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.     

The infrared emission lines of H2

The photodissociation regions located between ionized regions and molecular clouds are studied by using a one-dimensional model where molecular H₂ are formed on the dust grains, and destructed by photodissociation. The escape probability method is used for the line transfer. The excitation of infrared emission lines of H₂ by UV fluorescence in M17, by shock heating in Orion KL and mainly by UV fluorescence in NGC 2023 are discussed.     

A simple model for H2 line profiles in bow shocks

We present a model for empirically reproducing line profiles of molecular hydrogen emission in bow shocks. The model takes into account bow velocity, dissociation limit, a cooling function, viewing angle, bow shape and a limited form of extinction. Our results show that both geometrical factors and shock physics can significantly affect the profile morphology. In a companion paper we will apply this model to Fabry–Perot observations of bow shocks in the Orion BN–KL outflow.     

An interpretation of the semicircular structure of the supernova remnant CTB-109

The supernova exploded at the boundary of a dense molecular cloud in a diffuse gas. The eastern half of the shock wave entered the diffuse gas and is now in the stage of adiabatic expansion, forming the observed semicircular remnant; the western half entered the dense cloud, moved at a much slower speed and is now in the pressure-driven snowplow phase, the radiation it emits is in the uv, and is completely absorbed by the interstellar gas along the line of sight.     

Chemical evolution of the gas in C-type shocks in dark clouds

A magnetohydrodynamic model of a steady, transverse C-type shock in a dense molecular cloud is presented. A complete gas–grain chemical network is taken into account: the gas-phase chemistry, the adsorption of gas species on dust grains, various desorption mechanisms, the grain surface chemistry, the ion neutralization on dust grains, the sputtering of grain mantles. The population densities of energy levels of ions CI, CII and OI and molecules H₂, CO, H₂O are computed in parallel with the dynamical and chemical rate equations. The large velocity gradient approximation is used in the line radiative transfer calculations. The simulations consist of two steps: (i) modelling of the chemical and thermal evolution of a static molecular cloud and (ii) shock simulations. A comparison is made with the results of publicly available models of similar physical systems.The focus of the paper is on the chemical processing of gas material and ice mantles of dust grains by the shock. Sputtering of ice mantles takes place in the shock region close to the temperature peak of the neutral gas. At high shock speeds, molecules ejected from ice mantles are effectively destroyed in hot gas, and their survival time is low—of the order of dozens of years. After a passage of high-speed C-type shock, a zone of high abundance of atomic hydrogen appears in the cooling postshock gas that triggers formation of complex organic species such as methanol. It is shown that abundances of some complex organic molecules (COMs) in the postshock region can be much higher than in the preshock gas. These results are important for interpretation of observations of COMs in protostellar outflows.     

Shocks in Protostellar Outflows

We present our analysis of four molecular outflows from Class 0 (Cep E,L 1448) sources and higher mass objects (Cep A, DR 21). The emission line spectra of these outflows were observed in the mid- and far-infrared using the spectrometers (SWS, LWS) and the camera (ISOCAM) aboard the ISO satellite. We interpret the spectra using J- and C-type bow shock models and infer properties of both the outflow and surrounding gas. We find C-type bows with a shape parameter of s = 1.4 as the best interpretation of the measured line fluxes, independent of the object. The emission is partly caused by fluorescence.     

High-resolution observations of the Orion bright bar

High resolution strip maps of CS (J=1–0) and H51 line emission across the Orion bright bar are presented. They reveal the existence of a high density molecular layer (molecular sheet) plane parallel to the ionization front. This molecular sheet is redshifted relative to the ambient molecular cloud by about 2 km s^–1. The rapid decrease of the CS emission at about 50 arc sec from the bar suggests that a shock front exists here and the sheet is a post shock layer.Paper presented at the IAU Third Asian-Pacific Regional Meeting, held in Kyoto, Japan, between 30 September–6 October, 1984.This work was carried out under the common use observation program at the Nobeyama Radio Observatory (NRO). NRO, a branch of the Tokyo Astronomical Observatory, University of Tokyo, Japan, is a cosmic radio observing facility open to outside users.     

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.     

