Diffusive shock acceleration in extragalactic jets

We calculate the temporal evolution of distributions of relativistic electrons subject to synchrotron and adiabatic processes and Fermi-like acceleration in shocks. The shocks result from Kelvin-Helmholtz instabilities in the jet. Shock formation and particle acceleration are treated in a self-consistent way by means of a numerical hydrocode. We show that in our model the number of relativistic particles is conserved during the evolution, with no need of further injections of supra-thermal particles after the initial one. From our calculations, we derive predictions for values and trends of quantities like the spectral index and the cutoff frequency that can be compared with observations.Comment: 12 pages containing 7 postscript figures; uses A&A macros. Accepted for publication in Astronomy and Astrophysic

The effect of a stellar magnetic variation on the jet velocity

Stellar jets are normally constituted by chains of knots with some periodicity in their spatial distribution, corresponding to a variability of order of several years in the ejection from the protostar/disk system. A widely accepted theory for the presence of knots is related to the generation of internal working surfaces due to variations in the jet ejection velocity. In this paper we study the effect of variations in the inner disk-wind radius on the jet ejection velocity. We show that a small variation in the inner disk-wind radius produce a variation in the jet velocity large enough to generate the observed knots. We also show that the variation in the inner radius may be related to a variation of the stellar magnetic field.Comment: 5 pages, 3 figures, accepted for publication in Ap

Bow shock and radio halo in the merging cluster A520

Chandra observations of the merging galaxy cluster A520 reveal a prominent bow shock with M=2.1+0.4-0.3. This is only the second clear example of a substantially supersonic merger shock front in clusters. Comparison of the X-ray image with that of the previously known radio halo reveals a coincidence of the leading edge of the halo with the bow shock, offering an interesting experimental setup for determining the role of shocks in the radio halo generation. The halo in A520 apparently consists of two spatially distinct parts, the main turbulence-driven component and a cap-like forward structure related to the shock, where the latter may provide pre-energized electrons for subsequent turbulent re-acceleration. The radio edge may be caused by electron acceleration by the shock. If so, the synchrotron spectrum should have a slope of 1.2 right behind the edge with quick steepening further away from the edge. Alternatively, if shocks are inefficient accelerators, the radio edge may be explained by an increase in the magnetic field and density of pre-existing relativistic electrons due to gas compression. In the latter model, there should be radio emission in front of the shock with the same spectrum as that behind it, but 10-20 times fainter. If future sensitive radio measurements do not find such pre-shock emission, then the electrons are indeed accelerated (or re-accelerated) by the shock, and one will be able to determine its acceleration efficiency. We also propose a method to estimate the magnetic field strength behind the shock, based on measuring the dependence of the radio spectral slope upon the distance from the shock. In addition, the radio edge provides a way to constrain the diffusion speed of the relativistic electrons.Comment: 6 pages (emulateapj style), color figures. Minor text clarifications, improved figure. ApJ in pres

Herbig-Haro flows in L1641N

We have used the 2.56m Nordic Optical Telescope (NOT) to observe two deep fields in L1641N, selected on the basis of previous shock studies, using the 2.12 micron transition of H2 (and a Ks filter to sample the continuum) for a total exposure time of 4.6 h (72 min Ks) in the overlapping region. The resulting high-resolution mosaic shows numerous new shocks and resolves many known shocks into multiple components. Using previous observations taken 9 years earlier we calculate a proper motion map and combine this with Spitzer 24 micron observations of the embedded young stars. The combined H2 mosaic shows many new shocks and faint structures in the HH flows. From the proper motion map we find that most HH objects belong to two major bi-polar HH flows, the large-scale roughly North-South oriented flow from central L1641N and a previously unseen HH flow in eastern L1641N. Combining the tangential velocity map with the mid-IR Spitzer images, two very likely outflow sources are found. The outflow source of the eastern flow, L1641N-172, is found to be the currently brightest mid-IR source in L1641N and seem to have brightened considerably during the past 20 years. We make the first detection of this source in the near-IR (Ks) and also find a near-IR reflection nebula pointing at the source, probably the illuminated walls of a cone-shaped cavity cleared out by the eastern lobe of the outflow. Extending a line from the eastern outflow source along the proper motion vector we find that HH 301 and HH 302 (almost 1 pc away) belong to this new HH flow.Comment: 10 pages, 4 figures, Accepted for publication by A &

Can Protostellar Jets Drive Supersonic Turbulence in Molecular Clouds?

