The Fanno model for turbulent compressible flow

The paper considers the derivation and properties of the Fanno model for nearly unidirectional turbulent flow of gas in a tube. The model is relevant to many industrial processes. Approximate solutions are derived and numerically validated for evolving flows of initially small amplitude, and these solutions reveal the prevalence of localized large-time behaviour, which is in contrast to inviscid acoustic theory. The properties of large-amplitude travelling waves are summarized, which are also surprising when compared to those of inviscid theory

Chemistry of dense clumps near moving Herbig-Haro objects

Localised regions of enhanced emission from HCO+, NH3 and other species near Herbig-Haro objects (HHOs) have been interpreted as arising in a photochemistry stimulated by the HHO radiation on high density quiescent clumps in molecular clouds. Static models of this process have been successful in accounting for the variety of molecular species arising ahead of the jet; however recent observations show that the enhanced molecular emission is widespread along the jet as well as ahead. Hence, a realistic model must take into account the movement of the radiation field past the clump. It was previously unclear as to whether the short interaction time between the clump and the HHO in a moving source model would allow molecules such as HCO+ to reach high enough levels, and to survive for long enough to be observed. In this work we model a moving radiation source that approaches and passes a clump. The chemical picture is qualitatively unchanged by the addition of the moving source, strengthening the idea that enhancements are due to evaporation of molecules from dust grains. In addition, in the case of several molecules, the enhanced emission regions are longer-lived. Some photochemically-induced species, including methanol, are expected to maintain high abundances for ~10,000 years.Comment: 7 pages, 3 figure

Interactions of a Light Hypersonic Jet with a Non-Uniform Interstellar Medium

We present three dimensional simulations of the interaction of a light hypersonic jet with an inhomogeneous thermal and turbulently supported disk in an elliptical galaxy. We model the jet as a light, supersonic non-relativistic flow with parameters selected to be consistent with a relativistic jet with kinetic power just above the FR1/FR2 break. We identify four generic phases in the evolution of such a jet with the inhomogeneous interstellar medium: 1) an initial ``flood and channel'' phase, where progress is characterized by high pressure gas finding changing weak points in the ISM, flowing through channels that form and re-form over time, 2) a spherical, energy-driven bubble phase, were the bubble is larger than the disk scale, but the jet remains fully disrupted close to the nucleus, 3) a rapid, jet break--out phase the where jet breaks free of the last dense clouds, becomes collimated and pierces the spherical bubble, and 4) a classical phase, the jet propagates in a momentum-dominated fashion leading to the classical jet + cocoon + bow-shock structure. Mass transport in the simulations is investigated, and we propose a model for the morphology and component proper motions in the well-studied Compact Symmetric Object 4C31.04.Comment: 66 pages, 22 figures, PDFLaTeX, aastex macros, graphicx and amssymb packages, Accepted, to be published 2007 ApJ

Swift observations of the 2006 outburst of the recurrent nova RS Ophiuchi: II. 1D hydrodynamical models of wind driven shocks

Following the early Swift X-ray observations of the latest outburst of the recurrent nova RS Ophiuchi in February 2006 (Paper I), we present new 1D hydrodynamical models of the system which take into account all three phases of the remnant evolution. The models suggest a novel way of modelling the system by treating the outburst as a sudden increase then decrease in wind mass-loss rate and velocity. The differences between this wind model and previous Primakoff-type simulations are described. A more complex structure, even in 1D, is revealed through the presence of both forward and reverse shocks, with a separating contact discontinuity. The effects of radiative cooling are investigated and key outburst parameters such as mass-loss rate, ejecta velocity and mass are varied. The shock velocities as a function of time are compared to the ones derived in Paper I. We show how the manner in which the matter is ejected controls the evolution of the shock and that for a well-cooled remnant, the shock deceleration rate depends on the amount of energy that is radiated away.Comment: 9 pages, 5 figure

The Universe Was Reionized Twice

We show the universe was reionized twice, first at z~15-16 and second at z~6. Such an outcome appears inevitable, when normalizing to two well determined observational measurements, namely, the epoch of the final cosmological reionization at z~6 and the density fluctuations at z~6, which in turn are tight ly constrained by Lyman alpha forest observations at z~3. These two observations most importantly fix the product of star formation efficiency and ionizing photon escape fraction from galaxies at high redshift. To the extent that the relative star formation efficiencies in gaseous minihalos with H2 cooling and large halos with atomic cooling at high redshift are still unknown, the primary source for the first reionization could be Pop III stars either in minihalos or in large halos. We show that gas in minihalos can be cooled efficiently by H2 molecules and star formation can continue to take place largely unimpeded throughout the first reionization period, thanks to two new mechanisms for generating a high X-ray background during the Pop III era, put forth here. Moreover, an important process for producing a large number of H2 molecules in relic HII regions of Pop III galaxies, first pointed out by Ricotti, Gnedin, & Shull, is quantified here. It is shown that the Lyman-Werner background may never build up during the Pop III era. The long cosmological reionization and reheating history is complex. We discuss a wide range of implications and possible tests for this new reionization picture. In particular, Thomson scattering optical depth is increased to 0.10 +- 0.03, compared to 0.027 for the case of only one rapid reionization at z=6. Upcoming Microwave Anisotropy Probe observation of the polarization of the cosmic microwave background should be able to distinguish between these two scenarios.Comment: submitted to ApJ, 69 pages, substantial revision made and conclusions strengthene

