5224194
3D Magneto-hydrodynamic simulations of Parker instability with cosmic rays
This study investigates Parker instability in an interstellar medium (ISM) near the Galactic plane using three-dimensional magneto-hydrodynamic simulations. Parker instability arises from the presence of a magnetic field in a plasma, wherein the magnetic buoyant pressure expels the gas and cause the gas to move along the field lines. The process is thought to induce the formation of giant molecular clouds in the Galaxy. In this study, the effects of cosmic-ray (CR) diffusion are examined. The ISM at equilibrium is assumed to comprise a plasma fluid and a CR fluid at various temperatures, with a uniform magnetic field passing through it in the azimuthal direction of the Galactic disk. After a small perturbation, the unstable gas aggregates at the footpoint of the magnetic fields and forms dense blobs. The growth rate of the instability increases with the strength of the CR diffusion. The formation of dense clouds is enhanced by the effect of cosmic rays (CRs), whereas the shape of the clouds depends sensitively on the initial conditions of perturbation.
119432484
Open clusters in a dispersing molecular cloud
Star clusters are formed in molecular clouds which are believed to be the birth places of most stars. From recent observational data, Lada & Lada (2003) estimated that only 4 to 7% of the proto-clusters have survived. A possible reason is the dispersion of molecular clouds by UV radiation, stellar winds or supernova explosions. We study the cloud dispersion in the embedded clusters by N-body simulations. (© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)
16205821
Particle propagation in the galactic center and spatial distribution of non-thermal x-rays
We showed that if the non-thermal emission from the galactic center in the range 14–40 keV is due to inverse bremsstrahlung emission of subrelativistic protons, their interactions with hot and cold fractions of the interstellar medium are equally important. Our estimation shows that about 30% of the total non-thermal flux from the GC in the range 14–40 keV is generated in regions of cold gas while the rest is produced by proton interaction with hot plasma. From the spatial distribution of 6.7 keV iron line we concluded the spatial distribution of hot plasma is strongly non-uniform that should be taken into account in analysis of proton propagation in the GC. From the Suzaku data we got independent estimates for the diffusion coefficient of subrelativistic protons in the GC, which was in the range 10 26 –10 27 cm 2 s � 1 .
17286551
Indicators for cluster survivability in a dispersing cloud
We use N-body simulations to survey the response of embedded star clusters to the dispersal of their parent molecular cloud. The final stages of the clusters can be divided into three classes: the cluster (1) is destroyed, (2) has a loose structure, and (3) has a compact core. We are interested in three of the governing parameters of the parent cloud: (1) the mass, (2) the size, and (3) the dispersing rate. It is known that the final stage of the cluster is well correlated with the star formation efficiency (SFE) for systems with the same cluster and cloud profile. We deem that the SFE alone is not enough to address systems with clouds of different sizes. Our result shows that the initial cluster-cloud mass ratio at a certain Lagrangian radius and the initial kinetic energy are better indicators for the survivability of embedded clusters.
118673239
Hubble Constant, Lensing, and Time Delay in Relativistic Modified Newtonian Dynamics
The time delay in galaxy gravitational lensing systems has been used to determine the value of the Hubble constant. As with other dynamical phenomena on the galaxy scale, dark matter is often invoked in gravitational lensing to account for the "missing mass" (the apparent discrepancy between the dynamical mass and the luminous mass). Alternatively, modified gravity can be used to explain the discrepancy. In this paper, we adopt the tensor-vector-scalar gravity (TeVe S), a relativistic version of Modified Newtonian Dynamics, to study gravitational lensing phenomena and derive the formulae needed to evaluate the Hubble constant. We test our method on quasar lensing by elliptical galaxies in the literature. We focus on double-image systems with time delay measurement. Three candidates are suitable for our study: HE 2149–2745, FBQ J0951+2635, and SBS 0909+532. The Hubble constant obtained is consistent with the value used to fit the cosmic microwave background result in a neutrino cosmological model.
