Narrow Results

We implement the so-called "complex-plane iterative technique" (CIT) to the computation of classical differentially rotating magnetic white dwarf and neutron star models. The program has been written in SCILAB (© INRIA-ENPC), a matrix-oriented high-level programming language, which can be downloaded free of charge from the site . Due to the advanced capabilities of this language, the code is short and understandable. Highlights of the program are: (a) time-saving character, (b) easy use due to the built-in graphics user interface, (c) easy interfacing with Fortran via online dynamic link. We interpret our numerical results in various ways by extensively using the graphics environment of SCILAB.

In this paper, three methods for computing non-rotating neutron star models are discussed. The relativistic Oppenheimer–Volkoff (OV) equations of hydrostatic equilibrium are solved using (a) the well-known variable step method, (b) a method based on a modified form of the OV equations, where the thermodynamic enthalpy function is the independent variable, and (c) a new straightforward method, based on the ODEPACK Fortran package for solving systems of differential equations. We present results concerning several equations of state and conclusions concerning the applicability of each method.

Recollimation shocks (RS) appear associated with relativistic flows propagating through pressure mismatched atmospheres. Astrophysical scenarios invoking the presence of such shocks include jets from AGNs and X-ray binaries and GRBs. We shall start reviewing the theoretical background behind the structure of RS in overpressured jets. Next, basing on numerical simulations, we will focus on the properties of RS in relativistic steady jets threaded by helical magnetic fields depending on the dominant type of energy. Synthetic radio maps from the simulation of the synchrotron emission for a selection of models in the context of parsec-scale extragalactic jets will also be discussed.

We have investigated the influence of velocity shear on the linear and non-linear development of the CD kink instability. We follow temporal development of the instability within a periodic computational box. We find that helically distorted density structure propagates along the jet with speed and flow structure dependent on the location of the velocity shear relative to the characteristic radius of the helically twisted force-free magnetic field. At small radius the plasma flows through the kink. The kink propagation speed increases as the radius increases and the kink becomes more embedded in the plasma flow. Larger velocity shear radius leads to slower linear growth, makes a later transition to the nonlinear stage, and with larger maximum amplitude than occurs for a static plasma column. However, when the velocity shear radius is much greater than the characteristic radius of the helical magnetic field, linear and non-linear development become more similar to the development of a static plasma column.

We have investigated the influence of jet rotation and differential motion on the linear and nonlinear development of the current-driven (CD) kink instability of force-free helical magnetic equilibria via three-dimensional relativistic magnetohydrodynamic simulations. In this study, we follow the temporal development within a periodic computational box. Displacement of the initial helical magnetic field leads to the growth of the CD kink instability. In the rotating relativistic jet case, developing helical kink structure propagates along jet as it grows in amplitude. The growth rate of the CD kink instability does not depend on the jet rotation. The coupling of multiple unstable wavelengths is crucial to determining whether the jet is eventually disrupted in the nonlinear stage. The CD kink instability deformed magnetic field may trigger magnetic reconnection in the jet.

Magnetic fields play a crucial role in many astrophysical scenarios and, in particular, are of paramount importance in the emission mechanism and evolution of Neutron Stars (NSs). To understand the role of the magnetic field in compact objects it is important to obtain, as a first step, accurate equilibrium models for magnetized NSs. Using the conformally flat approximation we solve the Einstein's equations together with the GRMHD equations in the case of a static axisymmetric NS taking into account different types of magnetic configuration. This allows us to investigate the effect of the magnetic field on global properties of NSs such as their deformation.

The CHARA array is an optical/near infrared interferometer consisting of six 1-meter diameter telescopes with the longest baseline of 331 m. With high angular resolution, the CHARA array provides a unique and powerful way of studying nearby stellar systems. In 2011, the CHARA array was funded by NSF-ATI for an upgrade of adaptive optics systems to all six telescopes to improve the sensitivity by several magnitudes. The initial grant covers Phase I of the adaptive optics system, which includes an on-telescope Wavefront Sensor and fast tip/tilt correction. We are currently seeking funding for Phase II which will add fast deformable mirrors at the telescopes to close the loop. This paper will describe the design of the project, and show simulations of how much improvement the array will gain after the upgrade.

Calculations of the flux power spectrum of the Lyman α forest are performed as a means to quantify the possible effects of time-dependent dark energy. We use a parameterized version of the time-dependent dark energy equation of state consistent with the Planck analysis. We have run high-resolution, large-scale cosmological simulation with a modified version of the publicly available SPH code GADGET-2. These simulations were used to extract synthetic Lyman α forest spectra. These were then used to simulate the flux power spectrum at various observed quasar redshifts. We conclude that the effect of time-dependent dark energy on the flux power spectrum is of marginal statistical significance compared to the intrinsic cosmic variance.

