arXiv subjects

Ayumu Kuwahara

Publications and source records attributed to Ayumu Kuwahara.

11 recordsLinked to original sources

arXiv2026

Cooling-regulated gas accretion onto gap-opening planets

Gas accretion onto forming planets controls the final masses of giant planets and provides observable signatures of ongoing formation. How this process depends on the cooling properties of these newly attracted gas remains poorly constrained. We present long-term, three-dimensional global hydrodynamical simulations to quantify gas accretion onto gap-opening planets in the mass range between 1 and 3 Jupiter masses. We systematically vary the cooling time, $β$, from near-isothermal ($β=10^{-2}$ in units of orbital time) to near-adiabatic ($β=10^{2}$), and follow the evolution until a quasi-steady state is reached. Our simulations show that the gas accretion rate decreases monotonically with increasing $β$, as $\dot{M}_{\rm acc}\proptoβ^{-0.18}$, reaching values at $β=10^2$ that are approximately an order of magnitude lower than locally isothermal predictions, largely independent of planet mass. The reduction in accretion is traced to thermodynamic restructuring of the circumplanetary region: inefficient cooling weakens shocks, narrows the accretion bands feeding the circumplanetary disk. Our results imply that thermodynamic effects should be taken into account when interpreting observed accretion rates of young planets, and may introduce systematic uncertainties in commonly used locally isothermal assumptions.

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arXiv2026

Dust transport in envelopes of disk-embedded planets: I. Convectively stable envelopes

Planets embedded in protoplanetary disks accrete solids through their gaseous envelopes. The spatial distribution of these dust particles inside the envelopes of disk-embedded planets is poorly known. We present high-resolution two- and three-dimensional multifluid simulations that follow the dynamics of gas and dust around planets similar in mass to Earth. Our simulations resolve an outer recycling flow and an inner convectively stable envelope that is shielded from the recycling flow. We identify strongly dust-depleted envelopes: the dust-to-gas ratio decreases radially inward and is reduced by more than two to four orders of magnitude in the deep interior ($<0.1\,R_{\rm B}$; Bondi radius) compared to its value at the outer edge of the envelope. Small grains, with a dimensionless stopping time ${\rm St}\lesssim10^{-3}$, remain entrained in the recycling flow and do not enter the envelope, whereas large grains (${\rm St}\gtrsim10^{-2}$) penetrate the envelope but settle rapidly onto the core along the midplane. The resulting dust depletion in convectively-stable envelopes implies a substantial reduction in dust opacity throughout much of the envelope, facilitating cooling and a more rapid transition to runaway gas accretion. These results further suggest that enriching the deep envelope ($<0.1\,R_{\rm B}$) with dust or volatiles requires their delivery through large pebbles that then subsequently disintegrate, or sublimate, from their host grains in the deep envelope interior.

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arXiv2026

Dust transport in envelopes of disk-embedded planets: II. Fully convective envelopes

We perform 3D multifluid simulations of gas and dust to quantify how convection reshapes the spatial distribution of dust grains inside the envelopes of disk-embedded planets. Building on the first paper of this series, which explored convectively stable envelopes, we here consider envelopes in which convection is sustained by the accretion luminosity. The dust-to-gas ratio inside the planetary envelope is set primarily by the degree of dynamical isolation from the surrounding disk. When convection extends beyond the Bondi radius, the envelope remains connected to the disk flow and maintains a dust-rich state through continuous material exchange. Conversely, when an inner convective layer is separated from an outer recycling layer, the recycling flow filters incoming solids. The dust distribution inside the convective layer is then controlled by the competition between convective stirring and gravitational settling. This yields two regimes: a settling-dominated regime, in which large grains (typically $s\gtrsim0.1$ cm) fall at the terminal velocity and the envelope becomes dust depleted, and a convection-dominated regime, in which smaller grains are trapped in convective circulation and the envelope retains its dust. The delivery and release of volatile species from accreting solids at the envelope sublimation front, together with the efficiency of convective mixing, determine how this material is distributed between the core and envelope. Our findings imply that super-Earths and mini-Neptunes inside the water snowline have volatile-depleted cores and volatile-rich envelopes, while planets at wider orbits can have a wide diversity of envelope compositions from depleted to enriched. This compositional diversity for planetary envelopes appears to be consistent with the large diversity in observed atmospheric compositions of mini-Neptunes.

