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11.
Frank M. Rieger Valentí Bosch-Ramon Peter Duffy 《Astrophysics and Space Science》2007,309(1-4):119-125
We consider the acceleration of energetic particles by Fermi processes (i.e., diffusive shock acceleration, second order Fermi
acceleration, and gradual shear acceleration) in relativistic astrophysical jets, with particular attention given to recent
progress in the field of viscous shear acceleration. We analyze the associated acceleration timescales and the resulting particle
distributions, and discuss the relevance of these processes for the acceleration of charged particles in the jets of AGN,
GRBs and microquasars, showing that multi-component powerlaw-type particle distributions are likely to occur. 相似文献
12.
The magnetic field in an accretion disk is estimated assuming that all of the angular momentum within prescribed accretion
disk radii is removed by a jet. The magnetic field estimated at the base of the jet is extrapolated to the blazar emission
region using a model for a relativistic axisymmetric jet combined with some simplifying assumptions based on the relativistic
nature of the flow. The extrapolated magnetic field is compared with estimates based upon the synchrotron and inverse Compton
emission from three blazars, MKN 501, MKN 421 and PKS 2155-304. The magnetic fields evaluated from pure synchrotron self-Compton
models are inconsistent with the magnetic fields extrapolated in this way. However, in two cases inverse Compton models in
which a substantial part of the soft photon field is generated locally agree well, mainly because these models imply magnetic
field strengths consistent with an important Poynting Flux component. This comparison is based on estimating the mass accretion
rate from the jet energy flux. Further comparisons along these lines will be facilitated by independent estimates of the mass
accretion rate in blazars and by more detailed models for jet propagation near the black hole. 相似文献
13.
John Bally 《Astrophysics and Space Science》2007,311(1-3):15-24
Most stars produce spectacular jets during their formation. There are thousands of young stars within 500 pc of the Sun and
many power jets. Thus protostellar jets may be the most common type of collimated astrophysical outflow. Shocks powered by
outflows excite many emission lines, exhibit a rich variety of structure, and motions with velocities ranging from 50 to over
500 km s−1. Due to their relative proximity, proper motions and structural changes can be observed in less than a year. I review the
general properties of protostellar jets, summarize some results from recent narrow-band imaging surveys of entire clouds,
discuss irradiated jets, and end with some comments concerning outflows from high-mass young stellar objects. Protostellar
outflows are ideal laboratories for the exploration of the jet physics. 相似文献
14.
The role of collisions between extragalactic jets and dense clouds in determining the appearance of high-redshift radio galaxies is discussed and investigated through numerical hydrodynamic simulations in three dimensions. The code has the facility to track jet material separately from ambient material. This allows us to use simplifying assumptions to calculate synthetic radio images. The results indicate that the most powerful radio sources are likely to be observed during or shortly after an interaction, and that such interactions can explain both the radio structures and the spatial association between optical and radio light found in powerful radio galaxies. In some cases such a scenario may provide an alternative explanation of jet properties to mechanisms based on variations in the source or fluid-dynamical instabilities.This author is supported by a PPARC research studentship 相似文献
15.
本文提出，原则上所有活动星系核都能产生喷流，但是星系核心区域介质的渗入可能使得一部分喷流被减速以至于消失。这为喷流只见于一部分活动星系的事实提供了一种可能解释，也可能是建立活动星系核结构统一的模型的一个有意义的步骤。 相似文献
16.
Young stars produce both molecular outflows and, at a later evolutionary stage, well-collimated optical jets. The simplest explanation is that the molecular outflows are driven byobscured optical jets, rather than directly, by a disk wind for example, but the optical jets appear to have too small a momentum flux. Recent statistical studies however show that the molecular flows must be quasi-stationary, which means that the dynamical lifetime is a gross underestimate of the true age. As a consequence much less thrust is required. We present recent observations of RNO 43, which has well-defined optical and molecular outflows lying close to the plane of the sky. Excellent agreement with the observations is obtained with a simple kinematic model for the molecular material, which supposes that it lies in a parabolic shell around the optical jet with the highest velocities at the working surface. Together with our modelling of the NGC2024 outflow, this is very strong evidence that molecular outflows are produced by prompt entrainment of molecular material in a neutral or weakly-ionized jet. 相似文献
17.
