Excitation and kinematics in H2 bow shocks: near-infrared observations of HH 99 and VLA 1623A (HH 313)

We present a comprehensive near-infrared study of two molecular bow shocks in two protostellar outflows, HH 99 in R Coronae Australis and VLA 1623A (HH 313) in Rho Ophiuchi. New, high-resolution, narrow-band images reveal the well-defined bow shock morphologies of both sources. These are compared with two-dimensional MHD modelling of molecular bows from which we infer flow inclination angles, shock speeds and the magnetic field in the pre-shock gas in each system. With combined echelle spectroscopy and low-resolution K -band spectra we further examine the kinematics and excitation of each source. Bow shock models are used to interpret excitation (CDR) diagrams and estimate the extinction and, in the case of VLA 1623, the ortho–para ratio associated with the observed H₂ population. For the first time, morphology, excitation and kinematics are fitted with a single bow shock model.
Specifically, we find that HH 99 is best fitted by a C-type bow shock model (although a J-type cap is probably responsible for the [Fe  ii ] emission). The bow is flowing away from the observer (at an angle to the line of sight of ∼45°) at a speed of roughly 100 km s⁻¹. VLA 1623A is interpreted in terms of a C-type bow moving towards the observer (at an angle to the line of sight of ∼75°) at a speed of ∼80 km s⁻¹. The magnetic field associated with HH 99 is thought to be orientated parallel to the flow axis; in VLA 1623A the field is probably oblique to the flow axis, since this source is clearly asymmetric in our H₂ images.     

A gas dynamic model of the rotating solar atmosphere

A model is presented of a solar atmosphere which is heated by the periodic passage of shock waves. The outer atmosphere rotates and is assumed not to affect the strength of the shock waves. This constant shock strength hypothesis is used as the basis of the model of the outer solar atmosphere. From the model it is concluded that the chromospheric temperature rise and flow Mach number are slightly affected by the rotation of the atmosphere.     

Absolute and Convective Instability of Tangential Discontinuities in Viscous Fluids: Application to Heliopause

The stability of the heliopause, which is a tangential discontinuity separating the flow of the solar wind plasma compressed at the termination shock, from the flow of the insterstellar plasma compressed at the bow shock, is discussed. A brief review of the normal mode analysis is given. The recent results of the study of the absolute and convective instability of a tangential discontinuity in an incompressible plasma, viscous at one side of the discontinuity, and ideal at the other side, are presented. This equilibrium configuration can be considered as a crude model of the flow near the heliopause in its near-flank regions, where the flow is essentially subsonic. The obtained results suggest that the near flanks of the heliopause are only convectively unstable. The relation of these results with results of recent numerical investigations of the absolute and convective instability of the heliopause are discussed. This revised version was published online in July 2006 with corrections to the Cover Date.     

On the overtaking interaction of typical shock waves in the solar wind flow

The interaction of traveling fast solar shock waves with other fast shock waves generated previously is considered in terms of magnetohydrodynamics for various solar wind parameters. The shocks are not piston ones and move freely in the flow. The magnetic structure in the interplanetary magnetic field emerging after the shock interaction is shown to correspond to the well-known magnetic configuration commonly observed on spacecraft or the classical Hundhausen R model. A head-on collision of solar shock waves with the boundary of a magnetic cloud is considered. It is pointed out that a slow shockwave refracted into the magnetic cloud can appear at an oblique collision of the shock with the cloud boundary. The results clarify our understanding of the available spacecraft data.     

Flow behind an attached shock wave in a radiating gas

A simple method is used to determine the curvature of an attached shock wave and the flow variable gradients behind the shock curve at the tip of a straight-edged wedge placed symmetrically in a supersonic flow of a radiating gas near the optically thin limit. The shock curvature and the flow variable gradients along the wedge at the tip are computed for a wide range of upstream flow Mach numbers and wedge angles. Several interesting results are noted; in particular, it is found that the effect of an increase in the upstream flow Mach number or the radiative flux is to enhance the shock wave curvature which, however, decreases with an increase in the specific heat ratio or the wedge angle.     

MHD free-convection flow of an elasto-viscous fluid past an infinite vertical plate

A theoretical model of shock wave propagation in a self-gravitating radiative magneto-hydrodynamic medium has been studied. The effects of the magnetic field, radiation, and gravitation have been discussed separately. The results discussed depend upon the numerical variations of flow variables behind the shock.     