Near-infrared spectroscopy of (proto)-planetary nebulae: molecular hydrogen excitation as an evolutionary tracer

We present an in-depth analysis of molecular excitation in 11 H₂-bright planetary and protoplanetary nebulae (PN and PPN). From newly acquired K -band observations, we extract a number of spectra at positions across each source. H₂ line intensities are plotted on 'column density ratio' diagrams so that we may examine the excitation in and across each region. To achieve this, we combine the shock models of Smith, Khanzadyan & Davis with the photodissociation region (PDR) models of Black & van Dishoeck to yield a shock-plus-fluorescence fit to each data set.
Although the combined shock + fluorescence model is needed to explain the low- and high-energy H₂ lines in most of the sources observed (fluorescence accounts for much of the emission from the higher-energy H₂ lines), the relative importance of shocks over fluorescence does seem to change with evolutionary status. We find that shock excitation may well be the dominant excitation mechanism in the least evolved PPN (CRL 2688 – in both the bipolar lobes and in the equatorial plane) and in the most evolved PN considered (NGC 7048). Fluorescence, on the other hand, becomes more important at intermediate evolutionary stages (i.e. in 'young' PN), particularly in the inner core regions and along the inner edges of the expanding post-asymptotic giant branch (AGB) envelope. Since H₂ line emission seems to be produced in almost all stages of post-AGB evolution, H₂ excitation may prove to be a useful probe of the evolutionary status of PPN and PN alike. Moreover, shocks may play an important role in the molecular gas excitation in (P)PN, in addition to the low- and/or high-density fluorescence usually attributed to the excitation in these sources.     

Shocked molecular gas towards the supernova remnant G359.1–0.5 and the Snake

We have found a bar of shocked molecular hydrogen (H₂) towards the OH(1720 MHz) maser located at the projected intersection of supernova remnant (SNR)  G359.1–0.5  and the non-thermal radio filament known as the Snake. The H₂ bar is well aligned with the SNR shell and almost perpendicular to the Snake. The OH(1720 MHz) maser is located inside the sharp western edge of the H₂ emission, which is consistent with the scenario in which the SNR drives a shock into a molecular cloud at that location. The spectral line profiles of ¹²CO, HCO⁺ and CS towards the maser show broad-line absorption, which is absent in the ¹³CO spectra and most probably originates from the pre-shock gas. A density gradient is present across the region and is consistent with the passage of the SNR shock, while the H₂ filament is located at the boundary between the pre-shock and post-shock regions.     

Heterodyne spectroscopy of the J = 22-21 CO line in Orion

We have observed the J = 22-21 line of 12CO at 2528 GHz (118.8 micrometers) in the IRc2 region of Orion. The spectra at 0.6 km s-1 resolution show both plateau emission with FWHM approximately 35 km s-1 and a narrower component with FWHM approximately 8 km s-1. Comparison with heterodyne data of similar quality on the J = 17-16 line indicates that the broad and narrow components both originate in gas with an excitation temperature Tex approximately 600 K. The emission is consistent with the predictions of shock models in which the wide component arises from the heated outflow gas and postshock molecular material, while the narrow component comes from ambient material near the leading edge of the shock front where temperatures are high but significant acceleration has not yet occurred.     

Stratification of optical emission from NGC 6888 as a trace of the interaction between Wolf-Rayet stellar wind and the shell of a red supergiant

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.     

Ionization structure in accretion shocks with a composite cooling function

We have investigated the ionization structure of the post-shock regions of magnetic cataclysmic variables, using an analytic density and temperature structure model in which effects caused by bremsstrahlung and cyclotron cooling are considered. We find that in the majority of the shock-heated region where H- and He-like lines of the heavy elements are emitted, the collisional-ionization and corona-condition approximations are justified. We have calculated the line emissivity and ionization profiles for iron as a function of height within the post-shock flow. For low-mass white dwarfs, line emission takes place near the shock. For high-mass white dwarfs, most of the line emission takes place in regions well below the shock and hence it is less sensitive to the shock temperature. Thus, the line ratios are useful to determine the white dwarf masses for the low-mass white dwarfs, but the method is less reliable when the white dwarfs are massive. Line spectra can, however, be used to map the hydrodynamic structure of the post-shock accretion flow.     

Asymmetries of the solar Ca ii lines

A theoretical study of the influence of propagating acoustic pulses in the solar chromosphere upon the line profiles of the Ca ii resonance and infrared triplet lines has been made. The major objective has been to explain the observed asymmetries seen in the cores of the H and K lines and to predict the temporal behavior of the infrared lines caused by passing acoustic or shock pulses. The velocities in the pulses, calculated from weak shock theory, have been included consistently in the non-LTE calculations. The results of the calculations show that these lines are very sensitive to perturbations in the background atmosphere caused by the pulses. Only minor changes in the line shapes result from including the velocities consistently in the line source function calculations. The qualitative changes in the line profiles vary markedly with the strength of the shock pulses. The observed differences in the K line profiles seen on the quiet Sun can be explained in terms of a spectrum of pulses with different wave-lengths and initial amplitudes in the photosphere.     

Spectral line and white-light intensities in the corona in the presence of propagating or standing shocks

The effect of a propagating shock on the Hi L line and the polarization brightness in the inner solar wind region is investigated. We find that the shock produces measurable changes in both and, provided the measurements are made simultaneously, the alteration of the density and velocity across the shock can be derived. For a standing shock the effect on the L line and the white-light radiation is much smaller.     

