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17 pags., 18 figs., 3 tabs., Context. Interest in stellar feedback has recently increased because new studies suggest that radiative and mechanical feedback from young massive stars significantly regulates the physical and chemical composition of the interstellar medium. Recent SOFIA [C ii] 158 μm observations of the Orion Veil have revealed that the expanding bubble is powered by stellar winds and influenced by previously active molecular outflows of ionizing massive stars. Aims. We aim to investigate the mechanical feedback on the whole Veil shell by searching for jets and outflows that interact with the Veil shell and by determining the driving mechanisms of these collisions. Methods. We make use of the [C ii] 158 μm map of the Orion Nebula taken with the upGREAT instrument on board SOFIA. We image the [C ii] emission of more extreme local standard of rest velocities (ILSR) between 3 and 20 km s1 to pinpoint the high-velocity structures. Using position-velocity (PV) diagrams and high-velocity [C ii] emission, we search for spots of shock-accelerated [C ii]-emitting gas, so called dents. At these positions, we extract [C ii] line profiles to identify velocity components. We also compare the intensity distribution of the [C ii] emission with that of 8 μm PAH and 70 μm warm dust emission to see if there is a trend among these PDR tracers and to understand the origin of the dents. Results. We identify six dents on the Veil shell, with sizes between 0.3 and 1.35 pc and expansion velocities ranging from 4 to 14 km s1, relative to the expanding Veil shell. The [C ii] line widths toward the dents vary from 4 to 16 km s1 indicating that the dents are the result of interaction of the highly turbulent motions (e.g., shocked gas) with the Veil shell. Moreover, dents appear only in the [C ii] PV diagram, not in the 12CO or HI 21 cm diagrams. Furthermore, the intensity distribution of the [C ii] emission of the dents has a tight correlation with that of the 8 and 70 μm as long as the Orion Molecular Cloud or the Veil do not dominate its emission. Also, the observed dents do not have CO counterpart emission. These results indicate that the dents are made up of CO-dark H2 gas. In light of these findings, as well as the momenta of the dents and their dynamical timescales, we propose that the dents are created by the interaction of collimated jets and outflows from protostars in the Orion star-forming cloud with luminosities ranging from 103 to 104 L-, which indicates that they are B-type stars, with the surrounding Veil shell. However, it is challenging to pinpoint the driving stars as they may have moved from the original ejection points of the jets and outflows. Conclusions. We conclude that the dynamics of the expanding Veil shell is influenced not just by the O-type stars in the Trapezium cluster, but also by less massive stars, especially B-type stars, in the Orion Nebula. Mechanical feedback from protostars with a range of masses appears to play an important role in determining the morphology of H II regions and injecting turbulence into the medium., Studies of interstellar dust and gas at Leiden Observatory are supported by a Spinoza award from the Dutch Science agency, NWO. J.R.G. thanks the Spanish MICINN for funding support under grant PID2019-106110GB-I00. This study was based on observations made with the NASA/DLR Stratospheric Observatory for Infrared Astronomy (SOFIA). SOFIA is jointly operated by the Universities Space Research Association Inc. (USRA), under NASA contract NAS2-97001, and the Deutsches SOFIA Institut (DSI), under DLR contract 50 OK 0901 to the University of Stuttgart. upGREAT is a development by the MPI für Radioastronomie and the KOSMA/Universität zu Köln, in cooperation with the DLR Institut für Optische Sensorsysteme. We acknowledge the work, during the C+ upGREAT square degree survey of Orion, of the USRA and NASA staff of the Armstrong Flight Research Center in Palmdale, the Ames Research Center in Mountain View (California), and the Deutsches SOFIA Institut.

Strong downstream magnetic fields of order of $\sim 1$G, with large correlation lengths, are believed to cause the large synchrotron emission at the afterglow phase of gamma ray bursts (GRBs). Despite of the recent theoretical efforts, models have failed to fully explain the amplification of the magnetic field, particularly in a matter dominated scenario. We revisit the problem by considering the synchrotron emission to occur at the expanding shock front of a weakly magnetized relativistic jet over a magnetized surrounding medium. Analytical estimates and a number of high resolution 2D relativistic magneto-hydrodynamical (RMHD) simulations are provided. Jet opening angles of $\theta = 0^{\circ} - 20^{\circ}$, and ambient to jet density ratios of $10^{-4} - 10^2$ were considered. We found that most of the amplification is due to compression of the ambient magnetic field at the contact discontinuity between the reverse and forward shocks at the jet head, with substantial pile-up of the magnetic field lines as the jet propagates sweeping the ambient field lines. The pile-up is maximum for $\theta \rightarrow 0$, decreasing with $\theta$, but larger than in the spherical blast problem. Values obtained for certain models are able to explain the observed intensities. The maximum correlation lengths found for such strong fields is of $l_{\rm corr} \leq 10^{14}$ cm, $2 - 6$ orders of magnitude larger than the found in previous works., Comment: mnras accepted

We consider the transverse structure and stability properties of relativistic jets formed in the course of the collapse of a massive progenitor. Our numerical simulations show the presence of a strong shear in the bulk velocity of such jets. This shear can be responsible for a very rapid shear--driven instability that arises for any velocity profile. This conclusion has been confirmed both by numerical simulations and theoretical analysis. The instability leads to rapid fluctuations of the main hydrodynamical parameters (density, pressure, Lorentz factor, etc.). However, the perturbations of the density are effectively decoupled from those of the pressure because the beam of the jet is radiation--dominated. The characteristic growth time of instability is much shorter than the life time of the jet and, therefore, may lead to a complete turbulent beam. In the course of the non-linear evolution, these fluctuations may yield to internal shocks which can be randomly distributed in the jet. In the case that internal shocks in a ultrarelativistic outflow are responsible for the observed phenomenology of gamma-ray bursts, the proposed instability can well account for the short-term variability of gamma-ray light curves down to milliseconds., Comment: 8 pages, 5 figures. Received 2 July 2002 / Accepted 30 August 2002 in A&A