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To investigate impact vaporization for planetary atmosphere formation, we have studied the thermodynamic state generated by the shock wave due to a high-velocity impact, called the shock field. We have carried out iSALE simulations for high-velocity vertical impacts using ANEOS for an equation-of-state (EoS) model. To understand the shock fields obtained from simulations, we have investigated the contribution of the thermal and cold terms in the EoS model on the Hugoniot curves. Although the thermal and cold terms are important for the pressure, the internal energy is mainly determined by the thermal term. We thus assume a simple EoS determined by the thermal term and then analytically derive the shock internal-energy field, which reproduces the results of simulations well. Using the analytical solution of internal energy and the Hugoniot curve, we have derived the shock pressure field analytically as well. The analytical solutions for internal energy and pressure are valid even for impact velocities as low as the sound speed. The solution is good for the vertical direction or within the angles of about 60 degrees. We have applied the solution to impact vaporization for the formation of planetary atmospheres. This gives good estimation of reformation of the planetary atmospheres of Earth sized planet., Comment: Accepted for publication in A&A

More than a thousand warm debris disks have been detected as infrared excess at mid-infrared wavelengths, and their frequencies have been obtained for various spectral types of stars. However, the dependence of the frequencies on spectral type is still debated because the number of stars with significant and detectable infrared excess is limited. Herein, we present the largest systematic search for infrared excess using data from Gaia, WISE, and Spitzer. We identified 373, 485, and 255-reliable infrared excesses in the mid-infrared archival data at wavelengths of 12, 22, and 24 $\mu$m for WISE/$W3$, $W4$, and Spitzer/MIPS ch1, respectively. Although we confirmed that more massive stars tend to show higher frequencies of debris disks, these disk frequencies are relatively flat for both low- and intermediate-mass stars, with a jump at 7000 K for all three wavelengths. Assuming that bright, warm debris disks have lifetimes of a few to several hundred million years, the disk frequency can be understood as the ratio between the timescale and the upper limits of the sample ages. We also found that intermediate-mass stars with infrared excess tend to be bluer and fainter along the evolutionary track than those without, implying that massive stars hosting debris disks are relatively young, with an isochronal age of approximately 500 Myr. These tendencies are reasonably explained by a standard scenario in which debris disks are likely to be produced by collisions of planetesimals in early stages of stellar evolution, such as the Late Heavy Bombardment., Comment: Accepted for publication in AJ. 27 pages, 19 figures, 5 tables

While most debris disks consist of dust with little or no gas, a fraction has significant amounts of gas detected via emission lines of CO, ionized carbon, and/or atomic oxygen. Almost all such gaseous debris disks known are around A-type stars with ages up to 50 Myr. We show, using semi-analytic disk evolution modeling, that this can be understood if the gaseous debris disks are remnant protoplanetary disks that have become depleted of small grains compared to the interstellar medium. Photoelectric heating by the A-stars' FUV radiation is then inefficient, while the stars' EUV and X-ray emissions are weak owing to a lack of surface convective zones capable of driving magnetic activity. In this picture, stars outside the range of spectral types from A through early B are relatively hard to have such long-lived gas disks. Less-massive stars have stronger magnetic activity in the chromosphere, transition region, and corona with resulting EUV and X-ray emission, while more-massive stars have photospheres hot enough to produce strong EUV radiation. In both cases, primordial disk gas is likely to photoevaporate well before 50 Myr. These results come from 0D disk evolution models where we incorporate internal accretion stresses, MHD winds, and photoevaporation by EUV and X-ray photons with luminosities that are functions of the stellar mass and age. A key issue this work leaves open is how some disks become depleted in small dust so that FUV photoevaporation slows. Candidates include grains' growth, settling, radial drift, radiation force, and incorporation into planetary systems., Comment: Submitted to ApJL

Thanks to ``dust-to-planet'' simulations (DTPSs), which treat the collisional evolution directly from dust to giant-planet cores in a protoplanetary disk, we showed that giant-planet cores are formed in $\lesssim 10\,$au in several $10^5$ years, because porous pebbles grow into planetesimals via collisions prior to drift in 10 au (Kobayashi & Tanaka 2021, Paper I).However, such porous pebbles are unlikely to reproduce the polarized millimeter wavelength light observed from protoplanetary disks. We thus investigate gas-giant core formation with non-porous pebbles via DTPSs. Even non-porous bodies can grow into planetesimals and massive cores to be gas giants are also formed in several $10^5$ years. The rapid core formation is mainly via the accretion of planetesimals produced by collisional coagulation of pebbles drifting from the outer disk. The formation mechanism is similar to the case with porous pebbles, while core formation occurs in a wider region (5 - 10 au) than that with porous pebbles., Comment: Accepted for publication in ApJ

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

The faint CO gases in debris disks are easily dissolved into C by UV irradiation, while CO can be reformed via reactions with hydrogen. The abundance ratio of C/CO could thus be a probe of the amount of hydrogen in the debris disks. We conduct radiative transfer calculations with chemical reactions for debris disks. For a typical dust-to-gas mass ratio of debris disks, CO formation proceeds without the involvement of H$_2$ because a small amount of dust grains makes H$_2$ formation inefficient. We find that the CO to C number density ratio depends on a combination of $n_\mathrm{H}Z^{0.4}\chi^{-1.1}$, where $n_\mathrm{H}$ is the hydrogen nucleus number density, $Z$ is the metallicity, and $\chi$ is the FUV flux normalized by the Habing flux. Using an analytic formula for the CO number density, we give constraints on the amount of hydrogen and metallicity for debris disks. CO formation is accelerated by excited H$_2$ either when the dust-to-gas mass ratio is increased or the energy barrier of chemisorption of hydrogen on the dust surface is decreased. This acceleration of CO formation occurs only when the shielding effects of CO are insignificant. In shielded regions, the CO fractions are almost independent of the parameters of dust grains., Comment: 29pages, 13figures, accepted for ApJ

By performing $N$-body simulations, we investigated fundamental processes of collisions between dust aggregates composed of submicron-sized icy dust monomers. We examined the mass distribution of fragments in the collisional outcomes in a wide range of the mass ratio and the collision velocity between colliding dust aggregates. We derived analytic expressions of the mass distribution of large remnants and small fragments by numerical fitting to the simulation results. Our analytic formulae for masses of the large remnants can reproduce the contribution of mass transfer from a large target to a small projectile, which occurs for a mass ratio of $\gtrsim 3$ and is shown in a previous study (Hasegawa et al. 2021). We found that the power-law index of the cumulative mass distribution of the small fragments is independent of the mass ratio and only weakly dependent on the collision velocity. On the other hand, the mass fraction of fragments of individual dust monomers decreases with an increasing total mass of colliding aggregates for a fixed mass ratio. This tendency implies that multiple hierarchical disruptive collisions (i.e., collisions between fragments, collisions between fragments of fragments) are required for producing a large amount of individual dust monomers via collisional fragmentation. Our fragment model suggests that the total geometric cross section integrated over the fragments is estimated to be about the same order of the geometric cross section of the target., Comment: 30 pages, 23 figures, 4 tables, accepted for publication in ApJ

Planetesimal accretion is a key source for heavy-element enrichment in giant planets. It has been suggested that Jupiter's enriched envelope is a result of planetesimal accretion during its growth assuming it formed in a massive planetesimal disk. In this study, we simulate Jupiter's formation in this scenario. We assume in-situ formation and perform N-body simulations to infer the solid accretion rate. We find that tens-Earth masses of planetesimals can be captured by proto-Jupiter during the rapid gas accretion phase. However, if several embryos are formed near Jupiter's core, which is an expected outcome in the case of a massive planetesimal disk, scattering from the embryos increases the eccentricity and inclination of planetesimals and therefore significantly reduces the accretion efficiency. We also compare our results with published semi-analytical models and show that these models cannot reproduce the N-body simulations especially when the planetesimal disk has a large eccentricity and inclination. We show that when the dynamical evolution of planetesimals is carefully modelled, the total mass of captured planetesimals $M_\mathrm{cap,tot}$ is $2 M_\oplus \lesssim M_\mathrm{cap,tot}\lesssim 18 M_\oplus$. The metallicity of Jupiter's envelope can be explained by the planetesimal accretion in our massive disk model despite the low accretion efficiency coming from the high eccentricity and inclination of planetesimals. Our study demonstrates the importance of detailed modelling of planetesimal accretion during the planetary growth and its implications to the heavy-element mass in gaseous planets., Comment: 20 pages, 17 figures, accepted for publication in MNRAS

