Initial Density Profile Research Articles

FRAGMENTATION IN DUSTY LOW-METALLICITY STAR-FORMING HALOS

The first stars in the universe, termed Population III, are thought to have been very massive compared to the stars that form in the present epoch. As feedback from the first generation of stars altered the contents of the interstellar medium, the universe switched to a low-mass mode of star formation, which continues in the high metallicity stars formed in the present era. Several studies have investigated the transition between metal-free and metal-enriched star formation, with tentative evidence being found for a metallicity threshold near 10^-3.5 Z_sun due to atomic and molecular transitions and another threshold near 10^-5.5 Z_sun due to dust. In this work, we simulate the formation of stars in idealized low-metallicity halos using the AMR code Enzo. We conduct several simulations of 10^6 M_sun and 10^7 M_sun halos in which the metal content, initial rotation, and degree of turbulence are varied in order to study the effect of these properties on gas fragmentation over a range of densities. We find tentative support for the idea of a critical metallicity, but the effect of varying metallicity on the gas we observe is not as dramatic as what has been reported in earlier studies. We find no clear relation between the initial spin or the initial level of turbulence in the halo and the final properties of the gas contained therein. Additionally, we find that the degree to which the Jeans length is refined, the initial density profile of the gas, and the inclusion of deuterium chemistry each have a significant effect on the evolution and fragmentation of the gas in the halo - in particular, we find that at least 64 grid cells are needed to cover the Jeans length in order to properly resolve the fragmentation.

The virialization density of peaks with general density profiles under spherical collapse

We calculate the non-linear virialization density, Δc, of halos under spherical collapse from peaks with an arbitrary initial and final density profile. This is in contrast to the standard calculation of Δc which assumes top-hat profiles. Given our formalism, the non-linear halo density can be calculated once the shape of the initial peak's density profile and the shape of the virialized halo's profile are provided. We solve for Δc for halos in an Einstein de-Sitter and a ΛCDM universe. As examples, we consider power-law initial profiles as well as spherically averaged peak profiles calculated from the statistics of a Gaussian random field.We find that, depending on the profiles used, Δc is smaller by a factor of a few to as much as a factor of 10 as compared to the density given by the standard calculation ( ≈ 200). Using our results, we show that, for halo finding algorithms that identify halos through an over-density threshold, the halo mass function measured from cosmological simulations can be enhanced at all halo masses by a factor of a few. This difference could be important when using numerical simulations to assess the validity of analytic models of the halo mass function.

Elliptic and hexadecapole flow of charged hadrons in viscous hydrodynamics with Glauber and color glass condensate initial conditions for Pb–Pb collision at = 2.76 TeV

The experimentally measured elliptic (v2) and hexadecapole (v4) flow of charged particles as a function of transverse momentum (pT) at midrapidity in Pb–Pb collisions at = 2.76 TeV is compared with the relativistic viscous hydrodynamic model simulations. The simulations are carried out for two different initial energy density profiles obtained from (i) the Glauber model, and (ii) the color glass condensate (CGC) model. A comparison to experimental data for 10–20% to 40–50% centrality shows that a centrality dependent shear viscosity to entropy density (η/s) ratio with values ranging between 0.0 to 0.12 is needed to explain the v2 data for simulations with the Glauber based initial condition, whereas for the CGC based initial conditions a slightly higher value of η/s is preferred, around 0.08 to 0.16. From the comparison of the v4 simulated results to the corresponding experimental measurements we observe that for the centralities 20–30% to 40–50% the η/s values lie between 0.0 to 0.12 for both the initial conditions studied. The η/s values obtained from our studies for Pb–Pb collisions at = 2.76 TeV are compared to other studies which use both transport and hydrodynamic approaches.

Hot-electron generation from laser–pre-plasma interactions in cone-guided fast ignition

Two-dimensional particle-in-cell (PIC) simulations were performed for the cone-in-shell integrated fast-ignition experiments at the Omega Laser Facility [W. Theobald et al., Phys. Plasmas 18, 056305 (2011)]. The initial plasma density profile in the PIC simulations was taken from hydrodynamic simulations of the prepulse interaction with the gold cone. Hot-electron generation from laser–pre-plasma interactions and transport up to 100× the critical density (nc) was studied. The simulation showed a mean divergence half-angle of 68° and 50% absorption for the hot electrons. The simulation results show that the generated hot electrons were dominated in number by low-energy electrons but in energy by multi-MeV electrons. Electron transport between 5 and 100 nc was ballistic. In the late stage of the simulation, all the results were largely independent of polarization, indicating a stochastic hot-electron–generation mechanism.

