arXiv subjects

Hui Li

Publications and source records attributed to Hui Li.

At least 919 records · Page 51Linked to original sources

arXiv2014

Effects of dust feedback on vortices in protoplanetary disks

We carried out two-dimensional high-resolution simulations to study the effect of dust feedback on the evolution of vortices induced by massive planets in protoplanetary disks. Various initial dust to gas disk surface density ratios ($0.001$ -- $0.01$) and dust particle sizes (Stokes number $4\times10^{-4}$ -- $0.16$) are considered. We found that while dust particles migrate inwards, vortices are very effective in collecting them. When dust density becomes comparable to gas density within the vortex, a dynamical instability is excited and it alters the coherent vorticity pattern and destroys the vortex. This dust feedback effect is stronger with higher initial dust/gas density ratio and larger dust grain. Consequently, we found that the disk vortex lifetime can be reduced up to a factor of 10. We discuss the implications of our findings on the survivability of vortices in protoplanetary disks and planet formation.

astro-ph.EP↗

arXiv2014

Formation of Hard Power-laws in the Energetic Particle Spectra Resulting from Relativistic Magnetic Reconnection

Using fully kinetic simulations, we demonstrate that magnetic reconnection in relativistic plasmas is highly efficient at accelerating particles through a first-order Fermi process resulting from the curvature drift of particles in the direction of the electric field induced by the relativistic flows. This mechanism gives rise to the formation of hard power-law spectra in parameter regimes where the energy density in the reconnecting field exceeds the rest mass energy density $σ\equiv B^2/(4 πn m_ec^2) > 1$ and when the system size is sufficiently large. In the limit $σ\gg 1$, the spectral index approaches $p=1$ and most of the available energy is converted into non-thermal particles. A simple analytic model is proposed which explains these key features and predicts a general condition under which hard power-law spectra will be generated from magnetic reconnection.

astro-ph.HE↗

arXiv2014

Scaling of magnetic reconnection in relativistic collisionless plasmas

Using fully kinetic simulations, we study the scaling of the inflow speed of collisionless magnetic reconnection from the non-relativistic to ultra-relativistic limit. In the anti-parallel configuration, the inflow speed increases with the upstream magnetization parameter $σ$ and approaches the light speed when $σ> O(100)$, leading to an enhanced reconnection rate. In all regimes, the divergence of pressure tensor is the dominant term responsible for breaking the frozen-in condition at the x-line. The observed scaling agrees well with a simple model that accounts for the Lorentz contraction of the plasma passing through the diffusion region. The results demonstrate that the aspect ratio of the diffusion region remains $\sim 0.1$ in both the non-relativistic and relativistic limits.

astro-ph.HE↗

arXiv2014

The structure of TeV-bright shell-type supernova remnants

Aims. Two-dimensional MHD simulations are used to model the emission properties of TeV-bright shell-type supernova remnants (SNRs) and to explore their nature. Methods. In the leptonic scenario for the TeV emission, the $γ$-ray emission is produced via Inverse Compton scattering of background soft photons by high-energy electrons accelerated by the shocks of the SNRs. The TeV emissivity is proportional to the magnetic field energy density and MHD simulations can be used to model the TeV structure of such remnants directly. 2D MHD simulations for SNRs are then performed under the assumption that the ambient interstellar medium is turbulent with the magnetic field and density fluctuations following a Kolmogorov-like power-law spectrum. Results. (1) As expected, these simulations confirm early 1D and 2D modelings of these sources, namely the hydrodynamical evolution of the shock waves and amplification of magnetic field by Rayleigh-Taylor convective flows and by shocks propagating in a turbulent medium; (2) We reproduce rather complex morphological structure for $γ$-rays, suggesting intrinsic variations of the source morphology not related to the structure of the progenitor and environment; (3)The observed radial profile of several remnants are well reproduced with an ambient medium density of $0.1-1$ cm$^{-3}$. An even lower ambient density leads to a sharper drop of the TeV brightness with radius than what is observed near the outer edge of these remnants. Conclusions. In a turbulent background medium, we can reproduce the observed characteristics of several shell-type TeV SNRs with reasonable parameters except for a higher ambient density than that inferred from X-ray observations.