In our previous work (Paper I), we demonstrated that coagulation instability results in dust concentration against depletion due to the radial drift and accelerates dust growth locally. In this work (Paper II), we perform numerical simulations of coagulation instability taking into account effects of backreaction to gas and collisional fragmentation of dust grains. We find that the slowdown of the dust drift due to backreaction regulates dust concentration in the nonlinear growth phase of coagulation instability. The dust-to-gas surface density ratio increases from $10^{-3}$ up to $\sim10^{-2}$. Each resulting dust ring tends to have mass of $\simeq0.5M_{\oplus}-1.5M_{\oplus}$ in our disk model. In contrast to Paper I, the dust surface density profile shows a local plateau structure at each dust ring. In spite of the regulation at the nonlinear growth, the efficient dust concentration reduces their collision velocity. As a result, dust grains can grow beyond the fragmentation barrier, and the dimensionless stopping time reaches unity as in Paper I. The necessary condition for the efficient dust growth is (1) weak turbulence of $\alpha<1\times10^{-3}$ and (2) a large critical velocity for dust fragmentation ($> 1$ m/s). The efficient dust concentration in outer regions will reduce the inward pebble flux and is expected to decelerate the planet formation via the pebble accretion. We also find that the resulting rings can be unstable to secular gravitational instability (GI). The subsequent secular GI promotes planetesimal formation. We thus expect that a combination of these instabilities is a promising mechanism for dust-ring and planetesimal formation., Comment: 23 pages, 17 figures, accepted for publication in ApJ

Our previous linear analysis presents a new instability driven by dust coagulation in protoplanetary disks. The coagulation instability has the potential to concentrate dust grains into rings and assist dust coagulation and planetesimal formation. In this series of papers, we perform numerical simulations and investigate nonlinear outcome of coagulation instability. In this paper (Paper I), we first conduct local simulations to demonstrate the existence of coagulation instability. Linear growth observed in the simulations is in good agreement with the previous linear analysis. We next conduct radially global simulations to demonstrate that coagulation instability develops during the inside-out disk evolution due to dust growth. To isolate the various effects on dust concentration and growth, we neglect effects of backreaction to a gas disk and dust fragmentation in Paper I. This simplified simulation shows that either of backreaction or fragmentation is not prerequisite for local dust concentration via the instability. In most runs with weak turbulence, dust concentration via coagulation instability overcomes dust depletion due to radial drift, leading to the formation of multiple dust rings. The nonlinear development of coagulation instability also accelerates dust growth, and the dimensionless stopping time $\tau_{\mathrm{s}}$ reaches unity even at outer radii (>10 au). Therefore, coagulation instability is one promising process to retain dust grains and to accelerate dust growth beyond the drift barrier., Comment: 19 pages, 12 figures, accepted for publication in ApJ

For revealing the first step of the plant formation, it is important to understand how and when dust grains become larger in a disk around a protostar. To investigate the grain growth, we analyze dust continuum emission toward a disk around the Class I protostar, L1489 IRS at 0.9 and 1.3 mm wavelengths obtained by the Atacama Large Millimeter/submillimeter Array. The dust continuum emission extends to a disk radius ($r$) of $r\sim300$ au, and the spectral index ($\alpha$) is derived to be $\alpha\sim3.6$ at a radius of $r\sim100-300$ au, as similar to the interstellar dust. Therefore, the grain growth does not occur significantly in the outer disk ($r\sim100-300$ au). Furthermore, we tentatively identify a ring-like substructure at $r\sim90$ au even though the spatial resolution and sensitivity are not enough to determine this structure. If this is the real ring structure, the ring position and small dust in the disk outer part are consistent with the idea of the growth front. These results suggest that the L1489 protostellar disk may be the beginning of the planet formation., Comment: 8 pages, 5 figures, Accepted for publication in ApJ

We have investigated the protostellar disk around a Class 0/I protostar, L1527 IRS, using multi-wavelength observations of the dust continuum emission at $\lambda=0.87$, 2.1, 3.3, and 6.8 mm obtained by the Atacama Large Millimeter/submillimeter Array (ALMA) and the Jansky Very Large Array (VLA). Our observations achieved a spatial resolution of $3-13$ au and revealed an edge-on disk structure with a size of $\sim80-100$ au. The emission at 0.87 and 2.1 mm is found to be optically thick within a projected disk radius of $ r_{\rm proj}\lesssim50$ au. The emission at 3.3 and 6.8 mm shows that the power-law index of the dust opacity ($\beta$) is $\beta\sim1.7$ around $ r_{\rm proj}\sim 50$ au, suggesting that grain growth has not yet begun. The dust temperature ($T_{\rm dust}$) shows a steep decrease with $T_{\rm dust}\propto r_{\rm proj}^{-2}$ outside of the VLA clumps previously identified at $r_{\rm proj}\sim20$ au. Furthermore, the disk is gravitationally unstable at $r_{\rm proj}\sim20$ au, as indicated by a Toomre {\it Q} parameter value of $Q\lesssim1.0$. These results suggest that the VLA clumps are formed via gravitational instability, which creates a shadow on the outside of the substructure, resulting in the sudden drop in temperature. The derived dust masses for the VLA clumps are $\gtrsim0.1$ $M_{\rm J}$. Thus, we suggest that Class 0/I disks can be massive enough to be gravitationally unstable, which might be the origin of gas-giant planets in a 20 au radius. Furthermore, the protostellar disks can be cold due to shadowing., Comment: 23 pages, 18 figures, 1 table. Accepted for publication in ApJ

In this study, we demonstrate the dependence of atmospheric dust size on the near-infrared spectra of ten L dwarfs, and constrain the sizes of dust grains in each L dwarf atmosphere. In previous studies, by comparing observed and modeled spectra, it was suggested that the deviations of their spectral shapes from theoretical prediction are general characteristics. Here, we focus on the dust size in brown dwarf atmospheres to understand the observed spectra. We confirm that changing the dust size changes the temperature-pressure structure of the atmosphere, with the shape of the spectrum changing accordingly. At the wavelength at which dust is the main absorber of radiation (the dust-dominated regime), a large dust opacity combined with a medium grain size, e.g., 0.1 $\mu$m, results in a low photospheric temperature, and thus a small flux. Conversely, for the wavelength at which gas absorption is dominant (the gas-dominated regime), a large dust opacity modifies the temperature-pressure structure, resulting in a high photospheric temperature, which corresponds to large flux emissions. Taking into account the size effect, we compare the model spectral fluxes in the wavelength range 1-5 $\mu$m with the observational ones to constrain the main dust size in the atmosphere of each of the ten L dwarfs observed with AKARI and SpeX or CGS4. Ultimately, we reveal that the observed data are reproduced with higher fidelity by models based on a medium dust size of 0.1-3.0 $\mu$m for six of these L dwarfs; therefore, we suggest that such atmospheric dust sizes apply to the majority of L dwarfs., Comment: 43 pages, 11 figures, 3 tables, Accepted for publication in ApJ