Constraining the initial temperature and shear viscosity in a hybrid hydrodynamic model ofsNN=200GeV Au+Au collisions using pion spectra, elliptic flow, and femtoscopic radii

A new framework for evaluating hydrodynamic models of relativistic heavy ion collisions has been developed. This framework, a Comprehesive Heavy Ion Model Evaluation and Reporting Algorithm (CHIMERA) has been implemented by augmenting UVH 2+1D viscous hydrodynamic model with eccentricity fluctuations, pre-equilibrium flow, and the Ultra-relativistic Quantum Molecular Dynamic (UrQMD) hadronic cascade. A range of initial temperatures and shear viscosity to entropy ratios were evaluated for four initial profiles, $N_{part}$ and $N_{coll}$ scaling with and without pre-equilibrium flow. The model results were compared to pion spectra, elliptic flow, and femtoscopic radii from 200 GeV Au+Au collisions for the 0--20% centrality range.Two sets of initial density profiles, $N_{part}$ scaling with pre-equilibrium flow and $N_{coll}$ scaling without were shown to provide a consistent description of all three measurements.

Nonlocal Lagrangian bias

Halos are biased tracers of the dark matter distribution. It is often assumed that the patches from which halos formed are locally biased with respect to the initial fluctuation field, meaning that the halo-patch fluctuation field can be written as a Taylor series in that of the dark matter. If quantities other than the local density influence halo formation, then this Lagrangian bias will generically be nonlocal; the Taylor series must be performed with respect to these other variables as well. We illustrate the effect with Monte-Carlo simulations of a model in which halo formation depends on the local shear (the quadrupole of perturbation theory), and provide an analytic model which provides a good description of our results. Our model, which extends the excursion set approach to walks in more than one dimension, works both when steps in the walk are uncorrelated, as well as when there are correlations between steps. For walks with correlated steps, our model includes two distinct types of nonlocality: one is due to the fact that the initial density profile around a patch which is destined to form a halo must fall sufficiently steeply around it -- this introduces k-dependence to even the linear bias factor, but otherwise only affects the monopole of the clustering signal. The other is due to the surrounding shear field; this affects the quadratic and higher order bias factors, and introduces an angular dependence to the clustering signal. In both cases, our analysis shows that these nonlocal Lagrangian bias terms can be significant, particularly for massive halos; they must be accounted for in analyses of higher order clustering such as the halo bispectrum in Lagrangian or Eulerian space. Although we illustrate these effects using halos, our analysis and conclusions also apply to the other constituents of the cosmic web -- filaments, sheets and voids.

ORIGIN OF THE DENSE CORE MASS FUNCTION IN CONTRACTING FILAMENTS

Mass functions of starless dense cores (CMFs) may arise from contraction and dispersal of core-forming filaments. In an illustrative model, a filament contracts radially by self-gravity, increasing the mass of its cores. During this contraction, FUV photoevaporation and ablation by shocks and winds disperse filament gas and limit core growth. The stopping times of core growth are described by a waiting-time distribution. The initial filament column density profile and the resulting CMF each match recent Herschel observations in detail. Then low-mass cores have short growth ages and arise from the innermost filament gas, while massive cores have long growth ages and draw from more extended filament gas. The model fits the initial density profile and CMF best for mean core density 2 10^4 cm^-3 and filament dispersal time scale 0.5 Myr. Then the typical core mass, radius, mean column density, and contraction speed are respectively 0.8 solar masses, 0.06 pc, 6 10^21 cm^-2, and 0.07 km s^-1, also in accord with observed values.

Laser-Plasma Electron Acceleration with Initial Plasma Profile

An accelerated electron beam could be generated during the ultra short-high intensity laser pulse propagation in a plasma. It could be obtained by wake field acceleration, which highly depends on the plasma profile that may affect the electron energy spectrum. In this study, we use the Maxwell and hydrodynamic equations to obtain the electron density distribution and magnetic and electric field profiles in the plasma. According to our numerical calculations, the electrical field could be highly increased by fitting an initial plasma density profile. These considerations lead to the designing of a gas jet nozzle which could be useful for laser-plasma interaction experimental research. We have calculated the electric and magnetic field amplitudes in the laser propagation direction for various intensities and wavelengths. Our results show considerable field boosting when the laser wavelength or the laser intensity are increased.

