分类： 天文学 >> 天文学 提交时间： 2023-02-19
摘要： The nonlinear behaviour of low-viscosity warped discs is poorly understood. We verified a nonlinear bending-wave theory, in which fluid columns undergo affine transformations, with direct 3D hydrodynamical simulations. We employed a second-order Godunov-type scheme, Meshless Finite Mass (MFM), and also the Smoothed Particle Hydrodynamics (SPH) method, with up to 128M particles. For moderate nonlinearity, MFM maintains well the steady nonlinear warp predicted by the affine model for a tilted inviscid disc around a central object with a quadrupole moment. However, numerical dissipation in SPH is so severe that even a low-amplitude nonlinear warp degrades at a resolution where MFM performs well. A low-amplitude arbitrary warp tends to evolve towards a nonlinear steady state. However, no such state exists in our thin disc with an angular semi-thickness H/R = 0.02 when the outer tilt angle is beyond about 14 degrees. The warp breaks tenuously and reconnects in adiabatic simulations, or breaks into distinct annuli in isothermal simulations. The breaking radius lies close to the location with the most extreme nonlinear deformation. Parametric instability is captured only in our highest-resolution simulation, leading to ring structures that may serve as incubators for planets around binaries.
分类： 天文学 >> 天文学 提交时间： 2023-02-19
摘要： We study the initial development, structure and evolution of protoplanetary clumps formed in 3D resistive MHD simulations of self-gravitating disks. The magnetic field grows by means of the recently identified gravitational instability dynamo (Riols & Latter 2018; Deng et al. 2020). Clumps are identified and their evolution is tracked finely both backward and forward in time. Their properties and evolutionary path is compared to clumps in companion simulations without magnetic fields. We find that magnetic and rotational energy are important in the clumps' outer regions, while in the cores, despite appreciable magnetic field amplification, thermal pressure is most important in counteracting gravity. Turbulent kinetic energy is of a smaller scale than magnetic energy in the clumps. Compared to non-magnetized clumps, rotation is less prominent, which results in lower angular momentum in much better agreement with observations. In order to understand the very low sub-Jovian masses of clumps forming in MHD simulations, we revisit the perturbation theory of magnetized sheets finding support for a previously proposed magnetic destabilization in low-shear regions. This can help explaining why fragmentation ensues on a scale more than an order of magnitude smaller than that of the Toomre mass. The smaller fragmentation scale and the high magnetic pressure in clumps' envelopes explain why clumps in magnetized disks are typically in the super-Earth to Neptune mass regime rather than Super-Jupiters as in conventional disk instability. Our findings put forward a viable alternative to core accretion to explain widespread formation of intermediate-mass planets.
分类： 天文学 >> 天文学 提交时间： 2023-02-19
摘要： Intermediate mass planets, from Super-Earth to Neptune-sized bodies, are the most common type of planets in the galaxy. The prevailing theory of planet formation, core-accretion, predicts significantly fewer intermediate-mass giant planets than observed. The competing mechanism for planet formation, disk instability, can produce massive gas giant planets on wide-orbits, such as HR8799, by direct fragmentation of the protoplanetary disk. Previously, fragmentation in magnetized protoplanetary disks has only been considered when the magneto-rotational instability is the driving mechanism for magnetic field growth. Yet, this instability is naturally superseded by the spiral-driven dynamo when more realistic, non-ideal MHD conditions are considered. Here we report on MHD simulations of disk fragmentation in the presence of a spiral-driven dynamo. Fragmentation leads to the formation of long-lived bound protoplanets with masses that are at least one order of magnitude smaller than in conventional disk instability models. These light clumps survive shear and do not grow further due to the shielding effect of the magnetic field, whereby magnetic pressure stifles local inflow of matter. The outcome is a population of gaseous-rich planets with intermediate masses, while gas giants are found to be rarer, in qualitative agreement with the observed mass distribution of exoplanets.
分类： 天文学 >> 天文学 提交时间： 2023-02-19
摘要： In the core accretion model, planetesimals grow by mutual collisions and engulfing millimeter to centimeter particles, i.e., pebbles. Pebble accretion can significantly increase the accretion efficiency and help explain the presence of planets on wide orbits. However, the pebble supply is typically parameterized as a coherent pebble mass flux, sometimes being constant in space and time. Here we solve the dust advection and diffusion within viciously evolving protoplanetary disks to determine the pebble supply self-consistently. The pebbles are then accreted by planetesimals interacting with the gas disk via gas drags and gravitational torques. The pebble supply is variable with space and decays with time quickly with a pebble flux below 10 $M_\oplus$/Myr after 1 Myr in our models. As a result, only when massive planetesimals ($>$ 0.01 $M_\oplus$) are luckily produced by the streaming instability or the disk has low viscosity ($\alpha \sim 0.0001$), can the herd of planetesimals grows over Mars mass within 2 Myr. By then, planetesimals only capture pebbles about 50 times their mass and as little as 10 times beyond 20 au due to limited pebble supply. Further studies considering multiple dust species in various disk conditions are warranted to fully assess the realistic pebble supply and its influence on planetesimal growth.