astro-ph.HE↗

arXiv2014

Three-Dimensional MHD Simulation of Caltech Plasma Jet Experiment: First Results

Magnetic fields are believed to play an essential role in astrophysical jets with observations suggesting the presence of helical magnetic fields. Here, we present three-dimensional (3D) ideal MHD simulationsof the Caltech plasma jet experiment using a magnetic tower scenario as the baseline model. Magnetic fields consist of an initially localized dipole-like poloidal component and a toroidal component that is continuously being injected into the domain. This flux injection mimics the poloidal currents driven by the anode-cathode voltage drop in the experiment. The injected toroidal field stretches the poloidal fields to large distances, while forming a collimated jet along with several other key features. Detailed comparisons between 3D MHD simulations and experimental measurements provide a comprehensive description of the interplay among magnetic force, pressure and flow effects. In particular, we delineate both the jet structure and the transition process that converts the injected magnetic energy to other forms. With suitably chosen parameters that are derived from experiments, the jet in the simulation agrees quantitatively with the experimental jet in terms of magnetic/kinetic/inertial energy, total poloidal current, voltage, jet radius, and jet propagation velocity. Specifically, the jet velocity in the simulation is proportional to the poloidal current divided by the square root of the jet density, in agreement with both the experiment and analytical theory. This work provides a new and quantitative method for relating experiments, numerical simulations and astrophysical observation, and demonstrates the possibility of using terrestrial laboratory experiments to study astrophysical jets.

astro-ph.HE↗

arXiv2014

Resonances of Multiple Exoplanets and Implications for Their Formation

Among $\sim 160$ of the multiple exoplanetary systems confirmed, about $30\%$ of them have neighboring pairs with a period ratio $\leq 2$. A significant fraction of these pairs are around mean motion resonance (MMR), more interestingly, peak around 2:1 and 3:2, with a clear absence of more closely packed MMRs with period ratios less than 4:3, regardless of planet masses. Here we report numerical simulations demonstrating that such MMR behavior places important constraints on the disk evolution stage out of which the observed planets formed. Multiple massive planets (with mass $\geq 0.8$ $M_{\rm Jup}$) tend to end up with a 2:1 MMR mostly independent of the disk masses but low-mass planets (with mass $\leq 30$ $M_{\oplus}$) can have MMRs larger than 4:3 only when the disk mass is quite small, suggesting that the observed dynamical architecture of most low-mass-planet pairs was established late in the disk evolution stage, just before it was dispersed completely.

astro-ph.EP↗

arXiv2014

Basic Bell-MHD Turbulence

Nonresonant current instability was identified by Bell (2004) as an important mechanism for magnetic field amplification in supernova remnants. In this paper we focus on studying the nonlinear stage of this instability using the incompressible MHD formulation. We demonstrate that the evolution of magnetic turbulence driven by the Bell instability resembles turbulence driven on large scales. More importantly, we demonstrate that the energy-containing scale for magnetic fields is proportional to the square root of magnetic energy density. Given the observational constraints of the possible field amplification, this new relation allows us to directly estimate the maximum energy of particles scattered by such fields and this estimate is normally below the average particle energy. This implies that, without taking into account the feedback to cosmic rays, the typical scales of Bell fields, in either linear or nonlinear regime, will be too small to affect high energy particle acceleration. We mention several scenarios of back-reaction to cosmic rays that could be important.

astro-ph.GA↗

arXiv2014

Thermodynamical aspects of modified holographic dark energy model

We investigate the unified first law and the generalized second law in a modified holographic dark energy model. The thermodynamical analysis on the apparent horizon can work and the corresponding entropy formula is extracted from the systematic algorithm. The entropy correction term depends on the extra-dimension number of the brane as expected, but the interplay between the correction term and the extra dimensions is more complicated. With the unified first law of thermodynamics well-founded, the generalized second law of thermodynamics is discussed and it's found that the second law can be violated in certain circumstances. Particularly, if the number of the extra dimensions is larger than one, the generalized law of thermodynamics is always satisfied; otherwise, the validity of the second law can only be guaranteed with the Hubble radius greatly smaller than the crossover scale $r_c$ of the $5$-dimensional DGP model.

gr-qc↗

arXiv2014

Long Term Evolution of Planet-Induced Vortices in Protoplanetary Disks

Recent observations of large-scale asymmetric features in protoplanetary disks suggest that large-scale vortices exist in such disks. Massive planets are known to be able to produce deep gaps in protoplanetary disks. The gap edges could become hydrodynamically unstable to the Rossby wave/vortex instability and form large-scale vortices. In this study we examine the long term evolution of these vortices by carrying out high-resolution two dimensional hydrodynamic simulations that last more than $10^4$ orbits (measured at the planet's orbit). We find that the disk viscosity has a strong influence on both the emergence and lifetime of vortices. In the outer disk region where asymmetric features are observed, our simulation results suggest that the disk viscous $α$ needs to be low $\sim 10^{-5 }$ - $10^{-4}$ to sustain vortices to thousands and up to $10^{4}$ orbits in certain cases. The chance of finding a vortex feature in a disk then decreases with smaller planet orbital radius. For $α\sim 10^{-3}$ or larger, even planets with masses of 5 Jupiter-masses will have difficulty either producing or sustaining vortices. We have also studied the effects of different disk temperatures and planet masses. We discuss the implications of our findings on current and future protoplanetary disk observations.