A self-similar solution for a supernova explosion with allowance for accelerated relativistic particles

A self-similar solution to Sedov’s problem of a strong explosion in a homogeneous medium is generalized to the case of relativistic-particle generation in a supernova remnant; the particles are accelerated by Fermi’s mechanism at the shock front and in the perturbed post-shock region. Self-similarity takes place if the thickness of the prefront is small compared to its radius and if the pressure ratio of the relativistic and nonrelativistic components at the shock front is kept constant. In the presence of relativistic particles, the time dependence of the shock-front radius remains the same as that in their absence, but the plasma parameters in the inner perturbed region change appreciably. The shell of the matter raked up by the explosion is denser and thinner than that in the nonrelativistic case, the relativistic-particle pressure in the central region remains finite, and the nonrelativistic-gas pressure at the explosion center approaches zero. The influence of relativistic particles on the transition to the radiative phase of expansion of the supernova remnant and on its dynamics is studied. It is shown that relativistic particles can decrease several-fold the remnant radius at which the transition to the radiative phase occurs.     

A textbook case of bow shock entrainment in a YSO outflow

Near-infrared images in H₂ line emission and submillimetre maps in CO J  = 3–2 emission illustrate the remarkable association between a molecular bow shock and the redshifted molecular outflow lobe in W75N. The flow lobe fits perfectly into the wake of the bow, as one would expect if the lobe represented swept-up gas. Indeed, these observations strongly support the 'bow shock' entrainment scenario for molecular outflows driven by young stars.   The characteristics of the bow shock and CO outflow lobe are compared with those of numerical simulations of jet-driven flows. These models successfully reproduce the bulge and limb-brightening in the CO outflow, although the model H₂ bow exhibits more structure extending back along the flow axis. We also find that the size of the flow, the high mass fraction in the flow at low outflow velocities (low γ values) and the high CO/H₂ luminosity ratio indicate that the system is evolved. We also predict a correlation, in evolved systems, between outflow age and the CO/H₂ luminosity ratio.     

Similarity solution for the propagation of shock waves with varying strength in radiating medium

A model of similarity solution for the propagation of shock waves produced on account of an instantaneous release of energy in an inhomogeneous medium with the effect of radiation has been discussed. The disturbances of the medium are headed by a shock of variable strength. The variations of flow variables have been discussed for the different values of strength of the shock.     

Analysis of the H ring velocity distribution function near the Martian and Venusian bow shocks by an analytical model

We have studied the H⁺ velocity distribution function at Mars and Venus near the bow shock both in the solar wind and in the magnetosheath by a simple analytical one-dimensional model. We found that over half of the ions in the ring velocity distribution which moved towards the magnetosheath were scattered back into the bow shock. The original ring distribution is destroyed in less than an ion gyro period. Ions contained in the magnetosphere which hit the bow shock bounce back into the solar wind with a maximum energy exceeding twice the energy of solar wind protons. The ions finite gyroradius causes an asymmetric flow in the magnetosheath with respect to the direction of the convective electric field, which can be observed already a few ion gyroradius downstream of the bow shock.     

Propagation of spherical MHD shock-waves in self-gravitating gas

A theoretical model of shock-wave propagation has been studied in a heat-conducting and a self-gravitating medium. The effects of magnetic field has been taken into consideration. The shock is strong enough to neglect the ambient gas pressure. The variation of flow variables behind the shock have been investigated numerically.     

Self-similar model of magnetohydrodynamic radiative shock waves with increasing density

The magnetohydrodynamic model of shock waves has been discussed in an atmosphere with gravitation and radiation. The disturbance is headed by a strong shock of increasing density. The medium ahead of the shock is assumed to be inhomogeneous and at rest. Variation of magnetic field radiation flux, and other flow variables are given in tabular form.     

Theoretical analysis on the geoeffectiveness of a shock overtaking a preceding magnetic cloud

The shock compression of the preexisting southward directed magnetic field can enhance a geomagnetic disturbance. A simple theoretical model is proposed to study the geoeffectiveness of a shock overtaking a preceding magnetic cloud. Our aim is to answer theoretically the question how deep the shock enters into the cloud when the event just reaches the maximum geoeffectiveness. The results suggest that the minimum value of Dst ^* decreases initially, then increases again while the shock propagates from the border to the center of the cloud. There is a position where the shock compression of the preceding cloud obtains the maximum geoeffectiveness. In different situations, the position is different. The higher the overtaking shock speed is, the deeper is this position, and the smaller is the corresponding Dst ^* _min. Some shortcomings of this theoretical model are also discussed.     