Panoramic Views of the Cygnus Loop

We present a complete atlas of the Cygnus Loop supernova remnant in the light of [O III] (5007), H alpha, and [S II] (6717, 6731). Despite its shell-like appearance, the Cygnus Loop is not a current example of a Sedov-Taylor blast wave. Rather, the optical emission traces interactions of the supernova blast wave with clumps of gas. The surrounding interstellar medium forms the walls of a cavity through which the blast wave now propagates, including a nearly complete shell in which non-radiative filaments are detected. The Cygnus Loop blast wave is not breaking out of a dense cloud, but is instead running into confining walls. The interstellar medium dominates not only the appearance of the Cygnus Loop but also the continued evolution of the blast wave. If this is a typical example of a supernova remnant, then global models of the interstellar medium must account for such significant blast wave deceleration.Comment: 28 pages AAS Latex, 28 black+white figures, 6 color figures. To be published in The Astrophysical Journal Supplement Serie

Dynamical Simulations of Magnetically Channeled Line-Driven Stellar Winds: II. The Effects of Field-Aligned Rotation

Building upon our previous MHD simulation study of magnetic channeling in radiatively driven stellar winds, we examine here the additional dynamical effects of stellar {\em rotation} in the (still) 2-D axisymmetric case of an aligned dipole surface field. In addition to the magnetic confinement parameter $\eta_{\ast}$ introduced in Paper I, we characterize the stellar rotation in terms of a parameter $W \equiv V_{\rm{rot}}/V_{\rm{orb}}$ (the ratio of the equatorial surface rotation speed to orbital speed), examining specifically models with moderately strong rotation $W =$ 0.25 and 0.5, and comparing these to analogous non-rotating cases. Defining the associated Alfv\'{e}n radius R_{\rm{A}} \approx \eta_{\ast}^{1/4} \Rstar and Kepler corotation radius R_{\rm{K}} \approx W^{-2/3} \Rstar, we find rotation effects are weak for models with $R_{\rm{A}} < R_{\rm{K}}$, but can be substantial and even dominant for models with R_{\rm{A}} \gtwig R_{\rm{K}}. In particular, by extending our simulations to magnetic confinement parameters (up to $\eta_{\ast} = 1000$) that are well above those ($\eta_{\ast} = 10$) considered in Paper I, we are able to study cases with $R_{\rm{A}} \gg R_{\rm{K}}$; we find that these do indeed show clear formation of the {\em rigid-body} disk predicted in previous analytic models, with however a rather complex, dynamic behavior characterized by both episodes of downward infall and outward breakout that limit the buildup of disk mass. Overall, the results provide an intriguing glimpse into the complex interplay between rotation and magnetic confinement, and form the basis for a full MHD description of the rigid-body disks expected in strongly magnetic Bp stars like $\sigma$ Ori E.Comment: 14 pp, visit this http://shayol.bartol.udel.edu/massivewiki-media/publications/rotation.pdf for full figure version of the paper. MNRAS, in pres

The Thermal Composite Supernova Remnant Kes 27 as Viewed by CHANDRA: Shock Reflection from a Cavity Wall

We present a spatially resolved spectroscopic study of the thermal composite supernova remnant Kes 27 with Chandra. The X-ray spectrum of Kes 27 is characterized by K lines from Mg, Si, S, Ar, and Ca. The X-ray emitting gas is found to be enriched in sulphur and calcium. The broadband and tri-color images show two incomplete shell-like features in the northeastern half and brightness fading with increasing radius in the southwest. There are over 30 unresolved sources within the remnant. None show characteristics typical of young neutron stars. The maximum diffuse X-ray intensity coincides with a radio bright region along the eastern border. In general, gas in the inner region is at higher temperature and emission is brighter than from the outer region. The gas in the remnant appears to approach ionization equilibrium. The overall morphology can be explained by the evolution of the remnant in an ambient medium with a density enhancement from west to east. We suggest that the remnant was born in a pre-existing cavity and that the inner bright emission is due to the reflection of the initial shock from the dense cavity wall. This scenario may provide a new candidate mechanism for the X-ray morphology of other thermal composite supernova remnants.Comment: 11 emulateapj pages (including 10 figures), a few references adde

Thermal instability in ionized plasma

We study magnetothermal instability in the ionized plasmas including the effects of Ohmic, ambipolar and Hall diffusion. Magnetic field in the single fluid approximation does not allow transverse thermal condensations, however, non-ideal effects highly diminish the stabilizing role of the magnetic field in thermally unstable plasmas. Therefore, enhanced growth rate of thermal condensation modes in the presence of the diffusion mechanisms speed up the rate of structure formation.Comment: Accepted for publication in Astrophysics & Space Scienc

Catching the radio flare in CTA 102 I. Light curve analysis

Context: The blazar CTA 102 (z=1.037) underwent a historical radio outburst in April 2006. This event offered a unique chance to study the physical properties of the jet. Aims: We used multifrequency radio and mm observations to analyze the evolution of the spectral parameters during the flare as a test of the shock-in-jet model under these extreme conditions. Methods: For the analysis of the flare we took into account that the flaring spectrum is superimposed on a quiescent spectrum. We reconstructed the latter from archival data and fitted a synchrotron self-absorbed distribution of emission. The uncertainties of the derived spectral parameters were calculated using Monte Carlo simulations. The spectral evolution is modeled by the shock-in-jet model, and the derived results are discussed in the context of a geometrical model (varying viewing angle) and shock-shock interaction. Results: The evolution of the flare in the turnover frequency-turnover flux density plane shows a double peak structure. The nature of this evolution is dicussed in the frame of shock-in-jet models. We discard the generation of the double peak structure in the turnover frequency-turnover flux density plane purely based on geometrical changes (variation of the Doppler factor). The detailed modeling of the spectral evolution favors a shock-shock interaction as a possible physical mechanism behind the deviations from the standard shock-in-jet model.Comment: 15 pages, 12 figure