Residual turbulence from velocity shear stabilized interchange instabilities

The stabilizing effect of velocity shear on the macroscopic, broad bandwidth, ideal interchange instability is studied in linear and nonlinear regimes. A 2D dissipative magnetohydrodynamic (MHD) code is employed to simulate the system. For a given flow shear, V′, linear growth rates are shown to be suppressed to below the shear-free level at both the small and large wavelengths. With increasing V′, the unstable band in wavenumber-space shrinks so that the peak growth results for modes that correspond to relatively high wavenumbers, on the scale of the density gradient. In the nonlinear turbulent steady state, a similar turbulent spectrum obtains, and the convection cells are roughly circular. In addition, the density fluctuation level and the degree of flattening of the initial inverted density profile are found to decrease as V′ increases; in fact, unstable modes are almost completely stabilized and the density profile reverts to laminar when V′ is a few times the classic interchange growth rate. Moreover, the turbulent particle flux diminishes with increasing velocity shear such that all the flux is carried by the classical diffusive flux in the asymptotic limit. The simulations are compared with measurements of magnetic fluctuations from the Maryland Centrifugal Experiment, MCX, which investigated interchange modes in the presence of velocity shear. The experimental spectral data, taken in the plasma edge, are in general agreement with the numerical data obtained in higher viscosity simulations for which the level of viscosity is chosen consistent with MCX Reynolds numbers at the edge. In particular, the residual turbulence in both cases is dominated by elongated convection cells. Finally, concomitant Kelvin-Helmholtz instabilities in the system are also examined. Complete stability to interchanges is obtained only in the parameter space wherein the generalized Rayleigh inflexion theorem is satisfied.

A Lagrangian model for phototaxis-induced thin layer formation

We have developed a Lagrangian model to investigate a potential mechanism based on phototaxis behavior of phytoplankton cells for the formation of thin layers. We assume that all cells follow a time-regulated diurnal vertical migration during which they experience photo-acclimation based on the Denman and Marra (1986) model. When a cell experiences stress due to strong light that exceeds a threshold level, the cell swims downward, away from the light. We applied the Lagrangian model to a one dimensional second order turbulence closure model that generates a realistic surface mixing condition for a given set of physical parameters, such as wind and optical water type. For the chosen swimming velocities and prescribed behavior, we found that, in coastal water type and Jerlov III type, thin layer formation takes place up to 5ms−1 winds, while 10ms−1 winds cause sufficiently strong mixing to prevent the formation of thin layer. We have also investigated the effects of changing the irradiance threshold for the onset of the photoinhibition, the initial density profile and random walk swimming. In conclusion, thin layer formation due to photoinhibition may be possible for a low value of photoinhibition threshold that may occur either due to upwelling or strong light exposure.

Dynamical freeze-out in event-by-event hydrodynamics

In hydrodynamical modeling of the ultrarelativistic heavy-ion collisions the freeze-out is typically performed at a constant temperature or density. In this work we apply a dynamical freeze-out criterion, which compares the hydrodynamical expansion rate with the pion scattering rate. Recently many calculations have been done using event-by-event hydrodynamics where the initial density profile fluctuates from event to event. In these event-by-event calculations the expansion rate fluctuates strongly as well, and thus it is interesting to check how the dynamical freeze-out changes hadron distributions with respect to the constant temperature freeze-out. We present hadron spectra and elliptic flow calculated using (2+1)-dimensional ideal hydrodynamics, and show the differences between constant temperature and dynamical freeze-out criteria. We find that the differences caused by different freeze-out criteria are small in all studied cases.

Lunar-forming impacts: High-resolution SPH and AMR-CTH simulations

We present results of the highest-resolution simulations to date of potential Moon-forming impacts using a Lagrangian, particle-based method (smooth particle hydrodynamics, or SPH) and an Eulerian, grid-based method with adaptive mesh refinement (AMR-CTH). We consider a few candidate impacts advocated by recent works, directly comparing simulations performed at varying resolutions and with both numerical methods and their predictions for the properties of resulting protolunar disks. For a fixed set of impact conditions, simulations with either method and with different resolutions yield very similar results for the initial impact and the first few hours of the post-impact period. The subsequent disk properties in the ∼5–20h time period can vary substantially from case-to-case, depending on the orbits of and mutual interactions between large bound clumps of ejecta that often form after the initial impact. After such clumps have completed at least one orbit (which typically requires ∼25–50h), the predicted protolunar disk mass and its angular momentum converge to within about 10% for simulations of very similar impact conditions using different resolutions or methods. The disks produced by the CTH simulations are consistently about 10% less massive than those produced by SPH simulations, due presumably to inherent differences between the codes. The two methods predict broadly similar values for the fraction of the protolunar disk that originates from the target vs. the impactor, and for the initial disk radial surface density and temperature profiles.