分类： 天文学 >> 天文学 提交时间： 2023-02-19
摘要： In the core accretion model, planetesimals grow by mutual collisions and engulfing millimeter to centimeter particles, i.e., pebbles. Pebble accretion can significantly increase the accretion efficiency and help explain the presence of planets on wide orbits. However, the pebble supply is typically parameterized as a coherent pebble mass flux, sometimes being constant in space and time. Here we solve the dust advection and diffusion within viciously evolving protoplanetary disks to determine the pebble supply self-consistently. The pebbles are then accreted by planetesimals interacting with the gas disk via gas drags and gravitational torques. The pebble supply is variable with space and decays with time quickly with a pebble flux below 10 $M_\oplus$/Myr after 1 Myr in our models. As a result, only when massive planetesimals ($>$ 0.01 $M_\oplus$) are luckily produced by the streaming instability or the disk has low viscosity ($\alpha \sim 0.0001$), can the herd of planetesimals grows over Mars mass within 2 Myr. By then, planetesimals only capture pebbles about 50 times their mass and as little as 10 times beyond 20 au due to limited pebble supply. Further studies considering multiple dust species in various disk conditions are warranted to fully assess the realistic pebble supply and its influence on planetesimal growth.
分类： 天文学 >> 天文学 提交时间： 2023-02-19
摘要： We study the long-term radial transport of micron to mm-size grain in protostellar disks (PSDs) based on diffusion and viscosity coefficients measured from 3D global stratified-disk simulations with a Lagrangian hydrodynamic method. While gas-drag tend to transport dust species radially inwards, stochastic diffusion can spread a considerable fraction of dust radially outwards (upstream) depending on the nature of turbulence. In gravitationally unstable disks, we measure a high radial diffusion coefficient Dr with little dependence on altitude. This leads to strong and vertically homogeneous upstream diffusion in early PSDs. In the solar nebula, the robust upstream diffusion of micron to mm size grains not only efficiently transports highly refractory mocron-size grains (such as those identified in the samples of comet 81P/Wild 2) from their regions of formation inside the snow line out to the Kuiper Belt, but can also spread mm-size CAI formed in the stellar proximity to distances where they can be assimilated into chondritic meteorites. In disks dominated by magnetorotational instability (MRI), the upstream diffusion effect is generally milder, with a separating feature due to diffusion being stronger in the surface layer than the midplane. This variation becomes much more pronounced if we additionally consider a quiescent midplane with lower turbulence and larger characteristic dust size due to non-ideal MHD effects. This segregation scenario helps to account for dichotomy of two dust populations' spatial distribution as observed in scattered light and ALMA images.
分类： 天文学 >> 天文学 提交时间： 2023-02-19
Jian Ge
Hui Zhang
Weicheng Zang
Hongping Deng
Shude Mao
Ji-Wei Xie
Hui-Gen Liu
Ji-Lin Zhou
Kevin Willis
Chelsea Huang
Steve B. Howell
Fabo Feng
Jiapeng Zhu
Xinyu Yao
Beibei Liu
Masataka Aizawa
Wei Zhu
Ya-Ping Li
Bo Ma
Quanzhi Ye
摘要： We propose to develop a wide-field and ultra-high-precision photometric survey mission, temporarily named "Earth 2.0 (ET)". This mission is designed to measure, for the first time, the occurrence rate and the orbital distributions of Earth-sized planets. ET consists of seven 30cm telescopes, to be launched to the Earth-Sun's L2 point. Six of these are transit telescopes with a field of view of 500 square degrees. Staring in the direction that encompasses the original Kepler field for four continuous years, this monitoring will return tens of thousands of transiting planets, including the elusive Earth twins orbiting solar-type stars. The seventh telescope is a 30cm microlensing telescope that will monitor an area of 4 square degrees toward the galactic bulge. This, combined with simultaneous ground-based KMTNet observations, will measure masses for hundreds of long-period and free-floating planets. Together, the transit and the microlensing telescopes will revolutionize our understandings of terrestrial planets across a large swath of orbital distances and free space. In addition, the survey data will also facilitate studies in the fields of asteroseismology, Galactic archeology, time-domain sciences, and black holes in binaries.
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