arXiv subjects

Ji-Lin Zhou

Publications and source records attributed to Ji-Lin Zhou.

80 records · Page 5Linked to original sources

arXiv2008

Planetesimal Accretion in Binary Systems: The Effects of Gas Dissipation

Currently, one of major problems concerning planet formation theory in close binary systems is, the strong perturbation from the companion star can increase relative velocities ($\triangle V$) of planetesimals around the primary and thus hinder their growth. According to previous studies, while gas drag can reduce the $\triangle V$ between bodies of the same sizes by forcing orbital alignment to planetesimals, it increases the $\triangle V$ among bodies of different sizes. In this paper, focusing on the $γ$ Cephei binary system, we propose a mechanism that can overcome this difficulty. We show that in a dissipating gas disk (with a typical dissipating timescale of $\sim 10^5-10^6$ years), all the planetesimals eventually converge towards the same forced orbits regardless of their sizes, leading to much lower impact velocities among them. These $\triangle V$ decrease processes progressively increase net mass accretion and even trigger runaway growth for large bodies (radius $>15$ km). The effect of size distribution of planetesimals is discussed, and found to be one of the dominant factors that determine the outcome of collisional evolution. Anyway, it can be concluded that by including the gas dissipation in the early stage of disk evolution, the conditions for planetesimal accretion become much better, and the process from planetesimal to planet-embryo can be carried out in close binary systems like $γ$ Cephei.

astro-ph↗

arXiv2008

Migration and Final Location of Hot Super Earths in the Presence of Gas Giants

Based on the conventional sequential-accretion paradigm, we have proposed that, during the migration of first-born gas giants outside the orbits of planetary embryos, super Earth planets will form inside the 2:1 resonance location by sweeping of mean motion resonances (Zhou et al. 2005). In this paper, we study the subsequent evolution of a super Earth (m_1) under the effects of tidal dissipation and perturbation from a first-born gas giant (m_2) in an outside orbit. Secular perturbation and mean motion resonances (especially 2:1 and 5:2 resonances) between m_1 and m_2 excite the eccentricity of m_1, which causes the migration of m_1 and results in a hot super Earth. The calculated final location of the hot super Earth is independent of the tidal energy dissipation factor Q'. The study of migration history of a Hot Super Earth is useful to reveal its Q' value and to predict its final location in the presence of one or more hot gas giants. When this investigation is applied to the GJ876 system, it correctly reproduces the observed location of GJ876d around 0.02AU.

astro-ph↗

arXiv2007

Planetesimal Accretion onto Growing Proto-Gas-Giant Planets

The solar and extra solar gas giants appear to have diverse internal structure and metallicities. We examine a potential cause for these dispersions in the context of the conventional sequential accretion formation scenario. In principle, gas accretion onto cores with masses below several times that of the Earth is suppressed by the energy released from the bombardment of residual planetesimals. We show that these planetesimals generally cannot pass through the mean motion resonances of the cores, and the suppression of planetesimal bombardment rate enables the cores to accrete gas with little interruption. During growth from the cores to protoplanets, as the resonances overlap with each other, the trapped planetesimals become dynamically unstable and their eccentricity excitation is strongly enhanced. Subsequent gas drag induces the planetesimals to migrate and collide with the protoplanet. This process leads to the resumption and a surge of planetesimal bombardment during the advanced stage of the protoplanet growth. This mechanism may account for the diversity of the core-envelope structure between Jupiter, Saturn and the metallicity dispersion inferred from the transiting extra solar planets.

astro-ph↗

arXiv2007

Do N-planet systems have a boundary between chaotic and regular motions?

Planetary systems consisting of one star and n planets with equal planet masses μand scaled orbital separation are referred as EMS systems. They represent an ideal model for planetary systems during the post-oligarchic evolution. Through the calculation of Lyapunov exponents, we study the boundary between chaotic and regular regions of EMS systems. We find that for n > 2, there does not exist a transition region in the initial separation space, whereas for n=2, a clear borderline occurs with relative separation ~ μ^{2/7} due to overlap of resonances (Wisdom, 1980). This phenomenon is caused by the slow diffusion of velocity dispersion (~ t^{1/2}, t is the time) in planetary systems with n >2, which leads to chaotic motions at the time of roughly two orders of magnitude before the orbital crossing occurs. This result does not conflict with the existence of transition boundary in the full phase space of N-body systems.

astro-ph↗

arXiv2007

Post-Oligarchic Evolution of Protoplanetary Embryos and the Stability of Planetary Systems

We investigate the orbit-crossing time (T_c) of protoplanet systems both with and without a gas-disk background. The protoplanets are initially with equal masses and separation (EMS systems) scaled by their mutual Hill's radii. In a gas-free environment, we find log (T_c/yr) = A+B \log (k_0/2.3). Through a simple analytical approach, we demonstrate that the evolution of the velocity dispersion in an EMS system follows a random walk. The stochastic nature of random-walk diffusion leads to (i) an increasing average eccentricity ~ t^1/2, where t is the time; (ii) Rayleigh-distributed eccentricities (P(e,t)=e/σ^2 \exp(-e^2/(2σ^2)) of the protoplanets; (iii) a power-law dependence of T_c on planetary separation. As evidence for the chaotic diffusion, the observed eccentricities of known extra solar planets can be approximated by a Rayleigh distribution. We evaluate the isolation masses of the embryos, which determine the probability of gas giant formation, as a function of the dust and gas surface densities.

astro-ph↗

arXiv2003

Occurrence and Stability of Apsidal Resonance in Multiple Planetary Systems

With the help of the Laplace-Lagrange solution of the secular perturbation theory in a double-planet system, we study the occurrence and the stability of apsidal secular resonance between the two planets. The explicit criteria to predict whether two planets are in apsidal resonance is derived. The pobability of two planets falling in apsidal resonance is given in the initial element space. We verify the criteria with numerical integrations for the HD12661 system, and find they give good predictions except at the boundary of the criteria or when the planet eccentricities are too large. The nonlinear stability of the two planets in HD12661 system are studied by calculating the Lyapunov exponents of their orbits in a general three-body model. The applications of the criteria to the other multiple planetary systems are discussed.

astro-ph↗

arXiv1998

Squeezed state dynamics of kicked quantum systems

We study kicked quantum systems by using the squeezed state approach. Taking the kicked quantum harmonic oscillator as an example, we demonstrate that chaos in an underlying classical system can be enhanced as well as suppressed by quantum fluctuations. Three different energy diffusions are observed in the kicked quantum harmonic oscillator, namely, localization, linear diffusion, and quadratic diffusion.

chao-dyn↗

arXiv1998

Quantum fluctuations: Enhancement or suppression of chaos?

We have studied the effects of quantum fluctuations on dynamical behavior by using squeezed state approach. Our numerical results of the kicked harmonic oscillator demonstrate qualitatively and quantitatively that quantum fluctuations can not only enhance chaos but also suppress classical diffusion. In addition, the squeezed state approach also gives a simple picture of dynamical localization.

chao-dyn↗