Alexander Rosen

Relativistic Jet Response to Precession and Wave-Wave Interactions

Three-dimensional numerical simulations of the response of a Lorentz factor 2.5 relativistic jet to precession at three different frequencies have been performed. Low-, moderate-, and high-precession frequencies have been chosen relative to the maximally unstable frequency predicted by a Kelvin-Helmholtz stability analysis. The transverse motion and velocity decreases as the precession frequency increases. Although the helical displacement of the jet decreases in amplitude as the precession frequency increases, a helical shock is generated in the medium external to the jet at all precession frequencies. Complex pressure and velocity structure inside the jet are shown to be produced by a combination of the helical surface and first-body modes predicted by a normal mode analysis of the relativistic hydrodynamic equations. The surface and first-body modes have different wave speeds and wavelengths, are launched in phase by the periodic precession, and exhibit beat patterns in synthetic emission images. Wave (pattern) speeds range from 0.41c to 0.86c, but the beat patterns remain stationary. Thus, we find a mechanism that can produce differentially moving and stationary features in the jet.

Hydrodynamical simulations of the decay of high-speed molecular turbulence. I. Dense molecular regions

We present the results from three-dimensional hydrodynamical simulations of decaying high-speed turbulence in dense molecular clouds. We compare our results, which include a detailed cooling function, molecular hydrogen chemistry and a limited C and O chemistry, with those previously obtained for decaying isothermal turbulence. After an initial phase of shock formation, power-law decay regimes are uncovered, as in the isothermal case. We find that the turbulence decays faster than in the isothermal case because the average Mach number remains higher, owing to the radiative cooling. The total thermal energy, initially raised by the introduction of turbulence, decays only a little more slowly than the kinetic energy. We discover that molecule reformation, as the fast turbulence decays, is several times faster than that predicted for a non-turbulent medium. This is caused by moderate speed shocks which sweep through a large fraction of the volume, compressing the gas and dust. Through reformation, the molecular density and molecular column appear as complex patterns of filaments, clumps and some diffuse structure. In contrast, the molecular fraction has a wider distribution of highly distorted clumps and copious diffuse structure, so that density and molecular density are almost identically distributed during the reformation phase. We conclude that molecules form in swept-up clumps but effectively mix throughout via subsequent expansions and compressions.

Simulations of evolving or outbursting molecular protostellar jets

The kinematic and radiative power of molecular jets is expected to change as a protostar undergoes permanent or repeated changes in the rate at which it accretes. We study here the consequences of evolving jet power on the spatial and velocity structure, as well as the fluxes, of molecular emission from the bipolar outflow. We consider a jet of rapidly increasing density and a jet in which the mass input is abruptly cut off. We perform three-dimensional hydrodynamic simulations with atomic and molecular cooling and chemistry. In this work, highly collimated and sheared jets are assumed. We find that position-velocity diagrams, velocity-channel maps and the relative H 2 and CO fluxes are potentially the best indicators of the evolutionary stage. In particular, the velocity width of the CO lines may prove most reliable, although the often-quoted mass-velocity power-law index is probably not. We demonstrate how the relative H 2 1-0 S(1) and CO J = 1-0 fluxes evolve and apply this to interpret the phase of several outflows.

Expanding hydrodynamical jets crossing a galactic halo/intergalactic medium interface

Parameters within ranges that are plausible for radio sources are presently used to perform two-dimensional hydrodynamical calculations of axisymmetric, initially conical, jets whose initial propagation is through isothermal galactic halos with power-law density distributions; these emerge across a pressure-matched interface into a hotter, but less dense medium whose parameters are typical of an intracluster or intergalactic gas. Upon crossing this interface, the jets accelerate and focused toward cylindrical shapes having long, narrow cocoons. 64 refs.

BL Lacertae variability and superluminal motion via a helical filament/shock interface

The combination of a helical filament found by VLA observations at kpc scales in the M87 jet by Hardee, Owen, and Cornwell (1989) with a shock propagating down a jet suggests a kinematical model that could explain much of the flux variation in sources where the jet is aligned almost directly toward the observer, such as BL Lacertae objects. This model also naturally generates a superluminal pattern speed. The assumptions made by a first-order approximate model are discussed. Reasonable values for jet opening angle, viewing angle, shock speed, and radiative particle speed are used to match the model both to flux variations and superluminal motion of BL Lacertae itself with reasonable success. 33 refs.

Hydrodynamical Simulations of Molecular Dynamics in Supersonic Turbulent Flow

Here we present results from simulations of turbulence in star forming environments obtained by coupling three-dimensional hydrodynamical models with appropriate chemical processes. We investigate regimes of decaying high-speed molecular turbulence. Here we analyse PDFs of density for the volume, mass, molecular mass and the energy distribution over the range of scales. We compare our results to those previously obtained for isothermal turbulence and suggest possible explanations.

Beams crossing a galactic halo-intergalactic medium interface and the size of extragalactic radio sources

Numerical simulations of extragalactic jets which penetrate the boundary between a power-law galactic halo and a hotter intergalactic medium are carried out using a boundary-following code. Comparisons of these models are made with both simplified analytical models and observations of powerful radio sources. Good agreement is obtained with regard to the relationships between the overall linear size of such radio sources and both the cosmological redshift (at fixed power) and the total radio power (at fixed redshift). These computations support the basic idea that the interaction between jets and a cosmologically significant intergalactic medium is very important. 41 references.

Statistical analysis of power-size-redshift distributions of extragalactic jets

This paper investigates whether a hot, sparse, yet cosmologically significant intergalactic medium is consistent with data collected from extragalactic radio sources. This is done by use of Monte Carlo simulations which employ previously run pseudohydrodynamical simulations to cover an observational parameter space. These observational parameters include the scale height, central density, and temperature of a (isothermal) galactic halo, and the power of the central engine which drives the jet. The Monte Carlo simulations generate distribution of sizes in bins of (received) power and redshift, which have been compared with observational data using Kolmogorov-Smirnov tests. Results of this analysis are consistent with the existence of an IGM with temperature and density mentioned above. In addition, this analysis suggests that the active lifetime of powerful extragalactic radio sources decreases with increasing power. 38 refs.

Effect of an intergalactic medium on the average sizes of extragalactic jets

Jets emanating from active galactic nuclei are simulated here by a boundary-following code as they propagate through a bimodal external medium consisting of an isothermal galactic halo and a hotter, less dense, uniform intergalactic medium (IGM). The maximum linear sizes of the beams are found as functions of the beam and media parameters, and some of the observed radio source size statistics are produced in a theoretical model which includes an IGM with a constant local density and temperature. Monte Carlo simulations are used to find the median distance for plausible distributions of various input parameters for a number of constant values of IGM density and temperature. 31 refs.

Linear size versus redshift and linear size versus power for extended extragalactic radio sources

A three-dimensional boundary-following code, altered to allow for a two-phase external medium, is used to simulate extragalactic jets as they propagate through a power-law isothermal galactic halo and on into a hotter, less dense, and uniform intergalactic medium (IGM). Good agreement is obtained with regard to the relationships between the overall linear size of such radio sources and both the total radio power (at fixed redshift) and the cosmological redshift (at fixed power). The excellent agreement of the latter with the latest observational results gives strong support to the notion that much of the X-ray background is due to a pervasive diffuse thermal component which is dense enough to be of cosmological significance.