The flowing interstellar medium in the central parsec of an active nucleus plays a crucial role in determining its observed emission line features. Mass loss from the central stellar cluster acts as one source of material in this flow. Since the flow is likely to be hypersonic, shock waves will be created in it. Gas in thermal equilibrium with the central radiation field and at the stagnation pressure of the flow has an ionization parameter corresponding to that deduced from observations of the high-ionization broad emission lines (HIL). The thermal equilibrium is obtained as the shocked gas cools rapidly by Compton scattering of the central continuum. The entire flow back-scatters X-rays which can illuminate the outer regions of an accretion disc thus providing the deep X-ray heating needed to produce the low-ionization broad emission lines (LIL). The outer region where this disc becomes optically thin radiates most of these LIL and perhaps the observed near-infrared bump. This two-component model resolves the well-known cloud confinement problem by gravitational confinement for the LIL and by advocating a transient cloud population for the HIL. It also leads to a simple explanation for the systematic velocity shifts observed between highand low-ionization lines.

The interaction of an astrophysical shock with a cloud typically occurs at high Reynolds number, and in such cases will be highly turbulent. However, the formation of fully developed turbulence is usually prevented by the artificial viscosity inherent in hydrodynamical simulations. Upstream structures mean that the flow behind the shock is also likely to be turbulent, as it sweeps over such inhomogeneities. We study the nature of adiabatic shock-cloud interactions using a subgrid compressible k–ε turbulence model.

The interaction of a shock with a cloud has been extensively studied in the literature, where the effects of magnetic fields, radiative cooling and thermal conduction have been considered. In many cases, the formation of fully developed turbulence has been prevented by the artificial viscosity inherent in hydrodynamical simulations. This problem is particularly severe in some recent simulations designed to investigate the interaction of a flow with multiple clouds, where the resolution of individual clouds is necessarily poor. Furthermore, the shocked flow interacting with the cloud has been assumed to be completely uniform in all previous single-cloud studies. In reality, the flow behind the shock is also likely to be turbulent, with non-uniform density, pressure and velocity structure created as the shock sweeps over inhomogeneities upstream of the cloud (as seen in recent multiple cloud simulations). To address these twin issues we use a subgrid compressible k-epsilon turbulence model to estimate the properties of the turbulence generated in shock-cloud interactions and the resulting increase in the transport coefficients that the turbulence brings. A detailed comparison with the output from an inviscid hydrodynamical code puts these new results into context.Despite the above concerns, we find that cloud destruction in inviscid and k-epsilon models occurs at roughly the same speed when the post-shock flow is smooth and when the density contrast between the cloud and intercloud medium, chi less than or similar to 100. However, there are increasing and significant differences as chi increases. The k-epsilon models also demonstrate better convergence in resolution tests than inviscid models, a feature which is particularly useful for multiple-cloud simulations.Clouds which are over-run by a highly turbulent post-shock environment are destroyed significantly quicker as they are subject to strong 'buffeting' by the flow. The decreased lifetime and faster acceleration of the cloud material to the speed of the ambient flow leads to a reduction in the total amount of circulation (vorticity) generated in the interaction, so that the amount of vorticity may be self-limiting. Additional calculations with an inviscid code where the post-shock flow is given random, grid-scale, motions confirm the more rapid destruction of the cloud.Our results clearly show that turbulence plays an important role in shock-cloud interactions, and that environmental turbulence adds a new dimension to the parameter space which has hitherto been studied.

Many diffuse astrophysical media consist of multiple components, and their multicomponent natures affect the evolution of shocks in them. The focus here is on three problems concerning multicomponent models of media containing shocks. The subject of the first is the cosmic ray modification of shocks in supernova remnants and recently begun efforts to model observed shocks in the Tycho and Cygnus Loop remnants. The next involves shocks in the central 25 kpc of the Perseus cluster of galaxies which may contain an AGN-driven fountain flow in which multiple phase changes occur. The final one concerns multifluid models of shocks in magnetized, dense, dusty star forming regions. The exposition is mostly about work still in progress or that has not even fully begun.

Aims. We have examined a stream-source model for the production of the cometary tails observed in the Helix Nebula NGC 7293 in which a transonic or moderately supersonic stream of ionized gas overruns a source of ionized gas. We have compared the velocity structures calculated with the available observational data. We have also investigated the suggestion that faint striations visible in the ne bular gas are the decaying tails of now destroyed cometary globules. Methods. We have selected relevant results from extensive hydrodynamic calculations made with the COBRA code. Results. The velocities calculated are in good agreement with the observational data on tail velocities and are consistent with o bservations of the nebular structure. The results also are indicative of a stellar atmosphere origin for the cometary globules. Tail remnants persist for timescales long enough for their identification with striat ions to be plausible.

