12,637 research outputs found
Gap formation in a self-gravitating disk and the associated migration of the embedded giant planet
We present the results of our recent study on the interactions between a
giant planet and a self-gravitating gas disk. We investigate how the disk's
self-gravity affects the gap formation process and the migration of the giant
planet. Two series of 1-D and 2-D hydrodynamic simulations are performed. We
select several surface densities and focus on the gravitationally stable
region. To obtain more reliable gravity torques exerted on the planet, a
refined treatment of disk's gravity is adopted in the vicinity of the planet.
Our results indicate that the net effect of the disk's self-gravity on the gap
formation process depends on the surface density of the disk. We notice that
there are two critical values, \Sigma_I and \Sigma_II. When the surface density
of the disk is lower than the first one, \Sigma_0 < \Sigma_I, the effect of
self-gravity suppresses the formation of a gap. When \Sigma_0 > \Sigma_I, the
self-gravity of the gas tends to benefit the gap formation process and enlarge
the width/depth of the gap. According to our 1-D and 2-D simulations, we
estimate the first critical surface density \Sigma_I \approx 0.8MMSN. This
effect increases until the surface density reaches the second critical value
\Sigma_II. When \Sigma_0 > \Sigma_II, the gravitational turbulence in the disk
becomes dominant and the gap formation process is suppressed again. Our 2-D
simulations show that this critical surface density is around 3.5MMSN. We also
study the associated orbital evolution of a giant planet. Under the effect of
the disk's self-gravity, the migration rate of the giant planet increases when
the disk is dominated by gravitational turbulence. We show that the migration
timescale associates with the effective viscosity and can be up to 10^4 yr.Comment: 24 pages, 13 figures, accepted by RA
Migration and Growth of Protoplanetary Embryos I: Convergence of Embryos in Protoplanetary Disks
According to the core-accretion scenario, planets form in protostellar disks
through the condensation of dust, coagulation of planetesimals, and emergence
of protoplanetary embryos. At a few AU in a minimum mass nebula, embryos'
growth is quenched by dynamical isolation due to the depletion of planetesimals
in their feeding zone. However, embryos with masses () in the range of a
few Earth masses () migrate toward a transition radius between the
inner viscously heated and outer irradiated regions of their natal disk. Their
limiting isolation mass increases with the planetesimals surface density. When
, embryos efficiently accrete gas and evolve into cores of
gas giants. We use numerical simulation to show that, despite streamline
interference, convergent embryos essentially retain the strength of
non-interacting embryos' Lindblad and corotation torque by their natal disks.
In disks with modest surface density (or equivalently accretion rates), embryos
capture each other in their mutual mean motion resonances and form a convoy of
super Earths. In more massive disks, they could overcome these resonant
barriers to undergo repeated close encounters including cohesive collisions
which enable the formation of massive cores.Comment: 9 pages, 6 figures, accepted for publication in Ap
A Data Collecting Strategy for Farmland WSNs using a Mobile Sink
To the characteristics of large number of sensor nodes, wide area and unbalanced energy consumption in farmland Wireless Sensor Networks, an efficient data collection strategy (GCMS) based on grid clustering and a mobile sink is proposed. Firstly, cluster is divided based on virtual grid, and the cluster head is selected by considering node position and residual energy. Then, an optimal mobile path and residence time allocation mechanism for mobile sink are proposed. Finally, GCMS is simulated and compared with LEACH and GRDG. Simulation results show that GCMS can significantly prolong the network lifetime and increase the amount of data collection, especially suitable for large-scale farmland Wireless Sensor Networks
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