arXiv subjects

Hiroyuki Kurokawa

Publications and source records attributed to Hiroyuki Kurokawa.

At least 19 recordsLinked to original sources

arXiv2026

GREX-PLUS Science Book v2

GREX-PLUS (Galaxy Reionization EXplorer and PLanetary Universe Spectrometer) is a mission candidate for a JAXA strategic L-class mission to be launched in the 2030s. Its primary science goals are two-fold: galaxy formation and evolution, and planetary system formation and evolution. The GREX-PLUS spacecraft will carry a telescope with a 1 m primary mirror aperture cooled down to 50 K. The two science instruments will be onboard: a wide-field camera in the 2--8 $μ$m wavelength band and a high-resolution spectrometer with a wavelength resolution of 30,000 in the 10--18 $μ$m band. The GREX-PLUS wide-field camera aims to detect the first generation of galaxies at redshift $z>15$. The GREX-PLUS high-resolution spectrometer aims to identify the location of the water ``snowline'' in protoplanetary disks. Both instruments will provide unique datasets for a broad range of scientific topics, including galaxy mass assembly, the origin of supermassive blackholes, infrared background radiation, molecular spectroscopy in the interstellar medium, transit spectroscopy of exoplanet atmospheres, planetary atmospheres in the Solar System, and so on. This document is the second version of a collection of scientific themes that can be achieved with GREX-PLUS. Each section in Chapters~2 and 3 is based on presentations at several GREX-PLUS Science Workshops.

astro-ph.IM↗

arXiv2026

The mass of TOI-1883 b: A low density super-Neptune in the ridge regime transiting an early-M dwarf

Recent large-scale transit surveys conducted by space telescopes such as Kepler and TESS have revealed a vast number of exoplanets, uncovering the diversity of their population. One of the remarkable findings is the presence of a deficiency region in the period-radius distribution of short-period (< 10 days) Neptune-sized planets (4-8 Earth radii). This region is classified into the Neptune desert (< 3.2 days), the ridge (3.2-5.7 days), and the savanna (> 5.7 days) based on orbital period, each likely reflecting distinct evolutionary pathways. In this study, we used the InfraRed Doppler (IRD) instrument on the Subaru Telescope to determine the mass of the super-Neptune TOI-1883 b, which resides in the ridge region (P ~ 4.51 days) orbiting an M dwarf. We measured a planetary mass of Mp = 13.7 +6.8/-6.5 Earth masses and a mean density of 0.4 +0.3/-0.2 g cm^-3, with 3-sigma upper limits of 34.1 Earth masses, and 5-sigma upper limits of 47.7 Earth masses. These results suggest that TOI-1883 b is likely a low density super-Neptune. We also find that the boundary of the Neptune desert defined by planets orbiting FGK-type stars exhibits a similar distribution for planets around M-type stars. According to the population-based argument of Bourrier et al. (2025), this suggests that TOI-1883 b may have undergone disk-driven migration to reach its current orbit and experienced early atmospheric photoevaporation driven by strong stellar XUV irradiation. The derived planetary mass is comparable to or exceeds the conventional critical core mass. We suggest that the high metallicity of the host star ([Fe/H] = 0.32 +/- 0.18) may have suppressed the onset of runaway gas accretion. Furthermore, TOI-1883 b has a high Transmission Spectroscopy Metric (TSM > 140), making it an excellent target for future atmospheric characterization via transmission spectroscopy.

astro-ph.EP↗

arXiv2026

Revisiting the greenhouse effect of non-greenhouse gases in the atmospheres of Earth-like planets

