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1.
We study the physics of wave propagation in a weakly ionized plasma, as it applies to the formation of multifluid, magnetohydrodynamics (MHD) shock waves. We model the plasma as separate charged and neutral fluids which are coupled by ion–neutral friction. At times much less than the ion–neutral drag time, the fluids are decoupled and so evolve independently. At later times, the evolution is determined by the large inertial mismatch between the charged and neutral particles. The neutral flow continues to evolve independently; the charged flow is driven by and slaved to the neutral flow by friction. We calculate this driven flow analytically by considering the special but realistic case where the charged fluid obeys linearized equations of motion. We carry out an extensive analysis of linear, driven, MHD waves. The physics of driven MHD waves is embodied in certain Green functions which describe wave propagation on short time-scales, ambipolar diffusion on long time-scales and transitional behaviour at intermediate times. By way of illustration, we give an approximate solution for the formation of a multifluid shock during the collision of two identical interstellar clouds. The collision produces forward and reverse J shocks in the neutral fluid and a transient in the charged fluid. The latter rapidly evolves into a pair of magnetic precursors on the J shocks, wherein the ions undergo force-free motion and the magnetic field grows monotonically with time. The flow appears to be self-similar at the time when linear analysis ceases to be valid.  相似文献   

2.
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3.
Using a time-dependent multifluid, magnetohydrodynamic code, we calculated the structure of steady perpendicular and oblique C-type shocks in dusty plasmas. We included relevant processes to describe mass transfer between the different fluids, radiative cooling by emission lines and grain charging, and studied the effect of single- and multiple-sized grains on the shock structure. Our models are the first of oblique fast-mode molecular shocks in which such a rigorous treatment of the dust grain dynamics has been combined with a self-consistent calculation of the thermal and ionization structures including appropriate microphysics. At low densities, the grains do not play any significant rôle in the shock dynamics. At high densities, the ionization fraction is sufficiently low that dust grains are important charge and current carriers and, thus, determine the shock structure. We find that the magnetic field in the shock front has a significant rotation out of the initial upstream plane. This is most pronounced for single-sized grains and small angles of the shock normal with the magnetic field. Our results are similar to previous studies of steady C-type shocks showing that our method is efficient, rigorous and robust. Unlike the method employed in the previous most detailed treatment of dust in steady oblique fast-mode shocks, ours allow a reliable calculation even when chemical or other conditions deviate from local statistical equilibrium. We are also able to model transient phenomena.  相似文献   

4.
We present the first C-shock and radiative transfer model that calculates the evolution of the line profiles of neutral and ion species like SiO, H13CO+ and HN13C for different flow times along the propagation of the shock through the unperturbed gas. We find that the line profiles of SiO characteristic of the magnetic precursor stage have very narrow linewidths and are centered at velocities close to the ambient cloud velocity, as observed toward the young shocks in the L1448-mm outflow. Consistently with previous works, our model also reproduces the broad SiO emission detected in the high velocity gas in this outflow, for the downstream postshock gas in the shock. This implies that the different velocity components observed in L1448-mm are due to the coexistence of different shocks at different evolutionary stages.  相似文献   

5.
Y. C. Whang 《Solar physics》1994,149(2):347-362
We study a nonplanar model of magnetic reconnection associated with conical slow shocks, assuming that the shock surfaces are two identical cones with circular cross sections symmetrical about the ±x-axis. In the inflow region upstream of the shocks, two oppositely directed magnetic fields are separated by a current sheet. The model treats the current sheet as a tangential discontinuity and treats shocks and tangential discontinuity as surfaces of zero thickness. The dynamical structure of the global magnetic field in the continuous regions is studied using compressible, non-resistive MHD equations. In the inflow region, nonplanar magnetic field lines first move toward the current sheet. Near the sheet, the middle sections of the field lines become highly flattened, almost parallel to the sheet. Eventually, then oppositely directed field lines merge across the tangential discontinuity between the two shocks, and the magnetic lines are reconnected at the intersection of the shock and the tangential discontinuity. Reconnected magnetic lines are carried away at high speeds by the MHD flow in the outflow region, downstream of the shocks.  相似文献   

6.
Theoretical predictions by Farebrother et al. and Meijer et al. of rovibrational excitation probabilities in H2 arising from formation by Eley-Rideal processes on a graphite surface are incorporated into a model of the chemistry and excitation of interstellar H2. The model includes the usual radiative and collisional pumping of H2 rotational and vibrational states, in addition to the formation processes. Predictions are made for HH2 rovibrational emission line intensities for representative points in diffuse and in dark interstellar clouds. We find that – if all the interstellar HH2 is formed by this Eley-Rideal process – then the consequences of formation pumping, as distinct from collisional and radiative pumping, should be clearly evident in both cases. In particular, we predict a clear spectral signature of this direct HH2 formation process on graphite, distinct from radiative and collisional pumping; this signature should be evident in both diffuse and dark clouds; but the emissivity for dark clouds is predicted to be some 500 times greater than that in diffuse clouds in which the dense material may be embedded. An observational search for this signature in two dark cloud sources was made, but a preliminary analysis of the data did not yield a detection. The implications of and possible reasons for this preliminary conclusion are discussed. This revised version was published online in July 2006 with corrections to the Cover Date.  相似文献   