The observational study of Herbig-Haro shock waves

We review the basic shock properties and the origin and the geometry of Herbig-Haro (H-H) shock waves. We first discuss different aspects of “normal” H-H objects which are connected with working surfaces (including internal working surfaces) of jets from young stellar objects. The emphasis is on unsolved problems of the H-H shock waves and not on the problems of the jet. We study the line flux ratios of high excitation H-H objects (high velocity shocks) and low excitation HH objects (low velocity shocks) and carry out a comparison with theoretical predictions in both cases. We emphasize an unexplained deficit of higher ions (especially OIII and SIII, but also various other ions) in high excitation objects. This lets the line flux ratios of HH objects appear as if their shock velocities are almost never above 100 km s^?1, while other shock diagnostics (position-velocity diagrams, integrated line profiles, distributions of fluxes along the axis of the bow shock, etc.) definitely indicate higher shock velocities. Some aspects of the spectrum interpretation of the very low velocity shocks (like HH7) are explained quite well by the theory. A basic unsolved problem is, however, the explanation of the CI lines whose flux is up to a factor 10 times stronger than predicted for any model. Obviously we are very far from correctly predicting the ionization of C in shock models. In the last chapter we discuss, as one example of a very unusual HH-object, HH255 (Burnham's nebula). Detailed line fluxes in the immediate environment of T Tauri (the source of HH255) have shown that HH255 has a shock wave spectrum and is definitely an HH object. In the very narrow region between 3″ and 4″ S of T Tauri we find a sharp peak of the velocity dispersion, the centroid velocity, and N_e. In the same region there is an almost discontinous increase in ionization. Between 4″ and 10″ S (corresponding to 600-1600 a.u.) of T Tauri (the source of HH255) the ionization remains high but the centroid velocity is zero (with respect to T Tauri) and the velocity dispersion is very small. This result is completely surprising for a shock wave which according to the flux ratios must have ～90 km s^?1-1 shock velocity. Why should a cooling region of a shock have a centroid velocity of ～0 km s^?1 over a large range of distance from the stellar source? At present the geometry of the HH255 is enigmatic.     

Self-similar solutions for the blast wave problem in magnetogasdynamics,II

The problem of explosion along a line in a gas cloud in the presence of transverse magnetic field has been considered. Similarity solutions of the adiabatic motion of a gas behind an infinitely strong cylindrical shock wave propagating into an infinitely conducting medium at rest is obtained. Shock radius varies exponentially with time and density is inversely proportional to fourth power of shock radius just ahead of the shock front.     

Distribution of the Molecular Gas Around SN 1572

The early-stage structure and evolution of a supernova remnant (SNR) depends largely on its ambient interstellar medium, so the interstellar medium becomes the valid probe for investigating the evolution of SNRs. We have observed the ¹²CO (J = 1 − 0) line emission around the remnant of SN 1572 with the 13.7m millimeter-wave telescope at the Qinghai Station of PMO, in order to investigate the distribution of the CO molecular gas around SN 1572 and provide some observational basis for studying the relationship of SN 1572 with its ambient molecular gas and the evolution of this SNR. The observed result indicates that the molecular gas in the velocity range of V_LSR = −69∼ −58 km/s is associated with SN 1572, and this velocity component comes from a large-scale molecular cloud. The molecular gas is distributed along the periphery of the radio shell, continually but not uniformly, and forms a semi-closed molecular shell around the SNR. The enhanced emission exists in its whole eastern half, especially the CO emission is strongest on the northeastern edge. At the emission peak position, the spectral line exhibits a broadened velocity feature (>5 km/s). Combining with available observations in the optical, infrared, X-ray and other wavebands, it is demonstrated that the fast shock wave and ejecta are expanding into the molecular gas on the northeastern edge, and interacting with the dense gas. This interaction will have an important influence on the future evolution of SN 1572.     

Discovery of molecular hydrogen line emission associated with methanol maser emission

We report the discovery of H₂ line emission associated with 6.67-GHz methanol maser emission in massive star-forming regions. In our UNSWIRF/AAT observations, H₂ 1–0 S(1) line emission was found associated with an ultracompact H  ii region IRAS 14567–5846 and isolated methanol maser sites in G318.95–0.20 , IRAS 15278–5620 and IRAS 16076–5134 . Owing to the lack of radio continuum in the latter three sources, we argue that their H₂ emission is shock excited, while it is UV-fluorescently excited in IRAS 14567–5846 . Within the positional uncertainties of 3 arcsec, the maser sites correspond to the location of infrared sources. We suggest that 6.67-GHz methanol maser emission is associated with hot molecular cores, and propose an evolutionary sequence of events for the process of massive star formation.