We present inner-coma dust imaging of Comet Hyakutake (1996 B2) obtained on 11 consecutive nights in late March 1996, an interval including a major outburst and the comet’s closest approach to Earth. The evolution of the outburst morphology is followed, along with the motion along the tail of several outburst fragments. Two spiral dust jets—a primary jet, along with a much weaker secondary jet—are visible throughout the interval and are produced by two source regions on a rotating nucleus. These are examined as a function of rotational phase and viewing geometry, with their appearance changing from a nearly face-on view on March 18 to side-on by March 28. The dust outflow velocity as a function of distance from the nucleus is derived, with the dust continuing to accelerate to a distance of 4000 km or more and reaching an average outflow velocity of 0.38 km s−1 between 3000 and 8000 km. We present details of our Monte Carlo modeling of the jets and our methodology of fitting the model to the images. The modeling yields the pole orientation of the nucleus, with an obliquity of approximately 108°, corresponding to an RA of 13h41m and a Dec of −1.1°. For an assumed spherical nucleus, the primary active region is centered at approximately −66° latitude, has a radius of about 56°, and therefore covers about 22% of the surface. The source of the secondary jet is at a latitude of −28°, has a radius of about 16°, and is located at a longitude nearly 180° away from the primary source. Estimated uncertainties for the pole orientation and the source locations and sizes are each about 3°. This solution for the nucleus orientation and source locations explains the strong asymmetry in measured production rates before and after perihelion in radio observations (Biver et al., 1999, Astron. J. 118, 1850-1872). The modeling also tightly constrains the sidereal rotation period as 0.2618 ± 0.0001 day, completely consistent with the expected +0.0003 day difference from the observed solar rotation period of 0.2614 ± 0.0004 day determined by Schleicher and Osip (2002, Icarus 159, 210-233), given the pole orientation and position of the comet in its orbit. 相似文献
18.
Evolution of droplets in subsea oil and gas blowouts: Development and validation of the numerical model VDROP-J 总被引：1，自引：0，他引：1
Lin Zhao Michel C. Boufadel Scott A. Socolofsky Eric Adams Thomas King Kenneth Lee 《Marine pollution bulletin》2014
The droplet size distribution of dispersed phase (oil and/or gas) in submerged buoyant jets was addressed in this work using a numerical model, VDROP-J. A brief literature review on jets and plumes allows the development of average equations for the change of jet velocity, dilution, and mixing energy as function of distance from the orifice. The model VDROP-J was then calibrated to jets emanating from orifices ranging in diameter, D, from 0.5 mm to 0.12 m, and in cross-section average jet velocity at the orifice ranging from 1.5 m/s to 27 m/s. The d50/D obtained from the model (where d50 is the volume median diameter of droplets) correlated very well with data, with an R2 = 0.99. Finally, the VDROP-J model was used to predict the droplet size distribution from Deepwater Horizon blowouts. The droplet size distribution from the blowout is of great importance to the fate and transport of the spilled oil in marine environment. 相似文献
19.
We re-examine the maximum brightness temperature that a synchrotron source can sustain by adapting standard synchrotron theory
to an electron distribution that exhibits a deficit at low energy. The absence of low energy electrons reduces the absorption
of synchrotron photons, allowing the source to reach a higher brightness temperature without the onset of catastrophic cooling.
We find that a temperature of ∼1014 K is possible at GHz frequencies. In addition, a high degree of intrinsic circular polarisation is produced. We compute the
stationary, synchrotron and self-Compton spectrum arising from the continuous injection of such a distribution (modelled as
a double power-law) balanced by radiative losses and escape, and compare it with the simultaneously observed multi-wavelength
spectrum of the BL Lac object S5 0716+714. This framework may provide an explanation of other high brightness-temperature
sources without the need for mechanisms such as coherent emission or proton synchrotron radiation. 相似文献
20.
A strong spring Wyrtki jet(WJ) presents in May 2013 in the eastern equatorial Indian Ocean. The entire buildup and retreat processes of the spring WJ were well captured by two adjacent Acoustic Doppler Current Profilers mounted on the mooring systems. The observed zonal jet behaved as one intraseasonal event with the significant features of abrupt emergence as well as slow disappearance. Further research illustrate that the pronounced surface westerly wind burst during late-April to mid-May, associated with the active phase of a robust eastwardpropagating Madden–Julian oscillation in the tropical Indian Ocean, was the dominant reason for the rapid acceleration of surface WJ. In contrasting, the governing mechanism for the jet termination was equatorial wave dynamics rather than wind forcing. The decomposition analysis of equatorial waves and the corresponding changes in the ocean thermocline demonstrated that strong WJ was produced rapidly by the wind-generated oceanic downwelling equatorial Kelvin wave and was terminated subsequently by the westward-propagating equatorial Rossby wave reflecting from eastern boundaries of the Indian Ocean. 相似文献