.

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