Although non-greenhouse gases can vary substantially in abundance in Earth-like atmospheres, their climatic influences remain insufficiently understood. To investigate how such gases regulate climate, we vary the abundance of N$_2$ as a representative non-greenhouse component in one-dimensional N$_2$--CO$_2$--H$_2$O model atmospheres. Beyond pressure broadening of absorption lines and Rayleigh scattering emphasized in previous studies, our results show that changes in background N$_2$ pressure influence climate by modifying the amount of atmospheric H$_2$O, producing two effects: altering the thermodynamic lapse rate (H$_2$O-dilute warming) and changing the radiative contribution of H$_2$O to the greenhouse effect (H$_2$O-load warming). The resulting climate response to increasing N$_2$ depends on the CO$_2$ abundance. Under low CO$_2$ conditions, dilution of atmospheric H$_2$O leads to warming, whereas under high CO$_2$ conditions, increased H$_2$O loading also produces warming. At sufficiently high N$_2$ abundances, Rayleigh scattering induces cooling, an effect further amplified by the accompanying decrease in atmospheric H$_2$O. Under high CO$_2$ conditions, however, enhanced H$_2$O loading increases the absorption of stellar radiation and overwhelms the contribution of Rayleigh scattering, causing the cooling response to disappear. These results reveal multiple physical pathways through which non-greenhouse gases influence climate and provide a framework for understanding climate responses and habitability in diverse Earth-like atmospheres.

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arXiv2026

Testing the Icy Pebble Accretion Hypothesis with Primordial Main Belt Asteroids

Large main-belt asteroids (diameter $D \gtrsim 120\ \mathrm{km}$) exhibit a surface composition gradient as a function of heliocentric distance, ranging from anhydrous bodies to those rich in hydrated and, possibly, ammoniated materials. Their primordial nature holds key clues to the evolution of the Solar System. It has been suggested that the volatile-rich bodies formed in the outer Solar System and were implanted into the main belt. Alternatively, volatiles may have been delivered via inward-drifting icy pebbles in the protosolar disk. Here, we examine whether in-situ formed rocky embryos can acquire volatiles through pebble accretion as the snowline migrated inward. With the turbulence strength of the disk, radial pebble flux, and the dimensionless stopping time of pebbles (St) as parameters, we calculate the growth of large asteroids. The results are then compared with mass and compositional constraints based on asteroid observations. We find that a moderate pebble flux ($\lesssim18~M_\oplus / \text{Myr}$) is required to enable volatile delivery while preventing the largest asteroids from becoming more massive than Ceres. Water accretion is feasible with $\mathrm{St} \sim 10^{-3}$ ($\sim 1$ mm). However, only the largest asteroids (D > 200 km) can accumulate sufficient ammonia under such conditions. For most asteroids with D between 100 and 200 km, ammonia ice accretion requires $\mathrm{St} \sim 10^{-4}$ ($\sim 100\,μ$m). Such small particle sizes may pose both theoretical and observational challenges. Thus, we propose that the intermediate-sized, potentially ammonia-bearing asteroids serve as a record of the Solar System's dynamic evolution.

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arXiv2025

Cosmic-Ray Bath in a Past Supernova Gives Birth to Earth-Like Planets

A key question in astronomy is how ubiquitous Earth-like rocky planets are. The formation of terrestrial planets in our solar system was strongly influenced by the radioactive decay heat of short-lived radionuclides (SLRs), particularly $^{26}$Al, likely delivered from nearby supernovae. However, current models struggle to reproduce the abundance of SLRs inferred from meteorite analysis without destroying the protosolar disk. We propose the `immersion' mechanism, where cosmic-ray nucleosynthesis in a supernova shockwave reproduces estimated SLR abundances at a supernova distance ($\sim$1 pc), preserving the disk. We estimate that solar-mass stars in star clusters typically experience at least one such supernova within 1 pc, supporting the feasibility of this scenario. This suggests solar-system-like SLR abundances and terrestrial planet formation are more common than previously thought.

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arXiv2025

Helium Depletion in Escaping Atmospheres of Sub-Neptunes: A Signature of Primary-to-Secondary Transition

Short-period sub-Neptunes are common in extrasolar systems. These sub-Neptunes are generally thought to have primary atmospheres of protoplanetary-disk gas origin. However, atmospheric escape followed by degassing from their interiors can lead to the transition to secondary atmospheres depleted in gases less-soluble to magma, such as helium. These primary and secondary atmospheres can potentially be distinguished from observations of escaping hydrogen and helium. This study aims to elucidate the impact of the primary-secondary transition on atmospheric compositions of short-period sub-Neptunes. We simulate their evolution with atmospheric escape driven by stellar X-ray and extreme ultraviolet irradiation and degassing of hydrogen, helium, and water from their rocky interiors, with a one-dimensional structure model. We show that the transition takes place for low-mass, close-in planets which experience extensive atmospheric escape. These planets show the depletion of helium and enrichment of water in their atmospheres, because of their low and high abundances in the planetary interiors, respectively. A compilation of our parameter survey (the orbital period, planetary mass, envelope mass, and mantle FeO content) shows a correlation between the planet radius and the helium escape rate. We suggest that the transition from primary to secondary atmospheres may serve an explanation for helium non-detection for relatively-small ($\lesssim 2.5\ R_\oplus$) exoplanets.