A possible microjet from a low-mass but young star, which has already shed its cocoon, could be rendered observable by the Lyman photon flux in the interior of the Rosette Nebula. Outside this environment it may not have been observable at optical wavelengths. The kinematics of this proposed monopolar microjet from an F8 Ve star have been investigated by spatially resolved, long-slit, spectral observations with the Manchester Echelle Spectrometer on the San Pedro Martir telescope (Mexico). The flow is shown to be approaching to give a radial velocity difference from the host nebula of -56 km s-1. An outflow velocity of, at the most, a few hundreds of km s-1 is therefore indicated. If the flow velocity is taken as 200 km s-1, which is found in other microjets, then this jet's inclination to the sky is ≈16°. The mass in the outflowing ionized gas is estimated from the surface brightness of the Hα emission as ≈6 × 1027 g to give an estimated mass-loss rate of 10-8 M⊙ yr-1, which, along with the detection of the outflow velocity, confirms its microjet identification even though an uncertain filling factor was used in these calculations. The hottest cluster star, which is also in the neighborhood of the microjet, is found alone to emit marginally sufficient Lyman photons to account for the ionization of the jet, although direct observations of the local electron density from optical line ratios are required to confirm this point conclusively.

We present hydrodynamical calculations of radiative shocks with low Mach numbers and find that the well-known global overstability can occur if the temperature exponent ( a) of the cooling is sufficiently negative. We find that the stability of radiative shocks increases with decreasing Mach number, with the result that M = 2 shocks require alpha <= -1.2 in order to be overstable. Such values occur within a limited temperature range of many cooling curves. We observe that Mach numbers of order 100 are needed before the strong shock limit of alpha(cr) approximate to 0.4 is reached, and we discover that the frequency of oscillation of the fundamental mode also has a strong Mach number dependence. We find that feedback between the cooling region and the cold dense layer (CDL) further downstream is a function of Mach number, with stronger feedback and oscillation of the boundary between the CDL and the cooling region occuring at lower Mach numbers. This feedback can be quantified in terms of the reflection coefficient of sound waves, and in those cases where the cooling layer completely disappears at the end of each oscillation cycle, the initial velocity of the waves driven into the upstream pre-shock flow and into the downstream CDL, and the velocity of the the boundary between the CDL and the cooling layer, can be understood in terms of the solution to the Riemann problem. An interesting finding is that the stability properties of low Mach number shocks can be dramatically altered if the shocked gas is able to cool to temperatures less than the pre-shock value ( i.e. when chi < 1, where chi is the ratio of the temperature of the cold dense layer to the pre-shock temperature). In such circumstances, low Mach number shocks have values of acr which are comparable to values obtained for higher Mach number shocks when chi = 1. For instance, alpha(cr) = -0.1 when M = 2 and chi = 0.1, comparable to that when M = 10 and chi = 1. Thus, it is probable that low Mach number astrophysical shocks will be overstable in a variety of situations. We also explore the effect of different assumptions for the initial hydrodynamic set up and the type of boundary condition imposed downstream, and find that the properties of low Mach number shocks are relatively insensitive to these issues. The results of this work are relevant to astrophysical shocks with low Mach numbers, such as supernova remnants (SNRs) immersed in a hot interstellar medium ( e. g., within a starburst region), and shocks in molecular clouds, where time-dependent chemistry can lead to overstability.

Because of its key role in feedback in star formation and galaxy formation, we examine the nature of the interaction of a flow with discrete sources of mass injection. We show the results of two-dimensional numerical simulations in which we explore a range of configurations for the mass sources and study the effects of their proximity on the downstream flow. The mass sources act effectively as a single source of mass injection if they are so close together that the ratio of their combined mass injection rate is comparable to or exceeds the mass flux of the incident flow into the volume that they occupy. The simulations are relevant to many diffuse sources, such as planetary nebulae and starburst superwinds, in which a global flow interacts with material evaporating or being ablated from the surface of globules of cool, dense gas.

We have calculated the evolution of spherical accretion flows undergoing mass-loading from embedded clouds through either conduction or hydrodynamical ablation. We have observed the effect of varying the ratios of the mass-loading timescale and the cooling timescale to the ballistic crossing timescale through the mass-loading region. We have also varied the ratio of the potential energy of a particle injected into the flow near the outer region of mass-loading to the temperature at which a minimum occurs in the cooling curve. The two types of mass-loading produce qualitatively different types of behaviour in the accretion flow, since mass-loading through conduction requires the ambient gas to be hot, whereas mass ablation from clumps occurs throughout the flow. Higher ratios of injected to accreted mass typically occur with hydrodynamical ablation, in agreement with previous work on wind-blown bubbles and supernova remnants. We find that mass-loading damps the radiative overstability of such flows, in agreement with our earlier work. If the mass-loading is high enough it can stabilize the accretion shock at a constant radius, yielding an almost isothermal subsonic post-shock flow. Such solutions may be relevant to cooling flows onto massive galaxies. Mass-loading can also lead to the formation of isolated shells of high temperature material, separated by gas at cooler temperatures.