Fluctuations of current in nonstationary diffusive lattice gases

We employ the macroscopic fluctuation theory to study fluctuations of integrated current in one-dimensional lattice gases with a steplike initial density profile. We analytically determine the variance of the current fluctuations for a class of diffusive processes with a density-independent diffusion coefficient. Our calculations rely on a perturbation theory around the noiseless hydrodynamic solution. We consider both quenched and annealed types of averaging (the initial condition is allowed to fluctuate in the latter situation). The general results for the variance are specialized to a few interesting models including the symmetric exclusion process and the Kipnis-Marchioro-Presutti model [Kipnis, Marchioro, and Presutti, J. Stat. Phys. 27, 65 (1982)]. We also probe large deviations of the current for the symmetric exclusion process. This is done by numerically solving the governing equations of the macroscopic fluctuation theory using an efficient iteration algorithm.

ON THE DYNAMICAL FORMATION OF VERY YOUNG, X-RAY EMITTING BLACK HOLE BINARIES IN DENSE STAR CLUSTERS

We recently discovered a population of very young ({tau} {approx}< 6-8 Myr), X-ray emitting black hole binaries (BHBs) in the nearby starburst galaxy NGC 4449. These BHBs are located within or near to very young star clusters, indicating that they form within the clusters, but that some fraction are dynamically ejected. Here we present results from a suite of N-body simulations of N = 16,384 ({approx}6000 M{sub Sun }) star clusters, similar to the masses of BHB hosts in NGC 4449, through the first 10 Myr of their lives. Our goal is to determine whether dynamical interactions are responsible for the observed population of BHBs in NGC 4449. Our simulations span a wide range of initial size and density profiles, both with and without primordial mass segregation, testing both realistic initial conditions and extreme ones. We find that clusters without primordial mass segregation only dynamically produce BHBs within 10 Myr when they are extremely compact and centrally concentrated. Preliminary results that include primordial binaries support this conclusion. The introduction of strong primordial mass segregation, however, greatly increases the rapidity with which the binaries form, although these are still not tight enough that they will emit X-rays. We conclude that X-raymore » emitting BHBs are unlikely to form dynamically in clusters of this mass under realistic conditions. Instead, they probably originate from binaries that contain two massive stars with small orbital separations, which are present from the cluster's birth.« less

Comparison of results from a (2+1)-D relativistic viscous hydrodynamic model to elliptic and hexadecapole flow of charged hadrons measured in Au-Au collisions atsNN=200GeV

Simulated results from a (2+1)-D relativistic viscous hydrodynamic model have been compared to the experimental data on the centrality dependence of invariant yield, elliptic flow (${v}_{2}$), and hexadecapole flow (${v}_{4}$) as a function of transverse momentum (${p}_{T}$) of charged hadrons in Au-Au collisions at $\sqrt{{s}_{NN}}=200$ GeV. Results from two types of initial transverse energy density profile, one based on the Glauber model and other based on the color glass condensate (CGC) model, are presented. We observe no difference in the simulated results on the invariant yield of charged hadrons for the calculations with different initial conditions. The comparison to the experimental data on invariant yield of charged hadrons supports a shear-viscosity to entropy-density ratio ($\ensuremath{\eta}/s$) between 0 and 0.12 for the 0$%$--10$%$ to 40$%$--50$%$ collision centralities. The simulated ${v}_{2}({p}_{T})$ is found to be higher for a fluid with CGC based initial condition compared to Glauber based initial condition for a given collision centrality. Consequently the Glauber based calculations when compared to the experimental data require a lower value of $\ensuremath{\eta}/s$ relative to CGC based calculations. In addition, a centrality dependence of the estimated $\ensuremath{\eta}/s$ is observed from the ${v}_{2}({p}_{T})$ study. The ${v}_{4}({p}_{T})$ for the collision centralities 0$%$--10$%$ to 40$%$--50$%$ supports a $\ensuremath{\eta}/s$ value between 0 and 0.08 for a CGC based initial condition. Simulated results using the Glauber based initial condition for the ideal fluid evolution underestimate the ${v}_{4}({p}_{T})$ for collision centralities 0$%$--10$%$ to 30$%$--40$%$.