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arXiv2026

Interior dynamics of envelopes around disk-embedded planets

In the core accretion scenario, forming planets start to acquire gaseous envelopes while accreting solids. Conventional one-dimensional models assume envelopes to be static and isolated. However, recent three-dimensional simulations demonstrate dynamic gas exchange from the envelope to the surrounding disk. This process is controlled by the balance between heating, through the accretion of solids, and cooling, which is regulated by poorly-known opacities. In this work, we systemically investigate a wide range of cooling and heating rates, using three-dimensional hydrodynamical simulations. We identify three distinct cooling regimes. Fast-cooling envelopes ($β\lesssim 1$, with $β$ the cooling time in units of orbital time) are nearly isothermal and have inner radiative layers that are shielded from recycling flows. In contrast, slow cooling envelopes ($β\gtrsim10^3$) become fully convective. In the intermediate regime ($1\lesssimβ\lesssim300$), envelopes are characterized by a three-layer structure, comprising an inner convective, a middle radiative, and an outer recycling layer. The development of this radiative layer traps small dust and vapour released from sublimated species. In contrast, fully convective envelopes efficiently exchange material from inner to outer envelope. Such fully convective envelopes are likely to emerge in the inner parts of protoplanetary disks ($\lesssim$ 1 au) where cooling times are long, implying that inner-disk super-Earths may see their growth stalled and be volatile depleted.

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arXiv2024

Dust ring and gap formation by gas flow induced by low-mass planets embedded in protoplanetary disks $\rm II$. Time-dependent model

The observed dust rings and gaps in protoplanetary disks could be imprints of forming planets. Even low-mass planets in the one-to-ten Earth-mass regime, that do not yet carve deep gas gaps, can generate such dust rings and gaps by driving a radially-outwards gas flow, as shown in previous work. However, understanding the creation and evolution of these dust structures is challenging due to dust drift and diffusion, requiring an approach beyond previous steady state models. Here we investigate the time evolution of the dust surface density influenced by the planet-induced gas flow, based on post-processing three-dimensional hydrodynamical simulations. We find that planets larger than a dimensionless thermal mass of $m=0.05$, corresponding to 0.3 Earth mass at 1 au or 1.7 Earth masses at 10 au, generate dust rings and gaps, provided that solids have small Stokes numbers} (${\rm St}\lesssim10^{-2}$) and that the disk midplane is weakly turbulent ($α_{\rm diff}\lesssim10^{-4}$). As dust particles pile up outside the orbit of the planet, the interior gap expands with time, when the advective flux dominates over diffusion. Dust gap depths range from a factor a few, to several orders of magnitude, depending on planet mass and the level of midplane particle diffusion. We construct a semi-analytic model describing the width of the dust ring and gap, and then compare it with the observational data. We find that up to 65\% of the observed wide-orbit gaps could be explained as resulting from the presence of a low-mass planet, assuming $α_{\rm diff}=10^{-5}$ and ${\rm St}=10^{-3}$. However, it is more challenging to explain the observed wide rings, which in our model would require the presence of a population of small particles (${\rm St=10^{-4}}$). Further work is needed to explore the role of pebble fragmentation, planet migration, and the effect of multiple planets.

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arXiv2023

Analytic description of the gas flow around planets embedded in protoplanetary disks

A growing planet embedded in a protoplanetary disk induces three-dimensional gas flow, which exhibits a midplane outflow that can suppress dust accretion onto the planet and form global dust substructures (rings and gaps). Because analytic formulae for the planet-induced outflow are useful for modeling its influences on local and global dust surface densities and planet accretion, we derive the analytic formulae that describe the morphology and velocity of the planet-induced outflow. We first perform three-dimensional, nonisothermal hydrodynamical simulations of the gas flow past a planet, which enables us to introduce a fitting formula describing the morphology of the outflow. We then derive an analytic formula for the outflow speed using Bernoulli's theorem. We successfully derived a fitting formula for the midplane outflow morphology (the shape of the streamline), which is valid when the dimensionless thermal mass falls below $m\lesssim0.6$. The obtained analytic formulae for the outflow, such as the maximum outflow speed and the velocity distributions of the outflow in the radial and vertical directions to the disk, show good agreement with the numerical results. We find the following trends: (1) the maximum outflow speed increases with the planetary mass and has a peak of $\sim$30--40$\%$ of the sound speed when the dimensionless thermal mass is $m\sim0.3$, corresponding to a super-Earth mass planet at 1 au for the typical steady accretion disk model, and (2) the presence of the headwind (namely, the global pressure force acting in the positive radial direction of the disk) enhances (reduces) the outflow toward the outside (inside) of the planetary orbit. The planet-induced outflow of the gas affects the dust motion when the dimensionless stopping time of dust falls below ${\rm St}\lesssim\min(10m^2,0.1)$.

