3-D Simulations of Protostellar Jets in Stratified Ambient Media

We present fully three-dimensional hydrodynamical simulations of radiative cooling jets propagating into stratified isothermal ambient media with power-law density and pressure distributions. The parameters used are mainly suitable for protostellar jets but results applicable to extragalactic jets are also presented. Comparisons are made with previous simulations of jets through homogeneous media. We find that for radiative cooling jets propagating into regions where the ambient medium has an increasing density (and pressure) gradient, the ambient gas tends to compress the cold, low-pressure cocoon of shocked material that surrounds the beam and destroy the bow shock-like structure at the head. The compressing medium collimates the jet and promotes the development of Kelvin-Helmholtz instabilities which cause beam focusing, wiggling and the formation of internal traveling shocks, $close$ $to$ $the$ $head$, via pinching along the beam. This remarkably resembles the structure of some observed systems (e.g. Haro 6-5B northern and HH 24G jets). These effects are larger for jets with smaller density ratio between jet and environment $\eta$ (tested for $\eta$=1, 3, and 10) and larger Mach number $M_a=v_j/c_a$ (tested for $M_a=$12 and 24, where $v_j$ is the jet velocity and $c_a$ the ambient sound speed). In an ambient medium of decreasing density (and pressure), the beam is poorly collimated and relaxes, becoming faint. This could explain ''invisible'' jet sections, like the gap between the parent source and collimated beam (e.g., in HH30 jet). Although, on average, jets propagating into an increasing (decreasing) density environment are decelerated (accelerated) by the increasing (decreasing) ram pressure of the ambient medium, we find that their propagation velocities have an oscillating pattern.Comment: 33 pp, LaTeX file, 13 figures upon request. To appear in the Astrophys. J., vol 471, nov. 10t

Effects of Nose Bluntness and Shock-Shock Interactions on Blunt Bodies in Viscous Hypersonic Flows

A numerical study has been conducted to investigate the effects of blunt leading edges on the viscous flow field around a hypersonic vehicle such as the proposed National Aero-Space Plane. Attention is focused on two specific regions of the flow field. Analysis of these flow regions is required to accurately predict the overall flow field as well as to get necessary information on localized zones of high pressure and intense heating. The forebody is modeled by slender cones and ogives with spherically blunted nose. A combination of Navier-Stokes and parabolized Navier-Stokes equations is used to compute the flow field. The influence of entropy layer thickness on the extent of the leading edge effects is also considered. The extent of downstream effects of leading edge thickness are determined at Mach numbers of 10 and 20 for cone angles of 5°, 10°, and 20°. Three values of nose bluntness are considered with the smallest nose blunting (0.0025m) representing the sharp cone/ogive. For the flow region around the inlet the forebody shock can interact either with the blunt cowl leading edge shock or with the shock produced by the blunt leading edges of the swept sidewall compression surfaces of the inlet. For the interaction at the cowl leading edge, the forebody shock is assumed planar and the cowl is modeled by a two-dimensional cylindrically blunted wedge of infinite width. Use of the full Navier-Stokes equations is made on the cowl forebody and the thin-layer Navier-Stokes equations are suitably modified for space marching on the cowl afterbody. The results of the study show that the flow around the cowl is significantly altered by the impinging shock. The peak value of pressure is found to be nine times and heating rates eight times the stagnation point value for unimpinged case at Mach 8.03. The peak values were slightly lower for Mach 5.94 calculations. A three-dimensional thin-layer Navier-Stokes code has been used to calculate the flow field. The peak pressure for this case is found to be 2.25 times and the peak heating three times the unimpinged stagnation values. The results of the study are compared with the available experimental and numerical results

Electromagnetic Chirps from Neutron Star-Black Hole Mergers

We calculate the electromagnetic signal of a gamma-ray flare coming from the surface of a neutron star shortly before merger with a black hole companion. Using a new version of the Monte Carlo radiation transport code Pandurata that incorporates dynamic spacetimes, we integrate photon geodesics from the neutron star surface until they reach a distant observer or are captured by the black hole. The gamma-ray light curve is modulated by a number of relativistic effects, including Doppler beaming and gravitational lensing. Because the photons originate from the inspiraling neutron star, the light curve closely resembles the corresponding gravitational waveform: a chirp signal characterized by a steadily increasing frequency and amplitude. We propose to search for these electromagnetic chirps using matched filtering algorithms similar to those used in LIGO data analysis.Comment: 13 pages, 5 figures, submitted to Ap

The independent association of overweight and obesity with breathlessness in adults: a cross-sectional, population-based study

Obesity is an independent risk factor for chronic breathlessness and should be assessed in people with this symptom

Magnetic Field Effects on the Head Structure of Protostellar Jets

We present the results of 3-D SPMHD numerical simulations of supermagnetosonic, overdense, radiatively cooling jets. Two initial magnetic configurations are considered: (i) a helical and (ii) a longitudinal field. We find that magnetic fields have important effects on the dynamics and structure of radiative cooling jets, especially at the head. The presence of a helical field suppresses the formation of the clumpy structure which is found to develop at the head of purely hydrodynamical jets. On the other hand, a cooling jet embedded in a longitudinal magnetic field retains clumpy morphology at its head. This fragmented structure resembles the knotty pattern commonly observed in HH objects behind the bow shocks of HH jets. This suggests that a strong (equipartition) helical magnetic field configuration is ruled out at the jet head. Therefore, if strong magnetic fields are present, they are probably predominantly longitudinal in those regions. In both magnetic configurations, we find that the confining pressure of the cocoon is able to excite short-wavelength MHD K-H pinch modes that drive low-amplitude internal shocks along the beam. These shocks are not strong however, and it likely that they could only play a secondary role in the formation of the bright knots observed in HH jets.Comment: 14 pages, 2 Gif figures, uses aasms4.sty. Also available on the web page http://www.iagusp.usp.br/preprints/preprint.html. To appear in The Astrophysical Journal Letter

On weak convergence of locally periodic functions

We prove a generalization of the fact that periodic functions converge weakly to the mean value as the oscillation increases. Some convergence questions connected to locally periodic nonlinear boundary value problems are also considered.Comment: arxiv version is already officia