Gas-giant planets, such as Jupiter, Saturn and massive exoplanets, were formed via the gas accretion onto the solid cores each with a mass of roughly ten Earth masses. However, rapid radial migration due to disk-planet interaction prevents the formation of such massive cores via planetesimal accretion. Comparably rapid core growth via pebble accretion requires very massive protoplanetary disks because most pebbles fall into the central star. Although planetesimal formation, planetary migration, and gas-giant core formation have been studied with much effort, the full evolution path from dust to planets are still uncertain. Here we report the result of full simulations for collisional evolution from dust to planets in a whole disk. Dust growth with realistic porosity allows the formation of icy planetesimals in the inner disk (> 10 au), while pebbles formed in the outer disk drift to the inner disk and there grow to planetesimals. The growth of those pebbles to planetesimals suppresses their radial drift and supplies small planetesimals sustainably in the vicinity of cores. This enables rapid formation of sufficiently massive planetary cores within 0.2-0.4 million years, prior to the planetary migration. Our models shows first gas giants form at 2-7 au in rather common protoplanetary disks, in agreement with the exoplanet and solar systems., Comment: Accepted for publication in ApJ

We present a new instability driven by a combination of coagulation and radial drift of dust particles. We refer to this instability as ``coagulation instability" and regard it as a promising mechanism to concentrate dust particles and assist planetesimal formation in the very early stages of disk evolution. Because of dust-density dependence of collisional coagulation efficiency, dust particles efficiently (inefficiently) grow in a region of positive (negative) dust density perturbations, which lead to a small radial variation of dust sizes and as a result radial velocity perturbations. The resultant velocity perturbations lead to dust concentration and amplify dust density perturbations. This positive feedback makes a disk unstable. The growth timescale of coagulation instability is a few tens of orbital periods even when dust-to-gas mass ratio is of the order of $10^{-3}$. In a protoplanetary disk, radial drift and coagulation of dust particles tend to result in dust depletion. The present instability locally concentrates dust particles even in such a dust-depleted region. The resulting concentration provides preferable sites for dust-gas instabilities to develop, which leads to further concentration. Dust diffusion and aerodynamical feedback tend to stabilize short-wavelength modes, but do not completely suppress the growth of coagulation instability. Therefore, coagulation instability is expected to play an important role in setting up the next stage for other instabilities to further develop toward planetesimal formation, such as streaming instability or secular gravitational instability., Comment: 31 pages, 15 figures, accepted for publication in ApJ

Planets grow via the collisional accretion of small bodies in a protoplanetary disk. Such small bodies feel strong gas drag and their orbits are significantly affected by the gas flow and atmospheric structure around the planet. We investigate the gas flow in the protoplanetary disk perturbed by the gravity of the planet by three-dimensional hydrodynamic simulation. We then calculate the orbital evolutions of particles in the gas structure obtained from the hydrodynamic simulation. Based on the orbital calculations, we obtain the collision rate between the planet and centimeter to kilometer sized particles. Our results show that meter-sized or larger particles effectively collide with the planet due to the atmospheric gas drag, which significantly enhances the collision rate. On the other hand, the gas flow plays an important role for smaller particles. Finally, considering the effects of the atmosphere and gas flow, we derive the new analytic formula for the collision rate, which is in good agreement with our simulations. We estimate the growth timescale and accretion efficiency of drifting bodies for the formation of a gas-giant solid core using the formula. We find the accretion of sub-kilometer sized bodies achieve a short growth timescale (~0.005 Myr) and a high accretion efficiency (~1) for the core formation at 5 au in the minimum mass solar nebula model., Comment: Accepted for publication in The Astrophysical Journal

Asteroid Ryugu and asteroid Bennu, which were recently visited by spacecraft Hayabusa2 and OSIRIS-REx, respectively, are spinning top-shaped rubble piles. Other axisymmetric top-shaped near-Earth asteroids have been observed with ground-based radar, most of which rotate near breakup rotation periods of ~ 3 hours. This suggests that rotation-induced deformation of asteroids through rotational spinup produces top shapes. Although some previous simulations using the Discrete Element Method showed that spinup of rubble piles may produce oblate top shapes, it is still unclear what kinds of conditions such as friction angles of constituent materials and spinup timescales are required for top-shape formation. Here we show, through Smoothed Particle Hydrodynamics simulations of granular bodies spinning-up at different rates, that the rotation-induced deformation of spherical rubble piles before breakup can be classified into three modes according to the friction angle \phi_{d}: quasi-static and internal deformation for \phi_{d} < 40 degrees, dynamical and internal deformation for 50 degrees < \phi_{d} < 60 degrees, and surface landslides for \phi_{d} > 70 degrees. Note that these apparent large values of friction angle can be acceptable if we consider the effect of cohesion among blocks of a rubble pile under weak gravity. Bodies with \phi_{d} < 60 degrees evolve into oblate spheroids through internal deformation, but never form pronounced equators defining a top shape. In contrast, bodies with \phi_{d} > 70 degrees deform into axisymmetric top shapes through an axisymmetric surface landslides if spinup timescales are < a few days. In addition, through slow spinups with timescales > 1 month, bodies with \phi_{d} > 70 degrees deform into non-axisymmetric shapes via localized landslides. We suggest that rapid spinup mechanisms are preferable for the formation of axisymmetric top shapes., Comment: 29 pages, 11 figures, accepted for publication in Icarus

We investigated fundamental processes of collisional sticking and fragmentation of dust aggregates by carrying out N-body simulations of submicron-sized icy dust monomers. We examined the condition for collisional growth of two colliding dust aggregates in a wide range of the mass ratio, 1-64. We found that the mass transfer from a larger dust aggregate to a smaller one is a dominant process in collisions with a mass ratio of 2-30 and impact velocity of \approx 30-170 m s^-1. As a result, the critical velocity, v_fra, for fragmentation of the largest body is considerably reduced for such unequal-mass collisions; v_fra of collisions with a mass ratio of 3 is about half of that obtained from equal-mass collisions. The impact velocity is generally higher for collisions between dust aggregates with higher mass ratios because of the difference between the radial drift velocities in the typical condition of protoplanetary disks. Therefore, the reduced v_fra for unequal-mass collisions would delay growth of dust grains in the inner region of protoplanetary disks., Comment: 33 pages, 17 figures, 3 tables, accepted for publication in ApJ

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

Debris disks are classically considered to be gas-less systems, but recent (sub)millimeter observations have detected tens of those with rich gas content. The origin of the gas component remains unclear; namely, it can be protoplanetary remnants and/or secondary products deriving from large bodies. In order to be protoplanetary in origin, the gas component of the parental protoplanetary disk is required to survive for $\gtrsim10{\,\rm Myr}$. However, previous models predict $\lesssim 10{\,\rm Myr}$ lifetimes because of efficient photoevaporation at the late stage of disk evolution. In the present study, we investigate photoevaporation of gas-rich, optically-thin disks around intermediate-mass stars at a late stage of the disk evolution. The evolved system is modeled as those where radiation force is sufficiently strong to continuously blow out small grains ($\lesssim 4 {\,\rm \mu m}$), which are an essential component for driving photoevaporation via photoelectric heating induced by stellar far-ultraviolet (FUV). We find that the grain depletion reduces photoelectric heating, so that FUV photoevaporation is not excited. Extreme-ultraviolet (EUV) photoevaporation is dominant and yields a mass-loss rate of $2$--$5\times10^{-10}(\Phi_{\rm EUV}/10^{41}{\,\rm s}^{-1})^{1/2}\,M_\odot\,{\rm yr}^{-1}$, where $\Phi_{\rm EUV}$ is the EUV emission rate. The estimated lifetimes of the gas component are $\sim 50 (M_{\rm disk}/10^{-2}\,M_\odot)(\Phi_{\rm EUV}/10^{41}\,{\rm s}^{-1})^{1/2}\,{\rm Myr}$ and depend on the ``initial'' disk mass at the point small grains have been depleted in the system. With an order estimation, we show that the gas component can survive for a much longer time around A-type stars than lower-mass stars. This trend is consistent with the higher frequency of gas-rich debris disks around A-type stars, implying the possibility of the gas component being protoplanetary remnants., Comment: Accepted for publication in ApJ