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arXiv2023

Spin of protoplanets generated by pebble accretion: Influences of protoplanet-induced gas flow

We investigate the spin state of a protoplanet during the pebble accretion influenced by the gas flow in the gravitational potential of the protoplanet and how it depends on the planetary mass, the headwind speed, the distance from the host star, and the pebble size. We perform nonisothermal three-dimensional hydrodynamical simulations in a local frame to obtain the gas flow around the planet. We then numerically integrate three-dimensional orbits of pebbles under the obtained gas flow. Finally, assuming uniform spatial distribution of incoming pebbles, we calculate net spin by summing up specific angular momentum that individual pebbles transfer to the protoplanet at impacts. We find that a protoplanet with the envelope acquires prograde net spin rotation regardless of the planetary mass, the pebble size, and the headwind speed of the gas. This is because accreting pebbles are dragged by the envelope that commonly has prograde rotation. As the planetary mass or orbital radius increases, the envelope is thicker and the prograde rotation is faster, resulting in faster net prograde spin. When the dimensionless thermal mass of the planet, $m = R_{\mathrm{Bondi}} / H$, where $R_{\mathrm{Bondi}}$ and $H$ are the Bondi radius and the disk gas scale height, is larger than a certain critical mass ($m \gtrsim 0.3$ at $0.1 \, \mathrm{au}$ or $m \gtrsim 0.1$ at $1 \, \mathrm{au}$), the spin rotation exceeds the breakup one. The predicted spin frequency reaches the breakup one at the planetary mass $m_{\mathrm{iso,rot}} \sim 0.1 \, (a / 1 \, \mathrm{au})^{-1/2}$ (where $a$ is the orbital radius), suggesting that the protoplanet cannot grow beyond $m_{\mathrm{iso,rot}}$. It is consistent with the Earth's current mass and could help the formation of the Moon by a giant impact on fast-spinning proto-Earth.

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arXiv2022

Dust ring and gap formation by gas flow induced by low-mass planets embedded in protoplanetary disks $\rm I$. Steady-state model

Recent high-spatial-resolution observations have revealed dust substructures in protoplanetary disks such as rings and gaps, which do not always correlate with gas. Because radial gas flow induced by low-mass, non-gas-gap-opening planets could affect the radial drift of dust, it potentially forms these dust substructures in disks. We investigate the potential of gas flow induced by low-mass planets to sculpt the rings and gaps in the dust profiles. We first perform three-dimensional hydrodynamical simulations, which resolve the local gas flow past a planet. We then calculate the trajectories of dust influenced by the planet-induced gas flow. Finally, we compute the steady-state dust surface density by incorporating the influences of the planet-induced gas flow into a one-dimensional dust advection-diffusion model. The outflow of the gas toward the outside of the planetary orbit inhibits the radial drift of dust, leading to dust accumulation (the dust ring). The outflow toward the inside of the planetary orbit enhances the inward drift of dust, causing dust depletion around the planetary orbit (the dust gap). Under weak turbulence ($α_{\rm diff}\lesssim10^{-4}$, where $α_{\rm diff}$ is the turbulence strength parameter), the gas flow induced by the planet with $\gtrsim1\,M_{\oplus}$ (Earth mass) generates the dust ring and gap in the distribution of small dust grains ($\lesssim1$ cm) with the radial extent of $\sim1\text{--}10$ times gas scale height around the planetary orbit without creating a gas gap and pressure bump. The gas flow induced by low-mass, non-gas-gap-opening planets can be considered a possible origin of the observed dust substructures in disks. Our results may be helpful to explain the disks whose dust substructures were found not to correlate with those of the gas.

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arXiv2020

Influences of protoplanet-induced three-dimensional gas flow on pebble accretion $\rm\,I\hspace{-.1em}I\,$. Headwind regime