7.
We investigate numerically the contribution to the cosmic gamma-ray background from cosmic-ray ions and electrons accelerated at intergalactic shocks associated with cosmological structure formation. We show that the kinetic energy of accretion flows in the low-redshift intergalactic medium is thermalized primarily through moderately strong shocks, which allow for an efficient conversion of shock ram pressure into cosmic-ray pressure. Cosmic rays accelerated at these shocks produce a diffuse gamma-ray flux which is dominated by inverse Compton emission from electrons scattering off cosmic microwave background photons. Decay of neutral π mesons generated in p–p inelastic collisions of the ionic cosmic-ray component with the thermal gas contribute about 30 per cent of the computed emission. Based on experimental upper limits on the photon flux above 100 MeV from nearby clusters we constrain the efficiency of conversion of shock ram pressure into relativistic CR electrons to  ≲1 per cent  . Thus, we find that cosmic rays of cosmological origin can generate an overall significant fraction of order 20 per cent and no more than 30 per cent of the measured gamma-ray background.  相似文献   

8.
We utilize a 21/2-D MHD time-dependent model to perform a parametric study of interplanetary shock propagation to 1 AU. The input conditions are represented by the following variables:(1) initial shock velocity, (2) duration of the driving pulse, and (3) width of the pulse at the near-Sun position (18 solar radii). The total net energy added to the solar wind was calculated for each pulse. The forward shock's travel time to, and the peak dynamic pressure at, 1 AU as a function of location along the shock front have been studied over a range of total input pulse energies from 1029 to 1032 ergs. For input pulses with modest angular width and temporal duration, we find that the propagation of the resulting interplanetary fast forward shock waves depends primarily upon the net input energy. The dependence of the transit time upon energy is a power law with a -1/3 index which corresponds to the classical, piston driven case. Reverse shocks are also formed behind all but the lowest energy shocks. Their properties, although also a function of input energy, depend upon the specific values of the input pulse shock velocity, width and duration. We also briefly discuss the propagation of the shocks out to 1 AU, and the conditions for which the interplanetary shocks depart from being symmetric about the input pulse central meridian due to magnetic and dynamic effects.  相似文献   

9.
In this article we extend the study performed in our previous article of the collapse of primordial objects. We here analyse the behaviour of the physical parameters for clouds ranging from 107 to 1015 M⊙. We study the dynamical evolution of these clouds in two ways: as purely baryonic clouds and as clouds with non-baryonic dark matter included. We start the calculations at the beginning of the recombination era, following the evolution of the structure until the collapse (which we defined as the time when the density contrast of the baryonic matter is greater than 104). We analyse the behaviour of several physical parameters of the clouds (e.g. the density contrast and the velocities of the baryonic matter and the dark matter) as a function of time and radial position in the cloud. In this study all physical processes that are relevant to the dynamical evolution of the primordial clouds, such as for example photon drag (due to the cosmic background radiation) and hydrogen molecular production, besides the expansion of the Universe, are included in the calculations. In particular we find that the clouds with dark matter collapse at higher redshift when we compare the results with the purely baryonic models. As a general result we find that the distribution of the non-baryonic dark matter is more concentrated than the baryonic one. It is important to stress that we do not take into account the putative virialization of the non-baryonic dark matter; we just follow the time and spatial evolution of the cloud, solving its hydrodynamical equations. We also studied the role of cooling–heating processes in the purely baryonic clouds.  相似文献   

10.
This paper presents a semi-empirical model for variations of interstellar polarization curves based upon the Serkowski-Wilking law for optical and near-infrared wavebands. The model assumes that nonspherical dust grains producing interstellar polarization are core-mantle particles shaped like oblate spheroids. The physical picture is one in which large (a 0 0.1µm) particles in the dense cloud phase are deposited into the diffuse cloud medium and thereafter undergo mantle processing by galactic shocks and UV starlight. It is shown that polarization curves vary their widths mainly as a consequence of the nonthermal sputtering of mantles by low-velocity shocks. Mantle sputtering by shocks in low density clouds tends to broaden the curves, whereas mantle sputtering by shocks in denser clouds produce narrow curves. Hence, shock processing of grain mantles can explain the observed correlation between the width of polarization curves and the dust grain environment.  相似文献   