astro-ph.EP↗

arXiv2014

Twisted Topological Graph Algebras

We define the notion of a twisted topological graph algebra associated to a topological graph and a $1$-cocycle on its edge set. We prove a stronger version of a Vasselli's result. We expand Katsura's results to study twisted topological graph algebras. We prove a version of the Cuntz-Krieger uniqueness theorem, describe the gauge-invariant ideal structure. We find that a twisted topological graph algebra is simple if and only if the corresponding untwisted one is simple.

math.OA↗

arXiv2014

Temporal Evolution of the Magnetic Topology of the NOAA Active Region 11158

We studied the temporal evolution of the magnetic topology of the active region (AR) 11158 based on the reconstructed three-dimensional magnetic fields in the corona. The \nlfff\ extrapolation method was applied to the 12 minutes cadence data obtained with the \hmi\ (HMI) onboard the \sdo\ (SDO) during five days. By calculating the squashing degree factor Q in the volume, the derived quasi-separatrix layers (QSLs) show that this AR has an overall topology, resulting from a magnetic quadrupole, including an hyperbolic flux tube (HFT) configuration which is relatively stable at the time scale of the flare ($\sim 1-2$ hours). A strong QSL, which corresponds to some highly sheared arcades that might be related to the formation of a flux rope, is prominent just before the M6.6 and X2.2 flares, respectively. These facts indicate the close relationship between the strong QSL and the high flare productivity of AR 11158. In addition, with a close inspection of the topology, we found a small-scale HFT which has an inverse tear-drop structure above the aforementioned QSL before the X2.2 flare. It indicates the existence of magnetic flux rope at this place. Even though a global configuration (HFT) is recognized in this AR, it turns out that the large-scale HFT only plays a secondary role during the eruption. In final, we dismiss a trigger based on the breakout model and highlight the central role of the flux rope in the related eruption.

astro-ph.SR↗

arXiv2014

Priority-Aware Private Matching Schemes for Proximity-Based Mobile Social Networks

The rapid developments of mobile devices and online social networks have resulted in increasing attention to Mobile Social Networking (MSN). The explosive growth of mobile-connected and location-aware devices makes it possible and meaningful to do the Proximity-based Mobile Social Networks (PMSNs). Users can discover and make new social interactions easily with physical-proximate mobile users through WiFi/Bluetooth interfaces embedded in their smartphones. However, users enjoy these conveniences at the cost of their growing privacy concerns. To address this problem, we propose a suit of priority-aware private matching schemes to privately match the similarity with potential friends in the vicinity. Unlike most existing work, our proposed priority-aware matching scheme (P-match) achieves the privacy goal by combining the commutative encryption function and the Tanimoto similarity coefficient which considers both the number of common attributes between users as well as the corresponding priorities on each common attribute. Further, based on the newly constructed similarity function which takes the ratio of attributes matched over all the input set into consideration, we design an enhanced version to deal with some potential attacks such as unlimitedly inputting the attribute set on either the initiator side or the responder side, etc. Finally, our proposed E-match avoids the heavy cryptographic operations and improves the system performance significantly by employing a novel use of the Bloom filter. The security and communication/computation overhead of our schemes are thoroughly analyzed and evaluated via detailed simulations and implementation.

cs.CR↗

arXiv2013

Relativistic MHD Simulations of Poynting Flux-Driven Jets

Relativistic, magnetized jets are observed to propagate to very large distances in many Active Galactic Nuclei (AGN). We use 3D relativistic MHD (RMHD) simulations to study the propagation of Poynting flux-driven jets in AGN. These jets are assumed already being launched from the vicinity ($\sim 10^3$ gravitational radii) of supermassive black holes. Jet injections are characterized by a model described in Li et al. (2006) and we follow the propagation of these jets to ~ parsec scales. We find that these current-carrying jets are always collimated and mildly relativistic. When $α$, the ratio of toroidal-to-poloidal magnetic flux injection, is large the jet is subject to non-axisymmetric current-driven instabilities (CDI) which lead to substantial dissipation and reduced jet speed. However, even with the presence of instabilities, the jet is not disrupted and will continue to propagate to large distances. We suggest that the relatively weak impact by the instability is due to the nature of the instability being convective and the fact that the jet magnetic fields are rapidly evolving on Alfvénic timescale. We present the detailed jet properties and show that far from the jet launching region, a substantial amount of magnetic energy has been transformed into kinetic energy and thermal energy, producing a jet magnetization number $σ< 1$. In addition, we have also studied the effects of a gas pressure supported "disk" surrounding the injection region and qualitatively similar global jet behaviors were observed. We stress that jet collimation, CDIs, and the subsequent energy transitions are intrinsic features of current-carrying jets.