Beyond the snow line of protoplanetary discs and inside the dense core of molecular clouds, the temperature of gas is low enough for water vapour to condense into amorphous ices on the surface of preexisting refractory dust particles. Recent numerical simulations and laboratory experiments suggest that condensation of the vapour promotes dust coagulation in such a cold region. However, in the numerical simulations, cohesion of refractory materials is often underestimated, while in the laboratory experiments, water vapour collides with surfaces at more frequent intervals compared to the real conditions. Therefore, to re-examine the role of water ice in dust coagulation, we carry out systematic investigation of available data on coagulation of water ice particles by making full use of appropriate theories in contact mechanics and tribology. We find that the majority of experimental data are reasonably well explained by lubrication theories, owing to the presence of a quasi-liquid layer (QLL). Only exceptions are the results of dynamic collisions between particles at low temperatures, which are, instead, consistent with the JKR theory, because QLLs are too thin to dissipate their kinetic energies. By considering the vacuum conditions in protoplanetary discs and molecular clouds, the formation of amorphous water ice on the surface of refractory particles does not necessarily aid their collisional growth as currently expected. While crystallisation of water ice around but outside the snow line eases coagulation of ice-coated particles, sublimation of water ice inside the snow line is deemed to facilitate coagulation of bare refractory particles., Comment: 18 pages, 14 figures, published in Monthly Notices of the Royal Astronomical Society

We discovered a new growth mode of dust grains to km-sized bodies in protoplanetary disks that evolve by viscous accretion and magnetically driven disk winds (MDWs). We solved an approximate coagulation equation of dust grains with time-evolving disks that consist of both gas and solid components by a one-dimensional model. With the grain growth, all solid particles initially drift inward toward the central star by the gas drag force. However, the radial profile of gas pressure, $P$, is modified by the MDW that disperses the gas in an inside-out manner. Consequently, a local concentration of solid particles is created by the converging radial flux of drifting dust grains at the location with the convex upward profile of $P$. When the dimensionless stopping time, ${\rm St}$, there exceeds unity, the solid particles spontaneously reach the growth dominated state because of the positive feedback between the suppressed radial drift and the enhanced accumulation of dust particles that drift from the outer part. Once the solid particles are in the drift limited state, the above-mentioned condition of ${\rm St} \gtrsim 1$ for the dust growth is equivalent with \begin{equation} \Sigma_{\rm d}/\Sigma_{\rm g}\gtrsim \eta, \nonumber \end{equation} where $\Sigma_{\rm d}/\Sigma_{\rm g}$ is the dust-to-gas surface-density ratio and $\eta$ is dimensionless radial pressure-gradient force. As a consequence of the successful growth of dust grains, a ring-like structure containing planetesimal-sized bodies is formed at the inner part of the protoplanetary disks. Such a ring-shaped concentration of planetesimals is expected to play a vital role in the subsequent planet formation., Comment: 24 pages, 10 figures, 1 table, accepted for publication in The Astrophysical Journal

The members of asteroid families have various shapes. We investigate the origin of their shapes by high-resolution impact simulations for catastrophic disruptions using a Smoothed Particle Hydrodynamics code. Collisional remnants produced through our simulations of the catastrophic disruptions mainly have spherical or bilobed shapes. However, no flat remnants with the ratio of minor to major axis lengths c/a < 0.5 are formed. The results of the simulations provide various shapes of asteroids and explain most of the shapes in asteroid families that are supposed to be produced through catastrophic disruptions. However, the present simulations do not explain significantly flat asteroids. We suggest that these flat asteroids may be interlopers or formed through low-velocity collisions between member asteroids., Comment: 16 pages, 8 figures, accepted for publication in Planetary and Space Science

Terrestrial planets are believed to be formed via giant impacts of Mars-sized protoplanets. Planets formed via giant impacts have highly eccentric orbits. A swarm of planetesimals around the planets may lead to eccentricity damping for the planets via the equipartition of random energies (dynamical friction). However, dynamical friction increases eccentricities of planetesimals, resulting in high velocity collisions between planetesimals. The collisional cascade grinds planetesimals to dust until dust grains are blown out due to radiation pressure. Therefore, the total mass of planetesimals decreases due to collisional fragmentation, which weakens dynamical friction. We investigate the orbital evolution of protoplanets in a planetesimal disk, taking into account collisional fragmentation of planetesimals. For 100 km-sized or smaller planetesimals, dynamical friction is insignificant for eccentricity damping of planets because of collisional fragmentation. On the other hand, giant impacts eject collisional fragments. Although the total mass of giant impact ejecta is 0.1-0.3 Earth masses, the largest impact ejecta are ~ 1,000 km in size. We also investigate the orbital evolution of single planets with initial eccentricities 0.1 in a swarm of such giant impact ejecta. Although the total mass of giant impact ejecta decreases by a factor of 3 in 30 Myrs, eccentricities of planets are damped down to the Earth level (~0.01) due to interaction with giant impact ejecta. Therefore, giant impact ejecta play an important role for determination of terrestrial planet orbits., Comment: Accepted for publication in the Astrophysical Journal

We present the first sub-arcsecond images of 49 Ceti in the [C I]3P1-3P0 emission and the 614 micron dust continuum emission observed with ALMA, as well as that in the CO(J=3-2) emission prepared by using the ALMA archival data. The spatial distribution of the 614 micron dust continuum emission is found to have a broad-ring structure with a radius of about 100 au around the central star. A substantial amount of gas is also associated with 49 Ceti. The [C I] emission map shows two peaks inside the dust ring, and its overall extent is comparable to that of the dust continuum emission and the CO emission. We find that the [C I]/CO(J=3-2) intensity ratio significantly varies along the major axis. The ratio takes the minimum value of 1.8 around the dust peak position, and increases inwards and outwards. The enhanced ratio around the central star (~3) likely originates from the stellar UV radiation, while that in the outer disk (~10) from the interstellar UV radiation. Such complex distributions of the [C I] and CO(J=3-2) emission will be a key to understand the origin of the gas in 49 Ceti, and will also provide a stringent constraint on physical and chemical models of gaseous debris disks., Comment: 21 pages, 7 figures, 1 table; accepted for publication in ApJ

Light curve observations of a recently discovered interstellar object 1I/`Oumuamua suggest that this object has an extremely elongated shape with the axis ratio 0.3 or smaller. Planetesimal collisions can produce irregular shapes including elongated shapes. In this paper, we suggest that the extremely elongated shape of 1I/`Oumuamua may be the result of such an impact. To find detailed impact conditions to form the extremely elongated objects, we conduct numerical simulations of planetesimal collisions using Smoothed Particle Hydrodynamics method for elastic dynamics with self-gravity and interparticle friction. Impacts into strengthless target planetesimals with radius 50 m are conducted with various ratios of impactor mass to target mass q, friction angles phi_d, impact velocities v_imp, and impact angles theta_imp. We find that impacts with q \geq 0.5, phi_d \geq 40 degrees, v_imp \leq 40 degrees, and theta_imp \leq 30 degrees produce remnants with the ratio of intermediate to major axis length less than 0.3. This impact condition suggests that the parent protoplanetary disk in the planetesimal collision stage was weakly turbulent (alpha < 10^{-4} for the inner disk) and composed of planetesimals smaller than ~ 7 km to ensure small impact velocity., Comment: 21 pages, 9 figures, accepted to Icarus