High‐pressure polymorphs of magnesian orthopyroxene from a shock vein in the Yamato‐000047 lherzolitic shergottite

Abstract– We found a simple thin shock vein, less than or equal to about 60 μm in width and 1.8 mm in length, in the poikilitic area in the Yamato (Y‐) 000047 lherzolitic shergottite. The shock vein occurs only in magnesian Ca‐poor clinopyroxene, which may have transformed from orthopyroxene during the pressure increase at the shock event. The shock vein consists of (Mg_0.8,Fe_0.2)SiO₃ pyroxene polymorphs, such as columnar akimotoite, two kinds of pyroxene glasses, dendritic akimotoite, and framboidal pyroxene glass, in the order from the periphery to the center. The compositions and textures suggest that columnar akimotoite in the periphery of the shock vein crystallized from solid‐state phase transition of clinopyoroxene during the cooling of the vein, and the remains in the shock vein solidified from shock‐produced melt. The glass includes two kinds of massive glass in the vein and framboidal glass in the vein center. The framboidal glass is the most magnesian and may have been vitrified from perovskite crystallized from high‐pressure melt produced at high temperature ≥3000 °C and high‐pressure 23–40 GPa. Dendritic akimotoites in the vein center metastably crystallized from residual shock melt. The formation sequences of the constituent phases in the shock vein happen in the following order: columnar akimotoites, rim glass, center glass, framboidal glass, and dendritic akimotoites. The increase of the Raman intensity of 660–670 cm^?1 in the order of rim glass, center glass, and framboidal glass suggests that the formation of the pyroxene chain proceeds faster in the vein center than in the vein rim due to its slower cooling. The finding of the shock vein consisting merely of high‐pressure polymorphs of pyroxene, akimotoite, and framboidal glass (vitrified perovskite) is the first reported among all Martian meteorites.     

New, high-resolution, near-infrared observations of HH 1

We present new, high-resolution, near-infrared images of the HH 1 jet and bow shock. H₂ and [Fe  ii ] images are combined to trace excitation changes along the jet and across the many shock features in this flow. Echelle spectra of H₂ profiles towards a few locations in HH 1 are also discussed. Gas excitation in oblique, planar C-type shocks best explains the observations, although J-type shocks must be responsible for the observed [Fe  ii ] emission features. Clearly, no single shock model can account for all of the observations. This will probably be true of most, if not all, Herbig–Haro flows.     

A self-similar flow of self-gravitating gas behind a shock wave with increasing energy,I

Self-similar flows of self-gravitating gas behind a spherical shock wave which are driven out by a propelling contact surface, propagating in a nonuniform atmosphere at rest, are investigated. The total energy content of the flow between a shock front and contact surface is taken to be time-dependent. In brief, the self-similar homothermal flows of self-gravitating gas behind a shock wave and Roche's model case are also studied in the present paper.     

The termination shock in a striped pulsar wind

The origin of radio emission from plerions is considered. Recent observations suggest that radio-emitting electrons are presently accelerated rather than having been injected at early stages of the plerion evolution. The observed flat spectra without a low-frequency cut-off imply an acceleration mechanism that raises the average particle energy by orders of magnitude but leaves most of the particles at an energy of less than approximately a few hundred MeV. It is suggested that annihilation of the alternating magnetic field at the pulsar wind termination shock provides the necessary mechanism. Toroidal stripes of opposite magnetic polarity are formed in the wind emanating from an obliquely rotating pulsar magnetosphere (the striped wind). At the termination shock, the flow compresses and the magnetic field annihilates by driven reconnection. Jump conditions are obtained for the shock in a striped wind. It is shown that the post-shock magnetohydrodynamic parameters of the flow are the same as if the energy of the alternating field had already been converted into plasma energy upstream of the shock. Therefore, the available estimates of the ratio of the Poynting flux to the matter energy flux, σ, should be attributed not to the total upstream Poynting flux but only to that associated with the average magnetic field. A simple model for the particle acceleration in the shocked striped wind is presented.     

Revealing the “fingerprints” of the magnetic precursor of C-shocks

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.     

Radio Observations of Sgr B2

1 INTRODUCTIONFor the behavior of a molecular cloud in subsonic collision with another, Mao et al. (1992)have obtained simplified one-dimension traveling wave solutions for a plane-parallel s1ab. Chang-ing the sign in the transformation of variables in their case, we have the fOllowing results,1 rP = 2 l W op -- 1, (1)1 r =v = -- j W -- ry 1. (2)2 {V(N M)' 2z 2t -- W 1. (2)In Eq. (1), p increases with increasing t. Instability is expected to occur in strongly perturbedmolecular…     

The motion of the cylindrical-symmetrical relativistic shock wave in the presence of an axial magnetic field

Some observed astrophysical phenomena, such as the blast of a supernova, suggest the necessity to study the motion of shock waves in a relativistic fluid flow in the presence of a magnetic field. This paper deals with the motion of a special relativistic shock wave which propagates from the center line outwardly after an explosion with the assumption that the magnetic field which has an axial component only. Similarity solutions which depend on the parameter =r/t are constructed. Two special cases are then studied in detail. In the first case, there is an ultrarelativistic fluid in front of the shock and in the second case, there is a cold fluid in front of the shock.