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arXiv2025

Impacts of Atmospheric Carbon Species and Stellar Type on Climates of Terrestrial Planets

The climates of terrestrial planets are largely determined by the composition of their atmospheres and spectral types of their host stars. Previous studies suggest a wide range of carbon species abundances (CO\textsubscript{2}, CO, and CH\textsubscript{4}) can result from variations in reducing fluxes and stellar spectral types which influence photochemistry. However, a systematic investigation of how varying carbon species, particularly CO, affect planetary climates across wide parameter spaces remains limited. Here, we employ a one-dimensional radiative-convective equilibrium model to examine the dependence of planetary climate on the abundances of carbon species and host star type. We find that CO, due to weak absorption of stellar radiation, induces only moderate changes in stratospheric temperature, while its effect on surface temperature is negligible. Under Earth-like $p_\mathrm{N_2}$ (where $p_\mathrm{i}$ is the partial pressure on the surface of species i), for cases with fixed $p_\mathrm{CO_2}$, increase in CO leads to surface cooling on planets orbiting Sun-like stars unless the sum of $p_\mathrm{CO_2}$ and $p_\mathrm{CH_4}$ exceeds $\sim$1 bar. Whereas it results in surface warming for planets around M-type stars. When the total pressure of carbon species is fixed, converting CO\textsubscript{2} or CH\textsubscript{4} into CO always induces cooling. These effects arise from a combination of CO Rayleigh scattering, pressure broadening of greenhouse gas absorption lines, and \textcolor{red}{varying water vapor levels}. We further discuss how CO- and CH\textsubscript{4}-driven cooling (warming) can trigger positive (negative) climate-photochemistry feedback, influencing atmospheric evolution. Additionally, we suggest CO-rich planets may be less susceptible to water loss and atmospheric oxidation due to lower stratospheric water vapor content.

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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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arXiv2024

Self-consistent Conditions for $^{26}$Al Injection into Protosolar Disk from a Nearby Supernova

The early solar system contained a short-lived radionuclide, $^{26}$Al (its half-life time $t_{1/2} = 0.7$ Myr). The decay energy $^{26}$Al is thought to have controlled the thermal evolution of planetesimals and, possibly, the water contents of planets. Many hypotheses have been proposed for the origin of $^{26}$Al in the solar system. One of the possible hypotheses is the `disk injection scenario'; when the protoplanetary disk of the solar system had already formed, a nearby $(<1 \,\mathrm{pc})$ supernova injected radioactive material directly into the disk. Such a $^{26}$Al injection hypothesis has been tested so far with limited setups for disk structure and supernova distance, and treated disk disruption and $^{26}$Al injection separately. Here, we revisit this problem to investigate whether there are self-consistent conditions under which the surviving disk radius can receive enough $^{26}$Al which can account for the abundance in the early solar system. We also consider a range of disk mass and structure, $^{26}$Al yields from supernova, and a large dust mass fraction $η_\mathrm{d}$. We find that $^{26}$Al yields of supernova are required as $\gtrsim 2.1\times10^{-3}M_\odot(η_\mathrm{d}/0.2)^{-1}$, challenging to achieve with known possible $^{26}$Al ejection and dust mass fraction ranges. Furthermore, we find that even if the above conditions are met, the supernova flow changes the disk temperature, which may not be consistent with the solar-system record. Our results place a strong constraint on the disk injection scenario. Rather, we suggest that the fresh $^{26}$Al of the early solar system must have been synthesized/injected in other ways.

astro-ph.EP↗

arXiv2024

Near-mid infrared spectroscopy of carbonaceous chondrites: Insights into spectral variation due to aqueous alteration and thermal metamorphism in asteroids