In the core accretion scenario, gas giant planets are formed form solid cores with several Earth masses via gas accretion. We investigate the formation of such cores via collisional growth from kilometer-sized planetesimals in turbulent disks. The stirring by forming cores induces collisional fragmentation and surrounding planetesimals are ground down until radial drift. The core growth is therefore stalled by the depletion of surrounding planetesiamls due to collisional fragmentation and radial drift. The collisional strength of planetesimals determines the planetesimal-depletion timescale, which is prolonged for large planetesiamls. The size of planetesiamls around growing cores is determined by the planetesimal size distribution at the onset of runaway growth. Strong turbulence delays the onset of runaway growth, resulting in large planetesimals. Therefore, the core mass evolution depends on turbulent parameter $\alpha$; the formation of cores massive enough without significant depletion of surrounding planetesimals needs strong turbulence of $\alpha \gtrsim 10^{-3}$. However, the strong turbulence with $\alpha \gtrsim 10^{-3}$ leads to a significant delay of the onset of runaway growth and prevents the formation of massive cores within the disk lifetime. The formation of cores massive enough within several millions years therefore requires the several times enhancement of the solid surface densities, which is achieved in the inner disk $\lesssim 10$AU due to pile-up of drifting dust aggregates. In addition, the collisional strength $Q_{\rm D}^*$ even for kilometer-sized or smaller bodies affects the growth of cores; $Q_{\rm D}^* \gtrsim 10^7 \,{\rm erg/g}$ for bodies $\lesssim 1$ km is likely for this gas giant formation., Comment: 12 pages, 15 figures. accepted for publication in ApJ

In most of the previous studies related to collisional disruption of planetesimals in the gravity regime, Smoothed Particle Hydrodynamics (SPH) simulations have been used. On the other hand, impact simulations using grid-based hydrodynamic code have not been sufficiently performed. In the present study, we execute impact simulations in the gravity regime using the shock-physics code iSALE, which is a grid-based Eulerian hydrocode. We examine the dependence of the critical specific impact energy Q_RD* on impact conditions for a wide range of specific impact energy (Q_R) from disruptive collisions to erosive collisions, and compare our results with previous studies. We find collision outcomes of the iSALE simulation agree well with those of the SPH simulation. Detailed analysis mainly gives three results. (1) The value of Q_RD* depends on numerical resolution, and is close to convergence with increasing numerical resolution. The difference in converged value of Q_RD* between the iSALE code and the SPH code is within 30%. (2) Ejected mass normalized by total mass (M_ej/M_tot) generally depends on various impact conditions. However, when Q_R is normalized by Q_RD* that is calculated for each impact simulation, M_ej/M_tot can be scaled by Q_R/Q_RD*, and is independent of numerical resolution, impact velocity and target size. (3) This similarity law for Q_R/Q_RD* is confirmed for a wide range of specific impact energy. We also derive a semi-analytic formula for Q_RD* based on the similarity law and the crater scaling law. We find that the semi-analytic formula for the case with a non-porous object is consistent with numerical results., Comment: Accepted for publication in Icarus

More than a half of asteroids in the main belt have irregular shapes with the ratios of the minor to major axis lengths less than 0.6. One of the mechanisms to create such shapes is collisions between asteroids. The relationship between shapes of collisional outcomes and impact conditions such as impact velocities may provide information on the collisional environments and its evolutionary stages when those asteroids are created. In this study, we perform numerical simulations of collisional destruction of asteroids with radii 50 km and subsequent gravitational reaccumulation using Smoothed Particle Hydrodynamics for elastic dynamics with self-gravity, a model of fracture of rock, and a model of friction of completely damaged rock. We systematically vary the impact velocity from 50 m/s to 400 m/s and the impact angle from 5 degrees to 45 degrees. We investigate shapes of the largest remnants resulting from collisional simulations. As a result, various shapes (bilobed, spherical, flat, elongated, and hemispherical shapes) are formed through equal-mass and low-velocity (50 - 400 m/s) impacts. We clarify a range of the impact angle and velocity to form each shape. Our results indicate that irregular shapes, especially flat shapes, of asteroids with diameters larger than 80 km are likely to be formed through similar-mass and low-velocity impacts, which are likely to occur in the primordial environment prior to the formation of Jupiter., Comment: 11 pages, 15 figures, accepted to Astronomy and Astrophysics

We investigate the formation of hot and massive circumplanetary disks (CPDs) and the orbital evolution of satellites formed in these disks. Because of the comparatively small size-scale of the sub-disk, quick magnetic diffusion prevents the magnetorotational instability (MRI) from being well-developed at ionization levels that would allow MRI in the parent protoplanetary disk. In the absence of significant angular momentum transport, continuous mass supply from the parental protoplanetary disk leads to the formation of a massive CPD. We have developed an evolutionary model for this scenario and have estimated the orbital evolution of satellites within the disk. We find, in a certain temperature range, that inward migration of a satellite can be stopped by a change in the structure due to the opacity transitions. Moreover, by capturing second and third migrating satellites in mean motion resonances, a compact system in Laplace resonance can be formed in our disk models., Comment: 10 pages, 9 figures

In the Solar System, interplanetary dust particles (IDPs) originating mainly from asteroid collisions and cometary activities drift to the Earth orbit due to the Poynting-Robertson drag. We analyzed the thermal emission from IDPs that was observed by the first Japanese infrared astronomical satellite, AKARI. The observed surface brightness in the trailing direction of the Earth orbit is 3.7% greater than that in the leading direction in the $9{\rm \mu m}$ band and 3.0% in the $18{\rm \mu m}$ band. In order to reveal dust properties causing the leading-trailing surface brightness asymmetry, we numerically integrated orbits of the Sun, the Earth, and a dust particle as a restricted three-body problem including radiation from the Sun. The initial orbits of particles are determined according to the orbits of main-belt asteroids or Jupiter-family comets. The orbital trapping in mean motion resonances results in a significant leading-trailing asymmetry so that intermediate sized dust (~10-100${\rm \mu m}$) produces a greater asymmetry than the zodiacal light has. The leading-trailing surface brightness difference integrated over the size distribution of the asteroidal dust is obtained to be the values of 27.7% and 25.3% in the $9{\rm \mu m}$ and $18{\rm \mu m}$ bands, respectively. In contrast, the brightness difference for cometary dust is calculated as the values of 3.6% and 3.1% in the $9{\rm \mu m}$ and $18{\rm \mu m}$ bands, respectively, if the maximum dust radius is set to be $s_{\rm max} = 3000{\rm \mu m}$. Taking into account these values and their errors, we conclude that the contribution of asteroidal dust to the zodiacal infrared emission is less than ~10%, while cometary dust of the order of 1 mm mainly accounts for the zodiacal light in infrared., Comment: 31 pages, 8 figures, accepted for publication in AJ

Context: Debris disks are important observational clues for understanding planetary-system formation process. In particular, faint warm debris disks may be related to late planet formation near 1 AU. A systematic search of faint warm debris disks is necessary to reveal terrestrial planet formation. Aims: Faint warm debris disks show excess emission that peaks at mid-IR wavelengths. Thus we explore debris disks using the AKARI mid-IR all-sky point source catalog (PSC), a product of the second generation unbiased IR all-sky survey. Methods : We investigate IR excess emission for 678 isolated main-sequence stars for which there are 18 micron detections in the AKARI mid-IR all-sky catalog by comparing their fluxes with the predicted fluxes of the photospheres based on optical to near-IR fluxes and model spectra. The near-IR fluxes are first taken from the 2MASS PSC. However, 286 stars with Ks<4.5 in our sample have large flux errors in the 2MASS photometry due to saturation. Thus we have measured accurate J, H, and Ks band fluxes, applying neutral density (ND) filters for Simultaneous InfraRed Imager for Unbiased Survey (SIRIUS) on IRSF, the \phi 1.4 m near-IR telescope in South Africa, and improved the flux accuracy from 14% to 1.8% on average. Results: We identified 53 debris-disk candidates including eight new detections from our sample of 678 main-sequence stars. The detection rate of debris disks for this work is ~8%, which is comparable with those in previous works by Spitzer and Herschel. Conclusion: The importance of this study is the detection of faint warm debris disks around nearby field stars. At least nine objects have a large amount of dust for their ages, which cannot be explained by the conventional steady-state collisional cascade model., Comment: 17 pages, 21 figures, accepted for publication in Astronomy and Astrophysics