astro-ph.HE↗

arXiv2013

Agriculture driving male expansion in Neolithic Time

The emergence of agriculture is suggested to have driven extensive human population growths. However, genetic evidence from maternal mitochondrial genomes suggests major population expansions began before the emergence of agriculture. Therefore, role of agriculture that played in initial population expansions still remains controversial. Here, we analyzed a set of globally distributed whole Y chromosome and mitochondrial genomes of 526 male samples from 1000 Genome Project. We found that most major paternal lineage expansions coalesced in Neolithic Time. The estimated effective population sizes through time revealed strong evidence for 10- to 100-fold increase in population growth of males with the advent of agriculture. This sex-biased Neolithic expansion might result from the reduction in hunting-related mortality of males.

q-bio.PE↗

arXiv2013

Global patterns of sex-biased migrations in humans

A series of studies have revealed the among-population components of genetic variation are higher for the paternal Y chromosome than for the maternal mitochondrial DNA (mtDNA), which indicates sex-biased migrations in human populations. However, this phenomenon might be also an ascertainment bias due to nonrandom sampling of SNPs. To eliminate the possible bias, we used the whole Y chromosome and mtDNA sequence data of 491 individuals from the 1000 Genomes Project Phase I to address the sex-biased migration dispute. We found that genetic differentiation between populations was higher for Y chromosome than for the mtDNA at global scales. The migration rate of female might be three times higher than that of male, assuming the effective population size is the same for male and female.

q-bio.PE↗

arXiv2013

Discovery of Phylogenetic Relevant Y-chromosome Variants in 1000 Genomes Project Data

Current Y chromosome research is limited in the poor resolution of Y chromosome phylogenetic tree. Entirely sequenced Y chromosomes in numerous human individuals have only recently become available by the advent of next-generation sequencing technology. The 1000 Genomes Project has sequenced Y chromosomes from more than 1000 males. Here, we analyzed 1000 Genomes Project Y chromosome data of 1269 individuals and discovered about 25,000 phylogenetic relevant SNPs. Those new markers are useful in the phylogeny of Y chromosome and will lead to an increased phylogenetic resolution for many Y chromosome studies.

q-bio.PE↗

arXiv2013

Natural selection on human Y chromosomes

The paternally inherited Y chromosome has been widely used in population genetic studies to understand relationships among human populations. Our interpretation of Y chromosomal evidence about population history and genetics has rested on the assumption that all the Y chromosomal markers in the male-specific region (MSY) are selectively neutral. However, the very low diversity of Y chromosome has drawn a long debate about whether natural selection has affected this chromosome or not. In recent several years, the progress in Y chromosome sequencing has helped to address this dispute. Purifying selection has been detected in the X-degenerate genes of human Y chromosomes and positive selection might also have an influence in the evolution of testis-related genes in the ampliconic regions. Those new findings remind us to take the effect of natural selection into account when we use Y chromosome in population genetic studies.

q-bio.PE↗

arXiv2013

Convergence of Y chromosome STR haplotypes from different SNP haplogroups compromises accuracy of haplogroup prediction

Short tandem repeats (STRs) and single nucleotide polymorphisms (SNPs) are two kinds of commonly used markers in Y chromosome studies of forensic and population genetics. There has been increasing interest in the cost saving strategy by using the STR haplotypes to predict SNP haplogroups. However, the convergence of Y chromosome STR haplotypes from different haplogroups might compromise the accuracy of haplogroup prediction. Here, we compared the worldwide Y chromosome lineages at both haplogroup level and haplotype level to search for the possible haplotype similarities among haplogroups. The similar haplotypes between haplogroups B and I2, C1 and E1b1b1, C2 and E1b1a1, H1 and J, L and O3a2c1, O1a and N, O3a1c and O3a2b, and M1 and O3a2 have been found, and those similarities reduce the accuracy of prediction.

q-bio.PE↗