Session E2: Astrophysics

The influence of strong magnetic fields on neutron stars and proto-neutron stars

The magnetic field breaks spherical symmetry in stars causing the pressure transverse to the magnetic field direction to be different than the pressure parallel to it. We present explicit formulae appropriate at zero and finite temperature for both charged and uncharged particles including the effect of the anomalous magnetic moment. The inclusion of the anomalous magnetic moment increases the level of pressure anisotropy in both cases. We analyze different stages of magnetized star evolution incorporating baryon number conservation and the anisotropic energy momentum tensor. The first stages of the evolution are simulated through the inclusion of trapped neutrinos and fixed entropy per particle, while in the last stage the star is taken to be deleptonized and cold. We find that magnetic field effects, measured by the difference between the parallel and perpendicular pressures, are more pronounced in the beginning of the star evolution when there is a larger number of charged leptons and up quarks.   

A New Algorithm for the of Numerical Computation of Gravitational Waves

Gravitational waves appear as solutions of Einstein's equations for phenomena such as binary black hole collisions, supernovas, pulsars, and the big bang. The correct modeling of gravitational waves is a key requirement for a meaningful detection and interpretation of data collected by gravitational wave observatories like LIGO. The numerical simulation of Einstein's equations is a very difficult computational problem, requiring highly stable and accurate numerical methods that can be efficiently implemented in non-trivial geometries and on parallel super-computers. The numerical relativity community recognizes that a well-posed method fulfills these requirements, but the issue of constructing well-posed numerical algorithms of the Einstein's equation is not trivial and will be addressed in this project. We start with the quasilinear scalar waves propagating on an asymptotically flat curved space background with source, in Bondi null coordinates, and strive to demonstrate analytically and to verify numerically the well-posedness of our algorithm. Next, we endeavor to develop and test a new computational boundary and evolution algorithm based on the well-posedness of characteristic initial value and boundary problems for a scalar wave.   

Keeping it real: revisiting a real-space approach to running ensembles of cosmological N-body simulations

In setting up initial conditions for cosmological N-body simulations there are, fundamentally, two choices: either maximizing the correspondence of the initial density field to the assumed fourier-space clustering or, instead, matching to real-space statistics and allowing the overdensity to vary from box to box as it would in the real universe. Though very few comparisons of these methods exist in the literature, virtually all research groups initialize cosmological N-body simulations using the fourier space approach. As a stringent test of both methods, I perform ensembles of simulations using power law and a ``powerlaw times a bump'' model inspired by baryon acoustic oscillations, exploiting the self-similarity of these initial conditions to quantify the accuracy of the matter-matter two-point correlation results. The real-space method, which was originally proposed by Pen 1997 and implemented by Sirko 2005, performed well in producing the expected self-similar behavior and corroborated the evolution of the BAO feature observed in conventional simulations, even in the strongly non-linear regime. As a substantial improvement to the real-space approach, I present a ``better informed'' estimator for the correlation function that achieves precision measurements with fewer simulations.   

Quasar FeLoBAL Variability Studies from Multi-Year Observations

Broad Absorption Line (BAL) quasars provide a unique perspective in understanding the connections between galactic centers and their host galaxies. We are analyzing a specific type of quasar outflows called FeLoBALs, which are detected by the presence of low-ionization FeII BALs. FeLoBALs appear to be exceptionally powerful, with large column densities and kinetic energy yields, and they constitute $\sim$1\% of all BAL quasars, making them strong candidates for feedback to galaxy evolution. We conducted variability studies on 12 FeLoBALs within emission redshifts $0.69 \leq z \leq 1.93$ using up to 5 observation epochs on a given object, spanning both short $($down to $\sim 10.3$ days$)$ and long $($up to $\sim7.03$ years$)$ timescales in the quasar's rest frame. Our goals are to place new constraints on the physical properties of the gas (e.g., outflow locations, physical structures, and kinetic energies) and understand the underlying mechanisms producing the variability. The sample included spectra obtained using the MDM 2.4m Hiltner telescope at Kitt Peak, AZ, along with data from the 9.2m HET telescope at McDonald Observatory and SDSS data release 7. Results will be presented on the nature and characteristics of observed variability for the sample.   

Radiation Recoil Effects on the Dynamical Evolution of Asteroids

The thermal reemission from irregularly shaped bodies results in a torque that can change the rotation rate and the orientation of the spin axis. We present the results of the first simulations that self-consistently model the YORP effect on the spin states of dynamically evolving aggregates. We follow the evolution of aggregate objects computing the sequence of spin states and YORP torques through which they evolve as the changing spin alters their shape, which subsequently changes the YORP torques. The YORP effect has an extreme sensitivity to the topography of asteroids (Icarus 202, 501-513). If the spin-driven reconfiguration leads to a shape of the aggregate that is nearly symmetric, the YORP torques could become negligibly small or even vanish. This would imply a self-limitation in the evolution of the spin state. Moreover, subsequent reconfigurations could lead to a random walk making the evolution of the spin state completely stochastic. An extensive and statistical analysis of the simulations is conducted to determine whether or not the spin evolution is stochastic and whether the YORP effect is self-limiting.   

Weak Lensing Analysis of Ten High-z Galaxy Clusters

We present the results of our weak lensing analysis of ten massive, high redshift galaxy clusters imaged by the Hubble Space Telescope Advanced Camera for Surveys (ACS). We demonstrate the application of photometric redshift for background galaxy discrimination, explore the fitting of the mass profiles of the clusters using weak lensing shear, and present our creation of an initial test data set for the study of tomographic weak lensing with clusters. The use of tomographic techniques to constrain the dark energy equation of state parameter is a pivotal component of future large survey missions, however, the application of tomography to cosmic shear necessitates the exclusion of regions around galaxy clusters from analysis. We therefore test the applicability tomography to cluster-induced shear as a secondary, complementary sample through which estimates of the dark energy e.o.s. parameter can be made.   

On the Nature of the Chromosphere: Condensation and Line Emission

For a few seconds during an eclipse, the flash emission spectrum from gaseous atoms in the chromosphere can be observed. This layer of the Sun also has continuous emission, supporting the conjecture that condensed matter is also present. Contrary to the gaseous solar models which invoke collisional processes to account for the formation of emission lines, the simplest understanding of this behavior would be the evaporation of excited atoms from condensed surfaces existing within the chromosphere. This is reminiscent of the chemiluminescence which occurs during the condensation of silver clusters (Konig L., Rabin I., Schultze W., and Ertl G. Chemiluminescence in the Agglomeration of Metal Clusters. Science, v. 274, 1353). The condensation process associated with spicule formation, is an exothermic one, requiring the transport of energy away from the site of condensation. As atoms are leaving localized surfaces, their associated electrons could occupy any energy level, and hence a wide variety of emission lines could be produced. This also helps to explain the apparent heating of the upper chromosphere and corona and why spicules manifest a constant temperature over their entire length, even within a corona which appears much warmer.   

Quantum Topology of Particles having Exactly Three Generations

Discovery of a Higgs-like boson with a mass of 126 GeV has severely constrained theories of higher-generation and super-symmetric particles. It is natural, then, to examine theories which yield exactly the Standard color and charge quanta, and in three generations. The topology of tripartite particles (those have internal geometry with three-way symmetry), allow exactly three quantum generations. Such a theory, in which a closed string is replaced with a band (having torsion), offers a direct explanation for oscillatory particle states. Further, a theory of finite-dimensional particles has metrics, such as area and curvature, which are identified with terms of the instanton action. Such a particle theory is fundamentally consistent with Standard cosmology and compels a new line of research in the effort to explain dark matter.