Adhesion forces between submicrometer-sized silicate grains play a crucial role in the formation of silicate dust agglomerates, rocky planetesimals, and terrestrial planets. The surface energy of silicate dust particles is the key to their adhesion and rolling forces in a theoretical model based on the contact mechanics. Here we revisit the cohesion of amorphous silica spheres by compiling available data on the surface energy for hydrophilic amorphous silica in various circumstances. It turned out that the surface energy for hydrophilic amorphous silica in a vacuum is a factor of 10 higher than previously assumed. Therefore, the previous theoretical models underestimated the critical velocity for the sticking of amorphous silica spheres, as well as the rolling friction forces between them. With the most plausible value of the surface energy for amorphous silica spheres, theoretical models based on the contact mechanics are in harmony with laboratory experiments. Consequently, we conclude that silicate grains with a radius of $0.1~\mu$m could grow to planetesimals via coagulation in a protoplanetary disk. We argue that the coagulation growth of silicate grains in a molecular cloud is advanced either by organic mantles rather than icy mantles or, if there are no mantles, by nanometer-sized grain radius., Comment: in The Astrophysical Journal, 812:67 (12pp), 2015 October 10

When planetesimals grow via collisions in a turbulent disk, stirring through density fluctuation caused by turbulence effectively increases the relative velocities between planetesimals, which suppresses the onset of runaway growth. We investigate the onset of runaway growth in a turbulent disk through simulations that calculate the mass and velocity evolution of planetesimals. When planetesimals are small, the average relative velocity between planetesimals, $v_{\rm r}$, is much greater than their surface escape velocity, $v_{\rm esc}$, so that runaway growth does not occur. As planetesimals become large via collisional growth, $v_{\rm r}$ approaches $v_{\rm esc}$. When $v_{\rm r} \approx 1.5 v_{\rm esc}$, runaway growth of the planetesimals occurs. During the oligarchic growth subsequent to runaway growth, a small number of planetary embryos produced via runaway growth become massive through collisions with planetesimals with radii of that at the onset of runaway growth, $r_{\rm p,run}$. We analytically derive $r_{\rm p,run}$ as a function of the turbulent strength. Growing $\sim 10\,M_\oplus$ embryos that are suitable to become the cores of Jupiter and Saturn requires $r_{\rm p,run} \sim 100$\,km, which is similar to the proposed fossil feature in the size distribution of main belt asteroids. In contrast, the formation of Mars as quickly as suggested from Hf-W isotope studies requires small planetesimals at the onset of runaway growth. Thus, the conditions required to form Mars, Jupiter, and Saturn and the size distribution of the main-belt asteroids indicate that the turbulence increased in amplitude relative to the sound speed with increasing distance from the young Sun., Comment: accepted for publication in ApJ

The latest observation of HL Tau by ALMA revealed spectacular concentric dust rings in its circumstellar disk. We attempt to explain the multiple ring structure as a consequence of aggregate sintering. Sintering is known to reduce the sticking efficiency of dust aggregates and occurs at temperatures slightly below the sublimation point of their constituent material. We here present a dust growth model incorporating sintering and use it to simulate global dust evolution due to sintering, coagulation, fragmentation, and radial inward drift in a modeled HL Tau disk. We show that aggregates consisting of multiple species of volatile ices experience sintering, collisionally disrupt, and pile up at multiple locations slightly outside the snow lines of the volatiles. At wavelengths of 0.87--1.3 mm, these sintering zones appear as bright, optically thick rings with a spectral slope of $\approx 2$, whereas the non-sintering zones as darker, optically thinner rings of a spectral slope of $\approx$ 2.3--2.5. The observational features of the sintering and non-sintering zones are consistent with those of the major bright and dark rings found in the HL Tau disk, respectively. Radial pileup and vertical settling occur simultaneously if disk turbulence is weak and if monomers constituting the aggregates are $\sim 1~{\rm \mu m}$ in radius. For the radial gas temperature profile of $T = 310(r/1~{\rm AU})^{-0.57}~{\rm K}$, our model perfectly reproduces the brightness temperatures of the optically thick bright rings, and reproduces their orbital distances to an accuracy of $\lesssim$ 30%., Comment: 24 pages, 20 figures, discussion section added, accepted for publication in ApJ

We investigate the dust and gas distribution in the disk around HD 142527 based on ALMA observations of dust continuum, 13CO(3-2), and C18O(3-2) emission. The disk shows strong azimuthal asymmetry in the dust continuum emission, while gas emission is more symmetric. In this paper, we investigate how gas and dust are distributed in the dust-bright northern part of the disk and in the dust-faint southern part. We construct two axisymmetric disk models. One reproduces the radial profiles of the continuum and the velocity moments 0 and 1 of CO lines in the north and the other reproduces those in the south. We have found that the dust is concentrated in a narrow ring having ~50AU width (in FWHM; w_d=30AU in our parameter definition) located at ~170-200AU from the central star. The dust particles are strongly concentrated in the north. We have found that the dust surface density contrast between the north and south amounts to ~70. Compared to the dust, the gas distribution is more extended in the radial direction. We find that the gas component extends at least from ~100AU to ~250AU from the central star, and there should also be tenuous gas remaining inside and outside of these radii. The azimuthal asymmetry of gas distribution is much smaller than dust. The gas surface density differs only by a factor of ~3-10 between the north and south. Hence, gas-to-dust ratio strongly depends on the location of the disk: ~30 at the location of the peak of dust distribution in the south and ~3 at the location of the peak of dust distribution in the north. Despite large uncertainties, the overall gas-to-dust ratio is inferred to be ~10-30, indicating that the gas depletion may have already been under way., Comment: 39 pages, 37 figures, 9 tables, accepted for publication in PASJ

The bulk chemical compositions of planets are uncertain, even for major elements such as Mg and Si. This is due to the fact that the samples available for study all originate from relatively shallow depths. Comparison of the stable isotope compositions of planets and meteorites can help overcome this limitation. Specifically, the non-chondritic Si isotope composition of the Earth's mantle was interpreted to reflect the presence of Si in the core, which can also explain its low density relative to pure Fe-Ni alloy. However, we have found that angrite meteorites display a heavy Si isotope composition similar to the lunar and terrestrial mantles. Because core formation in the angrite parent-body (APB) occurred under oxidizing conditions at relatively low pressure and temperature, significant incorporation of Si in the core is ruled out as an explanation for this heavy Si isotope signature. Instead, we show that equilibrium isotopic fractionation between gaseous SiO and solid forsterite at 1370 K in the solar nebula could have produced the observed Si isotope variations. Nebular fractionation of forsterite should be accompanied by correlated variations between the Si isotopic composition and Mg/Si ratio following a slope of 1, which is observed in meteorites. Consideration of this nebular process leads to a revised Si concentration in the Earth's core of 3.6 (+6.0/-3.6) wt% and provides estimates of Mg/Si ratios of bulk planetary bodies., Comment: Accepted in Earth and Planetary Science Letters, 40 pages, 8 figures, 2 tables

Planets are formed from collisional growth of small bodies in a protoplanetary disk. Bodies much larger than approximately $1$\,m are mainly controlled by the gravity of the host star and experience weak gas drag; their orbits are mainly expressed by orbital elements: semimajor axes $a$, eccentricities $e$, and inclinations $i$, which are modulated by gas drag. In a previous study, $\dot a$, $\dot e$, and $\dot i$ were analytically derived for $e \ll 1$ and $i \ll H/a$, where $H$ is the scale height of the disk. Their formulae are valid in the early stage of planet formation. However, once massive planets are formed, $e$ and $i$ increase greatly. Indeed, some small bodies in the solar system have very large $e$ and $i$. Therefore, in this paper, I analytically derive formulae for $\dot a$, $\dot e$, and $\dot i$ for $1-e^2 \ll 1$ and $i \ll H/a$ and for $i \gg H/a$. The formulae combined from these limited equations will represent the results of orbital integration unless $e \geq 1$ or $i > \pi - H/a$. Since the derived formulae are applicable for bodies not only in a protoplanetary disk but also in a circumplanetary disk, I discuss the possibility of the capture of satellites in a circumplanetary disk using the formulae., Comment: accepted for publication in Earth, Planets and Space, 11 pages, 4 figures