Jets and outflows from young stellar objects are proposed candidates to drive supersonic turbulence in molecular clouds. Here, we present the results from multi-dimensional jet simulations where we investigate in detail the energy and momentum deposition from jets into their surrounding environment and quantify the character of the excited turbulence with velocity probability density functions. Our study include jet--clump interaction, transient jets, and magnetised jets. We find that collimated supersonic jets do not excite supersonic motions far from the vicinity of the jet. Supersonic fluctuations are damped quickly and do not spread into the parent cloud. Instead subsonic, non-compressional modes occupy most of the excited volume. This is a generic feature which can not be fully circumvented by overdense jets or magnetic fields. Nevertheless, jets are able to leave strong imprints in their cloud structure and can disrupt dense clumps. Our results question the ability of collimated jets to sustain supersonic turbulence in molecular clouds.Comment: 33 pages, 18 figures, accepted by ApJ, version with high resolution figures at: http://www.ita.uni-heidelberg.de/~banerjee/publications/jet_paper.pd

Influence of magnetic fields on pulsed, radiative jets

We present results of magnetohydrodynamic simulations of steady and time variable jets for a set of conditions applicable to outflows from young stellar objects (YSOs). As a first step in a detailed study of radiative magnetohydrodynamic jets, we study both steady and pulsed jets with a large-scale magnetic field oriented parallel to the jet flow axis. While toroidal components may be present in many jets, we have chosen in this initial study to focus solely on pure poloidal initial geometries. The range of magnetic field strengths studied is characterized by the dimensionless parameter beta = 8 pi P-gas/B-2 = 0.1-10(7). The results of our simulations show that the global characteristics are not strongly dependent on the strength of the magnetic field. Instead, we find that a predominantly poloidal field has more subtle effects, such as inhibiting instabilities, and increasing the "order" in the flow patterns. While the fields act to restrict "turbulent" gas motions, the pulse-induced internal shocks increase the likelihood of instabilities, complicate the global flow patterns, and increase the likelihood of magnetic reconnection. We detail the ways in which the magnetic pressure and tension forces affect the kinematics observed in these simulationsopen242

Structure and Stability of Keplerian MHD Jets

MHD jet equilibria that depend on source properties are obtained using a simplified model for stationary, axisymmetric and rotating magnetized outflows. The present rotation laws are more complex than previously considered and include a Keplerian disc. The ensuing jets have a dense, current-carrying central core surrounded by an outer collar with a return current. The intermediate part of the jet is almost current-free and is magnetically dominated. Most of the momentum is located around the axis in the dense core and this region is likely to dominate the dynamics of the jet. We address the linear stability and the non-linear development of instabilities for our models using both analytical and 2.5-D numerical simulation's. The instabilities seen in the simulations develop with a wavelength and growth time that are well matched by the stability analysis. The modes explored in this work may provide a natural explanation for knots observed in astrophysical jets.Comment: 35 pages, accepted by the Ap

Magnetized Kelvin-Helmholtz instability in the presence of a radiation field

The purpose of this study is to analyze the dynamical role of a radiation field on the growth rate of the unstable Kelvin - Helmholtz (KH) perturbations. As a first step toward this purpose, the analyze is done in a general way, irrespective of applying the model to a specific astronomical system. The transition zone between the two layers of the fluid is ignored. Then, we perform a linear analysis and by imposing suitable boundary conditions and considering a radiation field, we obtain appropriate dispersion relation. Unstable modes are studied by solving the dispersion equation numerically, and then growth rates of them are obtained. By analyzing our dispersion relation, we show that for a wide range of the input parameters, the radiation field has a destabilizing effect on KH instability. In eruptions of the galaxies or supermassive stars, the radiation field is dynamically important and because of the enhanced KH growth rates in the presence of the radiation; these eruptions can inject more momentum and energy into their environment and excite more turbulent motions.Comment: Accepted for publication in Astrophysics and Space Scienc