Formation of the regular satellites of giant planets in an extended gaseous nebula I: subnebula model and accretion of satellites

We model the subnebulae of Jupiter and Saturn wherein satellite accretion took place. We expect each giant planet subnebula to be composed of an optically thick (given gaseous opacity) inner region inside of the planet’s centrifugal radius (where the specific angular momentum of the collapsing giant planet gaseous envelope achieves centrifugal balance, located at rCJ ∼ 15R_J for Jupiter and rCS ∼ 22R_S for Saturn) and an optically thin, extended outer disk out to a fraction of the planet’s Roche-lobe (R_H), which we choose to be ∼R_H/5 (located at ∼150 R_J near the inner irregular satellites for Jupiter, and ∼200R_S near Phoebe for Saturn). This places Titan and Ganymede in the inner disk, Callisto and Iapetus in the outer disk, and Hyperion in the transition region. The inner disk is the leftover of the gas accreted by the protoplanet. The outer disk may result from the nebula gas flowing into the protoplanet during the time of giant planet gap-opening (or cessation of gas accretion). For the sake of specificity, we use a solar composition “minimum mass” model to constrain the gas densities of the inner and outer disks of Jupiter and Saturn (and also Uranus). Our model has Ganymede at a subnebula temperature of ∼250 K and Titan at ∼100 K. The outer disks of Jupiter and Saturn have constant temperatures of 130 and 90 K, respectively.Our model has Callisto forming in a time scale ∼10⁶ years, Iapetus in 10⁶-10⁷ years, Ganymede in 10³-10⁴ years, and Titan in 10⁴-10⁵ years. Callisto takes much longer to form than Ganymede because it draws materials from the extended, low density portion of the disk; its accretion time scale is set by the inward drift times of satellitesimals with sizes 300-500 km from distances ∼100R_J. This accretion history may be consistent with a partially differentiated Callisto with a ∼300-km clean ice outer shell overlying a mixed ice and rock-metal interior as suggested by Anderson et al. (2001), which may explain the Ganymede-Callisto dichotomy without resorting to fine-tuning poorly known model parameters. It is also possible that particulate matter coupled to the high specific angular momentum gas flowing through the gap after giant planet gap-opening, capture of heliocentric planetesimals by the extended gas disk, or ablation of planetesimals passing through the disk contributes to the solid content of the disk and lengthens the time scale for Callisto’s formation. Furthermore, this model has Hyperion forming just outside Saturn’s centrifugal radius, captured into resonance by proto-Titan in the presence of a strong gas density gradient as proposed by Lee and Peale (2000). While Titan may have taken significantly longer to form than Ganymede, it still formed fast enough that we would expect it to be fully differentiated. In this sense, it is more like Ganymede than like Callisto (Saturn’s analog of Callisto, we expect, is Iapetus). An alternative starved disk model whose satellite accretion time scale for all the regular satellites is set by the feeding of planetesimals or gas from the planet’s Roche-lobe after gap-opening is likely to imply a long accretion time scale for Titan with small quantities of NH₃ present, leading to a partially differentiated (Callisto-like) Titan. The Cassini mission may resolve this issue conclusively. We briefly discuss the retention of elements more volatile than H₂O as well as other issues that may help to test our model.     

Periodical light variations of t tauri stars as a result of disk accretion

I argue that temperatures of spots, responsible for observed periodical light variations of T Tauri stars (TTS), are not known with reliable accuracy to discriminate between chromospheric and accretion theories of TTS 's phenomenon. The hypothesis is set up that spots on classical TTS (CTTS) are due to heating of stellar surface by radiation from a collisional accretion shock, whereas spots on weak line TTS (WTTS), at least in some cases, are connected with a collisionless accretion shock rather than chromospheric activity. Possible scenarios of WTTS interaction with circumstellar matter are discussed.     

2D/3D Dust Continuum Radiative Transfer Codes to Analyze and Predict VLTI Observations

Radiative Transfer (RT) codes with image capability are a fundamental tool for preparing interferometric observations and for interpreting visibility data. In view of the upcoming VLTI facilities, we present the first comparison of images/visibilities coming from two 3D codes that use completely different techniques to solve the problem of self-consistent continuum RT. In addition, we focus on the astrophysical case of a disk distorted by tidal interaction with by-passing stars or internal planets and investigate for which parameters the distortion can be best detected in the mid-infrared using the mid-infrared interferometric device MIDI. This revised version was published online in July 2006 with corrections to the Cover Date.     