Recent three-dimensional magnetohydrodynamical simulations have identified a disk wind by which gas materials are lost from the surface of a protoplanetary disk, which can significantly alter the evolution of the inner disk and the formation of terrestrial planets. A simultaneous description of the realistic evolution of the gaseous and solid components in a disk may provide a clue for solving the problem of the mass concentration of the terrestrial planets in the solar system. We simulate the formation of terrestrial planets from planetary embryos in a disk that evolves via magnetorotational instability and a disk wind. The aim is to examine the effects of a disk wind on the orbital evolution and final configuration of planetary systems. We perform N-body simulations of sixty 0.1 Earth-mass embryos in an evolving disk. The evolution of the gas surface density of the disk is tracked by solving a one-dimensional diffusion equation with a sink term that accounts for the disk wind. We find that even in the case of a weak disk wind, the radial slope of the gas surface density of the inner disk becomes shallower, which slows or halts the type I migration of embryos. If the effect of the disk wind is strong, the disk profile is significantly altered (e.g., positive surface density gradient, inside-out evacuation), leading to outward migration of embryos inside ~ 1 AU. Disk winds play an essential role in terrestrial planet formation inside a few AU by changing the disk profile. In addition, embryos can undergo convergent migration to ~ 1 AU in certainly probable conditions. In such a case, the characteristic features of the solar system's terrestrial planets (e.g., mass concentration around 1 AU, late giant impact) may be reproduced., Comment: 8 pages, 4 figures, accepted for publication in A&A

Several hundred stars older than 10 million years have been observed to have infrared excesses. These observations are explained by dust grains formed by the collisional fragmentation of hidden planetesimals. Such dusty planetesimal discs are known as debris discs. In a dynamically cold planetesimal disc, collisional coagulation of planetesimals produces planetary embryos which then stir the surrounding leftover planetesimals. Thus, the collisional fragmentation of planetesimals that results from planet formation forms a debris disc. We aim to determine the properties of the underlying planetesimals in debris discs by numerically modelling the coagulation and fragmentation of planetesimal populations. The brightness and temporal evolution of debris discs depend on the radial distribution of planetesimal discs, the location of their inner and outer edges, their total mass, and the size of planetesimals in the disc. We find that a radially narrow planetesimal disc is most likely to result in a debris disc that can explain the trend of observed infrared excesses of debris discs around G-type stars, for which planet formation occurs only before 100 million years. Early debris disc formation is induced by planet formation, while the later evolution is explained by the collisional decay of leftover planetesimals around planets that have already formed. Planetesimal discs with underlying planetesimals of radii $\sim 100\,$km at $\approx 30$ AU most readily explain the Spitzer Space Telescope 24 and 70$ \mu$m fluxes from debris discs around G-type stars., Comment: 10 pages, 11 figures; accepted for publication in MNRAS

We investigate the formation of multiple-planet systems in the presence of a hot Jupiter using extended N-body simulations that are performed simultaneously with semi-analytic calculations. Our primary aims are to describe the planet formation process starting from planetesimals using high-resolution simulations, and to examine the dependences of the architecture of planetary systems on input parameters (e.g., disk mass, disk viscosity). We observe that protoplanets that arise from oligarchic growth and undergo type I migration stop migrating when they join a chain of resonant planets outside the orbit of a hot Jupiter. The formation of a resonant chain is almost independent of our model parameters, and is thus a robust process. At the end of our simulations, several terrestrial planets remain at around 0.1 AU. The formed planets are not equal-mass; the largest planet constitutes more than 50 percent of the total mass in the close-in region, which is also less dependent on parameters. In the previous work of this paper (Ogihara et al. 2013), we have found a new physical mechanism of induced migration of the hot Jupiter, which is called a crowding-out. If the hot Jupiter opens up a wide gap in the disk (e.g., owing to low disk viscosity), crowding-out becomes less efficient and the hot Jupiter remains. We also discuss angular momentum transfer between the planets and disk., Comment: 15 pages, 9 figures, 2 tables, accepted for publication in ApJ

We investigate formation of close-in terrestrial planets from planetary embryos under the influence of a hot Jupiter (HJ) using gravitational N-body simulations that include gravitational interactions between the gas disk and the terrestrial planet (e.g., type I migration). Our simulations show that several terrestrial planets efficiently form outside the orbit of the HJ, making a chain of planets, and all of them gravitationally interact directly or indirectly with the HJ through resonance, which leads to inward migration of the HJ. We call this mechanism of induced migration of the HJ as "crowding out." The HJ is eventually lost by collision with the central star, and only several terrestrial planets remain. We also find that the efficiency of the crowding-out effect depends on model parameters; for example, the heavier the disk is, the more efficient the crowding out is. When planet formation occurs in a massive disk, the HJ can be lost to the central star and is never observed. On the other hand, for a less massive disk, the HJ and terrestrial planets can coexist; however, the companion planets can be below the detection limit of current observations. In both cases, systems with the HJ and terrestrial planets have little chance for detection. Therefore, our model naturally explains the lack of companion planets in HJ systems regardless of the disk mass. In effect, our model provide a theoretical prediction for future observations; additional planets can be discovered just outside the HJ, and their masses should generally be small., Comment: 6 pages, 3 figures, accepted for publication in ApJL

We investigate the condition for capture into first-order mean motion resonances using numerical simulations with a wide range of various parameters. In particular, we focus on deriving the critical migration timescale for capture into the 2:1 resonance; additional numerical experiments for closely spaced resonances (e.g., 3:2) are also performed. We find that the critical migration timescale is determined by the planet-to-stellar mass ratio, and its dependence exhibits power-law behavior with index -4/3. This dependence is also supported by simple analytic arguments. We also find that the critical migration timescale for systems with equal-mass bodies is shorter than that in the restricted problem; for instance, for the 2:1 resonance between two equal-mass bodies, the critical timescale decreases by a factor of 10. In addition, using the obtained formula, the origin of observed systems that include first-order commensurabilities is constrained. Assuming that pairs of planets originally form well separated from each other and then undergo convergent migration and are captured in resonances, it is possible that a number of exoplanets experienced rapid orbital migration. For systems in closely spaced resonances, the differential migration timescale between the resonant pair can be constrained well; it is further suggested that several exoplanets underwent migration that can equal or even exceed the type I migration rate predicted by the linear theory. This implies that some of them may have formed in situ. Future observations and the use of our model will allow us to statistically determine the typical migration speed in a protoplanetary disk., Comment: 13 pages, 11 figures, 3 tables, accepted for publication in ApJ

Mars is likely to be a planetary embryo formed through collisions with planetesimals, which can explain its small mass and rapid formation timescale obtained from 182Hf-182$W chronometry. In the classical theory of planet formation, the final embryo mass is determined only by the solid surface density. However, embryos can stir surrounding planetesimals, leading to fragmentation through erosive (cratering) collisions. We find that radial drift of small fragments can drastically reduce the solid surface density. On the other hand, embryo growth is accelerated by fragment accretion. Since collisional fragmentation efficiency depends on the initial size of planetesimals, the final embryo mass and its growth time are determined by the initial planetesimal size and disk surface density. We have investigated the effect of these two parameters on the mass of Mars and the predicted radiogenic excess of 182W in the martian mantle. Two scenarios can explain the rapid formation of small Mars: (i) it formed by accretion of small planetesimals in a massive disk or (ii) it formed from large planetesimals but its growth was arrested by the inward then outward migration of Jupiter. Taking into account all constraints, we conclude that Mars is likely to have formed in a massive disk of about ~ 0.1 solar mass from planetesimals smaller than ~ 10 km in radius. Such small planetesimal size cannot explain core accretion of Jupiter, suggesting that there may have been a heliocentric gradient in planetesimal size in the solar nebula.