We present some preliminary results of our work about the close encounter of binary stars hosting planets on S-type orbits with the Sgr A* supermassive black hole in the center of our Galaxy.

We briefly describe and discuss the set-up of the project Mocca Survey Database I. The database contains more than 2000 Monte Carlo models of evolution of real star cluster performed with the Mocca code. Then, we very briefly discuss results of analysis of the database regarding the following projects: formation of intermediate mass black holes, abrupt cluster dissolution harboring black hole subsystems, retention fraction of black hole - black hole mergers, and tidal disruption events with intermediate mass black holes.

Nuclear Stellar Clusters are so dense stellar systems (containing up to 10⁷ stars in few parsec radius) that their dynamics cannot be followed with the high precision, direct summation available N -body codes over a long integration time. Here we present the main idea of our new computational approach to study the dynamics of the MW NSC exploiting the facilities of the high precision hybrid parallelized code NBODY6++GPU. This strategy will allow us to study the evolution of the NSC over its 2-body relaxation time with an acceptable numerical effort by mean of a massive computational platform.

Although the observed spectra for gamma-ray burst (GRB) prompt emission is well constrained, the underlying radiation mechanism is still not very well understood. We explore photospheric emission in GRB jets by modelling the Comptonization of fast cooled synchrotron photons whilst the electrons and protons are accelerated to highly relativistic energies by repeated energy dissipation events as well as Coulomb collisions. In contrast to the previous simulations, we implement realistic photon-to-particle number ratios of N_γ/N_e∼ 10⁵ or higher, that are consistent with the observed radiation efficiency of relativistic jets. Using our Monte Carlo radiation transfer (MCRaT) code, we can successfully model the prompt emission spectra when the electrons are momentarily accelerated to highly relativistic energies (Lorentz factor ∼ 50 − 100) after getting powered by ∼ 30 − 50 episodic dissipation events in addition to their Coulomb coupling with the jet protons, and for baryonic outflows that originate from moderate optical depths ∼ 20 − 30. We also show that the resultant shape of the photon spectrum is practically independent of the initial photon energy distribution and the jet baryonic energy content, and hence independent of the emission mechanism.

We present a matching procedure for rapidly calculating the non-linear mass power spectrum in dynamical dark energy cosmologies to percent level accuracy in the range of scales between 0.1 < k < 3. This procedure is verified by large N-body simulations and is valid at any redshift for cosmologies consistent with current observations. Such accuracy in the power spectrum is necessary for next generation cosmological mass probes. Our matching procedure reproduces the CMB distance to last scattering and delivers sub-percent level accuracy in the matter power spectra at z = 0 and z ≈ 3. We discuss the physical implications for probing dark energy with surveys of large scale structure.

We present results of a set of N-body simulations to model the future evolution of the 11 young massive clusters hosted in the central region of the dwarf starburst galaxy Henize 2-10, which contains at its center a massive black hole with a mass M_BH ⋍ 2×10⁶ M_⊙. Nuclear star clusters are present in a great quantity of galaxies of mass similar to Henize 2-10. Our results show that the orbital decay and merging of the Henize 2-10 clusters will likely lead to the formation of a nuclear star cluster with mass M_NSC ⋍ 4 − 6 × 10⁶ M_⊙ and effective radius r_NSC ⋍ 4.1 pc. Additionally, we found that this mechanism can lead to the formation of disky structures with global properties similar to those of nuclear stellar disks, which reside in many “middle-weight” galaxies. This work confirms and enlarge recent results that indicate how nuclear star clusters and super massive black holes are only partially correlated, since the formation process of nuclear star clusters is poorly affected by a black hole of the size of that in Henize 2-10. A new result is that nuclear star clusters and nuclear stellar disks may share the same formation path.

In Space Warps, a community of over 30 000 volunteers searched for lensed candidates in the Canada–France–Hawaii Telescope Legacy Survey (CFHTLS). 59 new lens candidates have been identified, along with rediscovery of 60% of the previously-known candidates.

We propose that volunteers should play an integral part in the modeling of lens candidates as well. We implemented SpaghettiLens, a method allowing non-professionals to create mass models for those discovered lens candidates and to be usable in a citizen-science environment. Tests with simulated lenses show that models by experienced volunteers are comparable to those by experts. We present models of most of the Space Warps lens candidates that were produced collaboratively by a small community of lens enthusiasts from the volunteer community. These models allow for further analysis.

We study the diffuse NIR background light associated with the cluster of high velocity stars at the Galactic Center, known as the S-stars. We use the luminosity function of the cluster members, the distribution of the diffuse background light and the stellar number density counts to investigate the amount of stellar and dark mass associated with the cluster in the immediate vicinity of the supermassive black hole. We show that both the total mass, and the number, of objects within an S-star's orbit can be constrained using observed changes in the star's orbital elements.