Forced oscillations in a hydrodynamical accretion disk and QPOs

This is the second of a series of papers aimed to look for an explanation on the generation of high frequency quasi-periodic oscillations (QPOs) in accretion disks around neutron star, black hole, and white dwarf binaries. The model is inspired by the general idea of a resonance mechanism in the accretion disk oscillations as was already pointed out by Abramowicz and Klu’zniak (2001). In a first paper (P'etri, 2005a, paper I), we showed that a rotating misaligned magnetic field of a neutron star gives rise to some resonances close to the inner edge of the accretion disk. In this second paper, we suggest that this process does also exist for an asymmetry in the gravitational potential of the compact object. We prove that the same physics applies, at least in the linear stage of the response to the disturbance in the system. This kind of asymmetry is well suited for neutron stars or white dwarfs possessing an inhomogeneous interior allowing for a deviation from a perfectly spherically symmetric gravitational field. After a discussion on the magnitude of this deformation applied to neutron stars, we show by a linear analysis that the disk initially in a cylindrically symmetric stationary state is subject to {three kinds of resonances: a corotation resonance, a Lindblad resonance due to a driven force and a parametric resonance}. In a second part, we focus on the linear response of a thin accretion disk in the 2D limit. {Waves are launched at the aforementioned resonance positions and propagate in some permitted regions inside the disk, according to the dispersion relation obtained by a WKB analysis}. In a last part, these results are confirmed and extended via non linear hydrodynamical numerical simulations performed with a pseudo-spectral code solving Euler's equations in a 2D cylindrical coordinate frame. {We found that for a weak potential perturbation, the Lindblad resonance is the only effective mechanism producing a significant density fluctuation}. In a last step, we replaced the Newtonian potential by the so called logarithmically modified pseudo-Newtonian potential in order to take into account some general-relativistic effects like the innermost stable circular orbit (ISCO). The latter potential is better suited to describe the close vicinity of a neutron star or a black hole. However, from a qualitative point of view, the resonance conditions remain the same. The highest kHz QPOs are then interpreted as the orbital frequency of the disk at locations where the response to the resonances are maximal. It is also found that strong gravity is not required to excite the resonances.     

Evolution of Polytropic Reimann Disks in Halos

The equilibrium of elliptical Riemann disks with a polytropic equation of state and their evolution under the influence of viscosity and gravitational radiation inside spheroidal halos with a relative surface mass density k. The evolutionary trajectory of a disk inside a halo with k<0.5, which is analogous to the evolution of an isolated disk, differs from that of a disk inside a denser halo.     

黑洞吸积盘的演化及温度分布的热不稳定性

本文在Ｔｈｏｒｎｅ工作的基础上讨论了吸积盘中黑洞的有关参量的演化，以及由Ｓｃｈｗａｒｚｓｃｈｉｌｄ黑洞吸积盘向Ｋｅｒｒ黑洞吸积盘演化过程中对吸积盘辐射通量的影响，最后针对几个典型的辐射过程，分别讨论了黑洞吸积盘在牛顿框架中的温度分布方程与广义相对论的温度分布方程的热不稳定性，并给出此类问题的热不稳定性的判据。     

Dissipative-acoustic instability in accretion disks at a nonlinear stage

Nonstationary hydrodynamic models of a viscous accretion disk around a central compact object were constructed. Two different numerical methods (TVD and SPH) are used to study the dynamics of dissipatively unstable acoustic perturbations at the nonlinear stage in terms of the standard α-disk model. The standard disk accretion in the Shakura-Sunyaev model is unstable against acoustic waves for various parameters of the system. If the α parameter, which specifies the level of turbulent viscosity, exceeds α?0.03, then a complex nonstationary system of small-scale weak shock waves is formed. The growth rate of the perturbations is higher in the central disk region. For α?0.2, the relative shock amplitude can exceed 50% of the equilibrium disk parameters. The reflection of waves from the disk boundaries and their nonlinear interaction are important factors that can produce unsteady accretion. The luminosity of such a disk undergoes quasi-periodic oscillations at a level of several percent (?5%) of the equilibrium level.     

Observational Test of an Accretion Model of Star Formation in Galactic Clusters

Theoretical predictions of an accretion model of star formation in galactic clusters, published quite long ago, which is found to have some relevance with the current picture of star formation have been tested with observation of seven clusters of young and intermediate ages. It is found that the agreement between the theory and observation is very good.     

Particle accretion and dissipation simulator: Collisional aggregation of icy particles

As a part of a project to study the cometary particle environment, a self-consistent ballistic rigid body particle colliding simulator, capable of producing fractal aggregates of icy particles modelled by hard or soft (metaball) spheres has been developed, to be used for creating test particles in future statistical studies of the cometary coma-particle interaction. Instead of a predefined sticking probability, actual surface interactions are modelled and a method for calculating internal stresses and fragmentation of an aggregate during the accretion phase is described. Simulations suggest that taking fragmentation into account has two major consequences, increase in the fractal dimension of resulting particles and a noticeable change in the mass spectrum of an ensemble.     

海州湾淤泥质海滩剖面堆积过程的计算模式

分别于1986年12月和1985年3月1日—1987年3月1日在海州湾进行了波浪和含沙量观测，运用所获资料以及本区长期水文、地形资料，采用流体力学、沉积学和泥沙运动力学相结合的研究方法，建立了海州湾淤泥质海滩剖面堆积过程的二维计算模式。结果表明，在堆积型淤泥质海滩，由于浮泥的经常性存在，使波浪急剧衰减，其对岸滩的作用甚为微弱，潮流成为塑造淤泥质海滩的主要动力，岸滩在淤涨过程中，在平均高潮位厂沿和平均低潮位附近出现两个凸点，上一个凸点外推速率大于下一个凸点，潮下带变化缓慢。