Pebble accretion is one of the major theories in planet formation. Aerodynamically small particles, called pebbles, are highly affected by the gas flow. A growing planet embedded in a protoplanetary disk induces three-dimensional (3D) gas flow. In our previous study, Paper I, we focused on the shear regime of pebble accretion, and investigated the influence of planet-induced gas flow on pebble accretion. In Paper I, we found that pebble accretion is inefficient in the planet-induced gas flow compared to that in the unperturbed flow, in particular when ${\rm St}\lesssim10^{-3}$, where St is the Stokes number. Following Paper I, we investigate the influence of planet-induced gas flow on pebble accretion. We consider the headwind of the gas, which is not included in Paper I. We extend our study to the headwind regime of pebble accretion in this study. Assuming a nonisothermal, inviscid sub-Keplerian gas disk, we perform 3D hydrodynamical simulations on the spherical polar grid which has a planet with the dimensionless mass, $m=R_{\rm Bondi}/H$, located at its center, where $R_{\rm Bondi}$ and $H$ are the Bondi radius and the disk scale height. We then numerically integrate the equation of motion of pebbles in 3D using hydrodynamical simulation data. Combining our results with the spacial variety of turbulence strength and pebble size in a disk, we conclude that the planet-induced gas flow still allows for pebble accretion in the early stage of planet formation. Suppression of pebble accretion due to the planet-induced gas flow occurs only in the late stage of planet formation, in particular in the inner region of the disk. This may be helpful to explain the distribution of exoplanets and the architecture of the Solar System, both of which have small inner and large outer planets.

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arXiv2019

Influences of three-dimensional gas flow induced by protoplanets on pebble accretion --$\rm\,I\,$. shear regime

The pebble accretion model has the potential to explain the formation of various types of planets. A growing planet embedded in a disk induces three-dimensional (3D) gas flow, which may influence pebble accretion. In this study, we investigate the influence of the 3D planet-induced gas flow on pebble accretion. Assuming a non-isothermal, inviscid gas disk, we perform 3D hydrodynamical simulations on the spherical polar grid. Then we numerically integrate the equation of motion of pebbles in 3D using hydrodynamical simulations data. We find that the trajectories of pebbles in the planet-induced gas flow differ significantly from those in the unperturbed shear flow for a wide range of pebble sizes investigated (${\rm St}=10^{-3}$--$10^{0}$, where ${\rm St}$ is the Stokes number). The horseshoe flow and outflow of the gas alter the motion of the pebbles, which leads to the reduction of the width of the accretion window, $w_{\rm acc}$, and the accretion cross section, $A_{\rm acc}$. On the other hand, the changes in trajectories also cause an increase in relative velocity of pebbles to the planet, which offsets the reduction of $w_{\rm acc}$ and $A_{\rm acc}$. As a consequence, in the Stokes regime, the accretion probability of pebbles, $P_{\rm acc}$, in the planet-induced gas flow is comparable to that in the unperturbed shear flow except when the Stokes number is small, ${\rm St}\sim10^{-3}$, in 2D accretion, or when the thermal mass of the planet is small, $m=0.03$ in 3D accretion. In contrast, in the Epstein regime, $P_{\rm acc}$ in the planet-induced gas flow becomes smaller than that in the shear flow in the Stokes regime in both 2D and 3D accretion, regardless of assumed ${\rm St}$ and $m$. Our results suggest that the 3D planet-induced gas flow may be helpful to explain the distribution of exoplanets as well as the architecture of the solar system.

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arXiv2019

Gas flow around a planet embedded in a protoplanetary disc: the dependence on the planetary mass

The three-dimensional structure of the gas flow around a planet is thought to influence the accretion of both gas and solid materials. In particular, the outflow in the mid-plane region may prevent the accretion of the solid materials and delay the formation of super-Earths' cores. However, it is not yet understood how the nature of the flow field and outflow speed change as a function of the planetary mass. In this study, we investigate the dependence of gas flow around a planet embedded in a protoplanetary disc on the planetary mass. Assuming an isothermal, inviscid gas disc, we perform three-dimensional hydrodynamical simulations on the spherical polar grid, which has a planet located at its centre. We find that gas enters the Bondi or Hill sphere at high latitudes and exits through the mid-plane region of the disc regardless of the assumed dimensionless planetary mass $m=R_{\rm Bondi}/H$, where $R_{\rm Bondi}$ and $H$ are the Bondi radius of the planet and disc scale height, respectively. The altitude from where gas predominantly enters the envelope varies with the planetary mass. The outflow speed can be expressed as $|u_{\rm out}|=\sqrt{3/2}mc_{\rm s}$ $(R_{\rm Bondi}\leq R_{\rm Hill})$ or $|u_{\rm out}|=\sqrt{3/2}(m/3)^{1/3} c_{\rm s}$ ($R_{\rm Bondi}\geq R_{\rm Hill}$), where $c_{\rm s}$ is the isothermal sound speed and $R_{\rm Hill}$ is the Hill radius. The outflow around a planet may reduce the accretion of dust and pebbles onto the planet when $m\gtrsim\sqrt{\rm St}$, where St is the Stokes number. Our results suggest that the flow around proto-cores of super-Earths may delay their growth and, consequently, help them to avoid runaway gas accretion within the lifetime of the gas disc.

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