We investigate the evolution of spherically symmetric supernova remnants in which mass loading takes place due to conductively driven evaporation of embedded clouds. Numerical simulations reveal significant differences between the evolution of conductively mass loaded and the ablatively mass loaded remnants studied in Paper I. A main difference is the way in which conductive mass loading is extinguished at fairly early times, once the interior temperature of the remnant falls below ~10 million K. Thus, at late times remnants that ablatively mass load are dominated by loaded mass and thermal energy, while those that conductively mass load are dominated by swept-up mass and kinetic energy. Simple approximations to the remnant evolution, complementary to those in Paper I, are given.

One aspect of supernova remnant evolution that is relatively unstudied is the inuence of an AGN environment. A high-density ambient medium and a nearby powerful continuum source will assist the cooling of shocked ejecta and swept-up gas, and recent results of simulations with appropriate heating and cooling terms are presented. A central nding is that the shocked ejecta rapidly cool to form cold dense clouds with a density, an ionization parameter, and a column density compatible with those inferred for the high-ionization component of the broad-emission-line regions in QSOs.

We briefly discuss the observational evidence for linear features in diffuse astronomical sources. We then discuss their formation either by radiation shadowing or by hydrodynamic processes.

We give an overview of the literature on ionization front structure and propagation and comment on recombination front structures. We discuss the formation of cometary tails by photoionization of globules and note the importance of including the diuse component of the ionizing radiation eld. We nally briey comment on the formation of small scale structures in H II regions.

We present accurate measurements of the physical conditions in five powerful radio galaxies, as derived from deep, long-slit spectroscopic observations. All five objects show prominent extended line emission, and have X-ray luminosities similar to those of isolated elliptical galaxies. The data are high enough quality that the electron density and temperature can be measured at several positions across the emission-line nebulae.We subtract a model continuum comprising a combination of a 15-Gyr stellar template, a young stellar template and a power law, so as to be better able to measure faint diagnostic lines. Electron temperatures measured from the [O III](4959 + 5007)/4363 line ratio are in the range 10 000 < T-e < 20 000 K, whilst [S II](6716/6731) densities fall between 100-500 cm(-3). Using these values, we find pressures within the line-emitting clouds a factor of 10-100 times higher than expected for pressure balance with the hot X-ray haloes of the host galaxies.Previous studies of sources that show significant evidence of jet-cloud interactions, both in terms of their kinematics and ionization, have concluded that the overpressure is a result of the warm, line-emitting gas being compressed by the radio cocoon; however, there is no evidence that the radio jet is influencing the emission-line regions in four of our five objects.We suggest that it is plausible that the line-emitting clouds have not yet relaxed into pressure equilibrium from their initial photoionization by the central active galactic nucleus.

Mass-loading of winds and jets occurs over a wide range of length scales and astrophysical circumstances. We briey review the general properties of simple steady spherically symmetric o ws and comment on the relevance of critical points to mass pick-up in them. We discuss the importance of intermediate scale structures. A brief overview of mass addition to jets is then given followed by comments on mass addition to bursting supernova driven blisters.

We summarize previous work on hydrodynamic ionization fronts and new work where magnetic fields are incorporated into their structures. We describe recent work on recombination front structures and outline refinements which need to be made to both them and ionization fronts.

We describe the effect of magnetic fields threading the interstellar medium on an ionization front (IF) moving through it. The standard classification of IF breaks down, with separate classes of R- and D-type Solutions appearing about each of the fast and slow magnetosonic critical speeds. Internal structure calculations confirm the results derived from evolutionary constraints.

We study the structure of shocks in clumpy media, using a multifluid formalism. As expected, shocks broaden as they weaken: for sufficiently weak shocks, no viscous subshock appears in the structure. This has significant implications for the survival of dense clouds in regions overrun by shocks in a wide range of astrophysical circumstances, from planetary nebulae to the nuclei of starburst galaxies.

We describe the evolution of spherically symmetric supernova remnants in which mass addition takes place from embedded clouds. We derive simple approximations to the remnant evolution which can be used for investigating the generation of supernova induced phenomena such as galactic superwinds.