11.
The shock waves associated with molecular outflows may be of continuous (C) type or jump (J) type, depending on conditions in the preshock gas, notably the degree of ionization. Intermediate situations also exist, in which a J-discontinuity terminates or is embedded in a C-type flow. We consider the results of recent modelling of several important cases where shock waves will comprise both C and J components, namely: (1) in the evolution of a planar J into a planar C type shock wave, prior to the attainment of a stationary state; (2) at shock speeds exceeding the maximum value for a planar C type shock, when the cooling flow encounters a sonic point; (3) in a curved bowshock, when the shock speed at the apex exceeds the maximum value for a C type shock while the bow wings remain of C type. We show that proper allowance for the departure of the chemistry from equilibrium, particularly the dissociation/reformation of H2, is crucial for an accurate treatment of the C to J transition. All the possible modes of shock propagation need to be considered when interpreting observations of molecular outflow sources. An important diagnostic tool in this context is the H2 excitation diagram, which plots the logarithm of the column densities of the H2 rotational levels, divided by the statistical weights, against their excitation energies. There can be large differences between the dynamical and physical conditions implied by J type shocks (with and without a precursor), C type shocks, and bowshocks which best fit the observed excitation diagram. We discuss, by means of examples, the use of the excitation diagram, in conjunction with our sophisticated shock model, to constrain conditions in shocks propagating in molecular outflows.  相似文献   

12.
The role of excited molecular hydrogen as a powerful observational tool is examined in the context of shock phenomena in molecular clouds, particularly in star forming regions. Conclusions that may be drawn from line intensities and line profiles, and the properties of J and C shocks in a bow shock structure are discussed.  相似文献   

13.
C-type shocks in the partially ionized ISM are modelled by numerical simulations. Under certain conditions the shocks are subject to the Wardle instability, which initially makes the shock front rippled, then in the non-linear stage can produce density variations in both the ion and neutral fluids. A systematic search in the numerically accessible parameter space is done to determine the wave vector kmax and the growth rates max of the fastest growing modes. The neutral Alfvén number, and the angle sbetween the shock normal and the upstream magnetic field determine the strength and obliqueness of the shock, as well as the dimensionless parameters of the fastest growing mode. The results confirm and extend Wardle's linear analysis.The non-linear evolution shows saturation of the instability and the formation of high density regions that detach from the shock front with the downstream flow. Numerical difficulties are partially solved by an implicit treatment of the ion-neutral friction terms, but strong shocks still can not be modelled efficiently. A fully implicit method for the ions and the magnetic field is used to model C-type shocks with low fractional ionization and high ion Alfvén speed.  相似文献   

14.
Galaxy merger simulations have explored the behaviour of gas within the galactic disc, yet the dynamics of hot gas within the galaxy halo have been neglected. We report on the results of high-resolution hydrodynamic simulations of colliding galaxies with metal-free hot halo gas. To isolate the effect of the halo gas, we simulate only the dark matter halo and the hot halo gas over a range of mass ratios, gas fractions and orbital configurations to constrain the shocks and gas dynamics within the progenitor haloes. We find that (i) a strong shock is produced in the galaxy haloes before the first passage, increasing the temperature of the gas by almost an order of magnitude to   T ∼ 106.3 K  . (ii) The X-ray luminosity of the shock is strongly dependent on the gas fraction; it is  ≳1039 erg s−1  for halo gas fractions larger than 10 per cent. (iii) The hot diffuse gas in the simulation produces X-ray luminosities as large as  1042 erg s−1  . This contributes to the total X-ray background in the Universe. (iv) We find an analytic fit to the maximum X-ray luminosity of the shock as a function of merger parameters. This fit can be used in semi-analytic recipes of galaxy formation to estimate the total X-ray emission from shocks in merging galaxies. (v) ∼10–20 per cent of the initial gas mass is unbound from the galaxies for equal-mass mergers, while 3–5 per cent of the gas mass is released for the 3:1 and 10:1 mergers. This unbound gas ends up far from the galaxy and can be a feasible mechanism to enrich the intergalactic medium with metals.  相似文献   

15.
We study the dynamical structure of a cooling dominated rotating accretion flow around a spinning black hole. We show that non-linear phenomena such as shock waves can be studied in terms of only three flow parameters, namely the specific energy     , the specific angular momentum (λ) and the accretion rate     of the flow. We present all possible accretion solutions. We find that a significant region of the parameter space in the     plane allows global accretion shock solutions. The effective area of the parameter space for which the Rankine–Hugoniot shocks are possible is maximum when the flow is dissipation-free. It decreases with the increase of cooling effects and finally disappears when the cooling is high enough. We show that shock forms further away when the black hole is rotating compared to the solution around a Schwarzschild black hole with identical flow parameters at a large distance. However, in a normalized sense, the flow parameters for which the shocks form around the rotating black holes are produced shocks closer to the black hole. The location of the shock is also dictated by the cooling efficiency in that higher the accretion rate     , the closer is the shock location. We believe that some of the high-frequency quasi-periodic oscillations may be due to the flows with higher accretion rate around the rotating black holes.  相似文献   