16029130
Line and Continuum Emission from the Galactic Center. III. Origin of 6.4 keV Line Emission from Molecular Clouds in the Galactic Center
We analyze the 6.4 keV line and continuum emission from the molecular cloud Sgr B2 and the source HESS J1745-303, which is supposed to be a complex of molecular gas. From the HESS results it follows that Sgr A is a source of high energy protons, which penetrate into molecular clouds producing there a TeV gamma-ray flux. We present arguments that Sgr A may also produce a flux of subrelativistic protons which generate the 6.4 keV line and bremsstrahlung continuum emission from the clouds.
15750292
Antikaon production and medium effects in heavy-ion collisions at AGS
Abstract Antikaon production in heavy-ion collisions at energies available from the Alternating Gradient Synchrotron (AGS) at the Brookhaven National Laboratory is studied in a relativistic transport (ART) model. We include contributions from the baryon-baryon, meson-baryon, and meson-meson interactions. The final-state interaction of antikaons via both absorption and elastic scattering by nucleons and pions are also considered. To compare with presently available or future experimental data, we have calculated the antikaon rapidity and transverse momentum distributions as well as its collective flow. Medium effects on these observables due to mean field potentials have also been investigated. It is found that both the ratio of antikaon transverse momentum spectrum to that of kaon and their transverse flow are most sensitive to the in-medium properties of kaons and antikaons.
39436824
J/psi absorption by nucleons in the meson-exchange model
We reinvestigate the $J/\ensuremath{\psi}$ dissociation processes induced by the reactions with nucleons, $J/\ensuremath{\psi}+N\ensuremath{\rightarrow}{\overline{D}}^{(*)}+{\ensuremath{\Lambda}}_{c}$, in the meson-exchange model. Main constraints used in this work are vector-meson dominance and charm vector-current conservation. We show that the cross section for $J/\ensuremath{\psi}+N\ensuremath{\rightarrow}\overline{D}+{\ensuremath{\Lambda}}_{c}$ can be larger than that for $J/\ensuremath{\psi}+N\ensuremath{\rightarrow}{\overline{D}}^{*}+{\ensuremath{\Lambda}}_{c}$ when these constraints are imposed. The dependence of the cross sections on the coupling constants is analyzed in detail, and the comparison with the recent quark-interchange model predictions is also made.
20283699
Momentum anisotropies in the quark coalescence model
Based on the quark coalescence model, we derive relations among the momentum anisotropies of mesons and baryons in relativistic heavy ion collisions from a given, but arbitrary azimuthal distribution for the partons. These relations allow us to estimate the higher-order anisotropies in the initial partonic matter from the measured hadronic anisotropies. Besides the familiar even Fourier coefficients such as the elliptic flow, we also pay attention to odd Fourier coefficients such as the directed flow, which has been observed at finite rapidity even at Relativistic Heavy Ion Collider energies.
4073106
Multiphase transport model for relativistic heavy ion collisions
We describe in detail how the different components of a multiphase transport (AMPT) model that uses the heavy ion jet interaction generator (HIJING) for generating the initial conditions, Zhang's parton cascade (ZPC) for modeling partonic scatterings, the Lund string fragmentation model or a quark coalescence model for hadronization, and a relativistic transport (ART) model for treating hadronic scatterings are improved and combined to give a coherent description of the dynamics of relativistic heavy ion collisions. We also explain the way parameters in the model are determined and discuss the sensitivity of predicted results to physical input in the model. Comparisons of these results to experimental data, mainly from heavy ion collisions at the BNL Relativistic Heavy Ion Collider, are then made in order to extract information on the properties of the hot dense matter formed in these collisions.
18778142
Determination of the stiffness of the nuclear symmetry energy from isospin diffusion.
With an isospin- and momentum-dependent transport model, we find that the degree of isospin diffusion in heavy-ion collisions at intermediate energies is affected by both the stiffness of the nuclear symmetry energy and the momentum dependence of the nucleon potential. Using a momentum dependence derived from the Gogny effective interaction, recent experimental data from NSCL-MSU on isospin diffusion are shown to be consistent with a nuclear symmetry energy given by E(sym)(rho) approximately 31.6(rho/rho(0))(1.05) at subnormal densities. This leads to a significantly constrained value of about -550 MeV for the isospin-dependent part of the isobaric incompressibility of isospin asymmetric nuclear matter.