Carbonaceous chondrites (CCs) are windows into the early Solar system and the histories of their parent bodies. Their infrared spectral signatures are powerful proxies for deciphering their composition and evolution history, but still present formidable challenges. In our study, we delved into the infrared spectra spanning 1-25 micron of 17 CCs, with distinct petrological characteristics and varying degrees of alteration. As aqueous alteration intensifies, the 3 micron-region absorption feature associated with OH-bearing minerals and water, and the 6 micron band indicative of water molecules, both grow in intensity. Simultaneously, their band centers shift towards shorter wavelengths. Moreover, as alteration progresses, a distinctive absorption feature emerges near 2.72 micron, resembling the OH absorption feature found in serpentine and saponite minerals. Comparison of aqueous alteration to laboratory-heated CCs suggests that the 3 micron region OH/H2O absorption feature differs between CC heated to less than or more than ~300C. The 12.4 micron/11.4 micron reflectance ratio diminishes, and the reflectance peak in the 9-14 micron range shifts towards shorter wavelengths. These changes are attributed to the transformation of anhydrous silicates into phyllosilicates. In the 15-25 micron region, the influence of thermal metamorphism becomes evident and results in the appearance of more spectral features, the single reflectance peak at 22.1 micron undergoes a transformation into two distinct peaks at 19 micron and 25 micron, which is primarily attributed to the increased presence of anhydrous silicates and olivine recrystallization. These findings offer novel insights into the volatile-rich compositions of C-complex asteroids and the thermal evolution histories of their parent bodies.

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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

GREX-PLUS Science Book

GREX-PLUS (Galaxy Reionization EXplorer and PLanetary Universe Spectrometer) is a mission candidate for a JAXA's strategic L-class mission to be launched in the 2030s. Its primary sciences are two-fold: galaxy formation and evolution and planetary system formation and evolution. The GREX-PLUS spacecraft will carry a 1.2 m primary mirror aperture telescope cooled down to 50 K. The two science instruments will be onboard: a wide-field camera in the 2-8 $μ$m wavelength band and a high resolution spectrometer with a wavelength resolution of 30,000 in the 10-18 $μ$m band. The GREX-PLUS wide-field camera aims to detect the first generation of galaxies at redshift $z>15$. The GREX-PLUS high resolution spectrometer aims to identify the location of the water ``snow line'' in proto-planetary disks. Both instruments will provide unique data sets for a broad range of scientific topics including galaxy mass assembly, origin of supermassive blackholes, infrared background radiation, molecular spectroscopy in the interstellar medium, transit spectroscopy for exoplanet atmosphere, planetary atmosphere in the Solar system, and so on.

astro-ph.CO↗

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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arXiv2021

Numerous chondritic impactors and oxidized magma ocean set Earth's volatile depletion

Earth's surface environment is largely influenced by its budget of major volatile elements: carbon (C), nitrogen (N), and hydrogen (H). Although the volatiles on Earth are thought to have been delivered by chondritic materials, the elemental composition of the bulk silicate Earth (BSE) shows depletion in the order of N, C, and H. Previous studies have concluded that non-chondritic materials are needed for this depletion pattern. Here, we model the evolution of the volatile abundances in the atmosphere, oceans, crust, mantle, and core through the accretion history by considering elemental partitioning and impact erosion. We show that the BSE depletion pattern can be reproduced from continuous accretion of chondritic bodies by the partitioning of C into the core and H storage in the magma ocean in the main accretion stage and atmospheric erosion of N in the late accretion stage. This scenario requires a relatively oxidized magma ocean ($\log_{10} f_{\rm O_2}$ $\gtrsim$ $\rm{IW}$$-2$, where $f_{\rm O_2}$ is the oxygen fugacity, ${\rm IW}$ is $\log_{10} f_{\rm O_2}^{\rm IW}$, and $f_{\rm O_2}^{\rm IW}$ is $f_{\rm O_2}$ at the iron-wüstite buffer), the dominance of small impactors in the late accretion, and the storage of H and C in oceanic water and carbonates in the late accretion stage, all of which are naturally expected from the formation of an Earth-sized planet in the habitable zone.

astro-ph.EP↗

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.

astro-ph.EP↗

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.

astro-ph.EP↗

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.

astro-ph.EP↗