The trajectories of dust particles ejected from a comet are affected by solar radiation pressure as a function of their ratios of radiation pressure cross section to mass. Therefore, a study on the orbital evolution of the particles caused by the radiation pressure reveals the physical properties of dust on the surface of the comet nucleus. In the course of NASA's Deep Impact mission, the ejecta plume evolved under the influence of the radiation pressure. From the evolution and shape of the plume, we have succeeded in obtaining $\beta \approx 0.4$, where $\beta$ is the ratio of the radiation pressure to the solar gravity. Taking into account $\beta \approx 0.4$ as well as the observational constraints of a high color temperature and a small silicate-feature strength, dust particles ejected from the surface of comet 9P/Tempel 1 are likely compact dust aggregates of sizes $\approx 20\,\mu$m (mass $\sim 10^{-8}$\,g). This is comparable to the major dust on the surface of comet 1P/Halley ($\sim 10\mu$m) inferred from in-situ measurements and theoretical considerations. Since such dust aggregates with $\beta \approx 0.4$ must have survived on the surface against jets due to ice sublimation, the temperature of ice in the nucleus must be kept below 145\,K, which is much lower than equilibrium temperature determined by solar irradiation and thermal emission. These facts indicate that 9P/Tempel 1 has a dust mantle composed of $20\,\mu$m-sized dust aggregates with low thermal conductivities $\sim 1 \, {\rm erg\, cm}^{-1} \, {\rm K}^{-1}\,{\rm s}^{-1}$., Comment: 4 pages, 3 figures, accepted for publication in Astronomy and Astrophysics

We have investigated Saturn's core formation at a radial pressure maximum in a protoplanetary disk, which is created by gap opening by Jupiter. A core formed via planetesimal accretion induces the fragmentation of surrounding planetesimals, which generally inhibits further growth of the core by removal of the resulting fragments due to radial drift caused by gas drag. However, the emergence of the pressure maximum halts the drift of the fragments, while their orbital eccentricities and inclinations are efficiently damped by gas drag. As a result, the core of Saturn rapidly grows via accretion of the fragments near the pressure maximum. We have found that in the minimum-mass solar nebula, kilometer sized planetesimals can produce a core exceeding 10 Earth masses within two million years. Since Jupiter may not have undergone significant type II inward migration, it is likely that Jupiter's formation was completed when the local disk mass has already decayed to a value comparable to or less than Jovian mass. The expected rapid growth of Saturn's core on a timescale comparable to or shorter than observationally inferred disk lifetime enables Saturn to acquire the current amount of envelope gas before the disk gas is completely depleted. The high heat energy release rate onto the core surface due to the rapid accretion of the fragments delays onset of runaway gas accretion until the core mass becomes somewhat larger than that of Jupiter, which is consistent with the estimate based on interior modeling. Therefore, the rapid formation of Saturn induced by gap opening of Jupiter can account for the formation of multiple gas giants (Jupiter and Saturn) without significant inward migration and larger core mass of Saturn than that of Jupiter., Comment: Accepted for publication in ApJ

Rapid orbital drift of macroscopic dust particles is one of the major obstacles against planetesimal formation in protoplanetary disks. We reexamine this problem by considering porosity evolution of dust aggregates. We apply a porosity model based on recent N-body simulations of aggregate collisions, which allows us to study the porosity change upon collision for a wide range of impact energies. As a first step, we neglect collisional fragmentation and instead focus on dust evolution outside the snow line, where the fragmentation has been suggested to be less significant than inside the snow line because of a high sticking efficiency of icy particles. We show that dust particles can evolve into highly porous aggregates (with internal densities of much less than 0.1 g/cm^3) even if collisional compression is taken into account. We also show that the high porosity triggers significant acceleration in collisional growth. This acceleration is a natural consequence of particles' aerodynamical property at low Knudsen numbers, i.e., at particle radii larger than the mean free path of the gas molecules. Thanks to this rapid growth, the highly porous aggregates are found to overcome the radial drift barrier at orbital radii less than 10 AU (assuming the minimum-mass solar nebula model). This suggests that, if collisional fragmentation is truly insignificant, formation of icy planetesimals is possible via direct collisional growth of submicron-sized icy particles., Comment: 17 pages, 13 figures, ApJ, in press; typos corrected, references updated

(Abridged) We access the expected EKB dust disk properties by modeling. We treat the debiased population of the known transneptunian objects (TNOs) as parent bodies and generate the dust with our collisional code. The resulting dust distributions are modified to take into account the influence of gravitational scattering and resonance trapping by planets on migrating dust grains as well as the effect of sublimation. A difficulty is that the amount and distribution of dust are largely determined by sub-kilometer-sized bodies. These are directly unobservable, and their properties cannot be accessed by collisional modeling, because objects larger than 10...60m in the present-day EKB are not in a collisional equilibrium. To place additional constraints, we use in-situ measurements of the New Horizons spacecraft within 20AU. We show that the TNO population has to have a break in the size distribution at s<70km. However, even this still leaves us with several models that all correctly reproduce a nearly constant dust impact rates in the region of giant planet orbits and do not violate the constraints from the non-detection of the EKB dust thermal emission by the COBE spacecraft. The modeled EKB dust disks, which conform to the observational constraints, can either be transport-dominated or intermediate between the transport-dominated and collision-dominated regime. The in-plane optical depth of such disks is tau(r>10AU)~10^-6 and their fractional luminosity is f_d~10^-7. Planets and sublimation are found to have little effect on dust impact fluxes and dust thermal emission. The spectral energy distribution of an EKB analog, as would be seen from 10pc distance, peaks at wavelengths of 40...50\mum at F~0.5mJy, which is less than 1% of the photospheric flux at those wavelengths. Therefore, exact EKB analogs cannot be detected with present-day instruments such as Herschel/PACS., Comment: 10 pages, 8 figures, accepted for publication in Astronomy and Astophysics

The formation of giant planets requires accumulation of ~10 Earth mass in solids; but how do protoplanets acquire their mass? There are many, often competing processes that regulate the accretion rate of protoplanets. To assess their effects we present a new, publicly-available toy model. The rationale behind the toy model is that it encompasses as many physically-relevant processes as possible, but at the same time does not compromise its simplicity, speed, and physical insight. The toy model follows a modular structure, where key features -- e.g. planetesimal fragmentation, radial orbital decay, nebula turbulence -- can be switched on or off. Our model assumes three discrete components (fragments, planetesimals, and embryos) and is zero dimensional in space. We have tested the outcomes of the toy model against literature results and generally find satisfactory agreement. We include, for the first time, model features that capture the three-way interactions among small particles, gas, and protoplanets. Collisions among planetesimals will result in fragmentation, transferring a substantial amount of the solid mass to small particles, which couple strongly to the gas. Our results indicate that the efficiency of the accretion process then becomes very sensitive to the gas properties -- especially to the turbulent state and the magnitude of the disk headwind (the decrease of the orbital velocity of the gas with respect to Keplerian) -- as well as to the characteristic fragment size., Comment: accepted for publication in ApJ

Massive planetary cores ($\sim 10$ Earth masses) trigger rapid gas accretion to form gas giant planets \rev{such as} Jupiter and Saturn. We investigate the core growth and the possibilities for cores to reach such a critical core mass. At the late stage, planetary cores grow through collisions with small planetesimals. Collisional fragmentation of planetesimals, which is induced by gravitational interaction with planetary cores, reduces the amount of planetesimals surrounding them, and thus the final core masses. Starting from small planetesimals that the fragmentation rapidly removes, less massive cores are formed. However, planetary cores acquire atmospheres that enlarge their collisional cross section before rapid gas accretion. Once planetary cores exceed about Mars mass, atmospheres significantly accelerate the growth of cores. We show that, taking into account the effects of fragmentation and atmosphere, initially large planetesimals enable formation of sufficiently massive cores. On the other hand, because the growth of cores is slow for large planetesimals, a massive disk is necessary for cores to grow enough within a disk lifetime. If the disk with 100\,km-sized initial planetesimals is 10 times as massive as the minimum mass solar nebula, planetary cores can exceed 10 Earth masses in the Jovian planet region ($>5\,$AU)., Comment: accepted for publication in ApJ