Counting hot/cold spots in quark-gluon plasma

We study how local fluctuations in the initial states of relativistic heavy-ion collisions manifest in the correlations between different orders of harmonic moments of the density profiles, particularly those involving only odd harmonics that arise purely from initial-state fluctuations. We find the strengths of those correlations are sensitive to the number of hot and cold spots in the initial states. Hydrodynamic evolution of the fireball translates initial-state geometric anisotropies as well as their correlations into final-state momentum anisotropies and correlations. We conclude that the measurement of the correlations between different harmonic moments of final-state azimuthal distribution can be employed to quantify the inhomogeneity of the initial density profiles such as the population of hot and cold spots that are produced in high-energy nuclear collisions.

2D particle-in-cell simulations of ion acceleration in laser irradiated submicron clusters including field ionization

The interaction of femtosecond laser pulses with submicron water clusters is studied here by two-dimensional particle-in-cell simulations. Field ionization is included in our simulations using Ammosov–Delone–Krainov ionization rate. We search for optimum laser and cluster parameters to obtain quasimonoenergetic beam of protons accelerated from the cluster. For the laser amplitude a0≈3 used in recent experiments, the optimum cluster size is about 150 nm for the generation of pronounced peak in proton energy distribution function at maximum energy and the optimum laser pulse duration is about 40–80 fs. Various initial density profiles of cluster plasma, formed due to insufficient laser pulse contrast and prepulses, are involved in this study, including underdense clusters.

Exact Relaxation Dynamics of a Localized Many-Body State in the 1D Bose Gas

Through an exact method, we numerically solve the time evolution of the density profile for an initially localized state in the one-dimensional bosons with repulsive short-range interactions. We show that a localized state with a density notch is constructed by superposing one-hole excitations. The initial density profile overlaps the plot of the squared amplitude of a dark soliton in the weak coupling regime. We observe the localized state collapsing into a flat profile in equilibrium for a large number of particles such as N=1000. The relaxation time increases as the coupling constant decreases, which suggests the existence of off-diagonal long-range order. We show a recurrence phenomenon for a small number of particles such as N=20.

Effect of the initial density and angular-velocity profiles of pre-stellar cores on the properties of young stellar objects

The physical properties of young stellar objects are studied as functions of the initial spatial distributions of the gas surface density Σ and angular velocity Ω in pre-stellar cores using numerical hydrodynamic simulations. Two limiting cases are considered: spatially homogeneous cores with Σ = const and Ω = const and centrally concentrated cores with radius-dependent densities Σ ∝ r−1 and Ω ∝ r−1. The degree of gravitational instability and protostellar disk fragmentation is mostly determined by the initial core mass and the ratio of the rotational to the gravitational energy, and depends only weakly on the initial spatial configuration of pre-stellar cores, except for the earliest stages of evolution, when models with spatially homogeneous cores can be more gravitationally unstable. The accretion of disk matter onto a protostar also depends weakly on the initial distributions of Σ and Ω, with matter from the collapsing core falling onto the disk at a rate that is slightly higher in models with spatially homogeneous cores. An appreciable dependence of the disk mass, disk radius, and the disk-to-protostar mass ratio on the initial density and angular velocity profiles of the parent core is found only for class 0 young objects; this relationship is not systematic in the later I and II stages of stellar evolution. The mass of the central protostar depends weakly on the initial core configuration in all three evolutionary stages.

Importance of the initial conditions for star formation - III. Statistical properties of embedded protostellar clusters

We investigate the formation of protostellar clusters during the collapse of dense molecular cloud cores with a focus on the evolution of potential and kinetic energy, the degree of substructure, and the early phase of mass segregation. Our study is based on a series of hydrodynamic simulations of dense cores, where we vary the initial density profile and the initial turbulent velocity. In the three-dimensional adaptive mesh refinement simulations, we follow the dynamical formation of filaments and protostars until a star formation efficiency of 20%. Despite the different initial configurations, the global ensemble of all protostars in a setup shows a similar energy evolution and forms sub-virial clusters with an energy ratio $E_\mathrm{kin}/|E_\mathrm{pot}|\sim0.2$. Concentrating on the innermost central region, the clusters show a roughly virialised energy balance. However, the region of virial balance only covers the innermost $\sim10-30%$ of all the protostars. In all simulations with multiple protostars, the total kinetic energy of the protostars is higher than the kinetic energy of the gas cloud, although the protostars only contain 20% of the total mass. The clusters vary significantly in size, mass, and number of protostars, and show different degrees of substructure and mass segregation. Flat density profiles and compressive turbulent modes produce more subclusters then centrally concentrated profiles and solenoidal turbulence. We find that dynamical relaxation and hence dynamical mass segregation is very efficient in all cases from the very beginning of the nascent cluster, i.e., during a phase when protostars are constantly forming and accreting.