16.
We examine the behaviour of accretion flow around a rotating black hole in presence of cooling. We obtain global flow solutions for various accretion parameters that govern the accreting flow. We show that standing isothermal shock wave may develop in such an advective accretion flow in presence of cooling. This shocked solution has observational consequences as it successfully provides the possible explanations of energy spectra as well as generation of outflows/jets of various galactic and extra-galactic black hole candidates. We study the properties of isothermal shock wave and find that it strongly depends on the cooling efficiency. We identify the region in the parameter space spanned by the specific energy and specific angular momentum of the flow for standing isothermal shock as a function of cooling efficiencies and find that parameter space gradually shrinks with the increase of cooling rates. Our results imply that accretion flow ceases to contain isothermal shocks when cooling is beyond its critical value.  相似文献   

17.
We study the stability properties of hydrodynamic shocks with finite Mach numbers. The linear analysis supplements previous analyses which took the strong shock limit. We derive the linearized equations for a general specific heat ratio as well as temperature and density power-law cooling functions, corresponding to a range of conditions relevant to interstellar atomic and molecular cooling processes. Boundary conditions corresponding to a return to the upstream temperature  ( R = 1)  and to a cold wall  ( R = 0)  are investigated. We find that for Mach number   M > 5  , the strong shock overstability limits are not significantly modified. For   M < 3  , however, shocks are considerably more stable for most cases. In general, as the shock weakens, the critical values of the temperature power-law index (below which shocks are overstable) are reduced for the overtones more than for the fundamental, which signifies a change in basic behaviour. In the   R = 0  scenario, however, we find that the overstability regime and growth rate of the fundamental mode are increased when cooling is under local thermodynamic equilibrium. We provide a possible explanation for the results in terms of a stabilizing influence provided downstream but a destabilizing effect associated with the shock front. We conclude that the regime of overstability for interstellar atomic shocks is well represented by the strong shock limit unless the upstream gas is hot. Although molecular shocks can be overstable to overtones, the magnetic field provides a significant stabilizing influence.  相似文献   

18.
We suggest a model that explains the stratification peculiarities of the [O III] and Hα line emission from some of the ring nebulae around Wolf-Rayet stars. These peculiarities lie in the fact that the [O III] line emission regions are farther from the central star than the Hα regions, with the distance between them reaching several tenths of a parsec. We show that the radiative shock produced by a Wolf-Rayet stellar wind and propagating with a velocity of ～100 km s?1 cannot explain such large distances between these regions due to the low velocity of the gas outflow from the shock front. The suggested model takes into account the fact that the shock produced by a Wolf-Rayet stellar wind propagates in a two-phase medium: a rarefied medium and dense compact clouds. The gas downstream of a fast shock traveling in a rarefied gas compresses the clouds. Slow radiative shocks are generated in the clouds; these shocks heat the latter to temperatures at which ions of doubly ionized oxygen are formed. The clouds cool down, radiating in the lines of this ion, to temperatures at which Balmer line emission begins. The distance between the [O III] and Hα line emission regions is determined by the cooling time of the clouds downstream of the slow shock and by the velocity of the fast shock. Using the ring nebula NGC 6888 as an example, we show that the gas downstream of the fast shock must be at the phase of adiabatic expansion rather than deceleration with radiative cooling, as assumed previously.  相似文献   

19.
We discuss a special case of formation of axisymmetric shocks in the accretion flow of ideal gas on to a Schwarzschild black hole: when the total energy of the flow is negative. The result of our analysis enlarges the parameter space for which these steady shocks are exhibited in the accretion of gas rotating around relativistic stellar objects. Since Keplerian discs have negative total energy, we guess that, in this energy range, the production of the shock phenomenon might be easier than in the case of positive energy. So our outcome reinforces the view that sub-Keplerian flows of matter may significantly affect the physics of the high energy radiation emission from black hole candidates. We give a simple procedure to obtain analytically the position of the shocks. The comparison of the analytical results with the data of one-dimensional (1D) and two-dimensional (2D) axisymmetric numerical simulations confirms that the shocks form and are stable.  相似文献   

20.
Qualitative considerations of mass and particle loading in plasma-neutral gas interaction are made by the help of a one-dimensional steady model. In the case of particle loading the plasma flow behind a strong shock is accelerated as in the case of mass loading regarded by Biermannet al. (1967) whereas behind a weak shock the flow is decelerated. As an example, one-dimensional time-dependent flows with ionization of the neutral gas by Alfvén's critical velocity effect are calculated. Because of the acceleration of the subsonic flow by loading processes, the existence of a tangential discontinuity in the flow around comets seems questionable.  相似文献   

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