首页 | 本学科首页   官方微博 | 高级检索  
相似文献
 共查询到20条相似文献，搜索用时 31 毫秒
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.  相似文献   

2.
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.  相似文献   

3.
We present the jump conditions for ionization fronts with oblique magnetic fields. The standard nomenclature of R- and D-type fronts can still be applied, but in the case of oblique magnetization there are fronts of each type about each of the fast- and slow-mode speeds. As an ionization front slows, it will drive first a fast- and then a slow-mode shock into the surrounding medium. Even for rather weak upstream magnetic fields, the effect of magnetization on ionization front evolution can be important.  相似文献   

4.
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   

5.
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   

6.
We discuss a new type of dust acceleration mechanism that acts in a turbulent magnetized medium. The magnetohydrodynamic turbulence can accelerate grains through resonant as well as non-resonant interactions. We show that the magnetic compression provides higher velocities for super-Alfvénic turbulence and can accelerate an extended range of grains in warm media compared to gyroresonance. While fast modes dominate the acceleration for the large grains, slow modes can be important for submicron grains. We provide comprehensive discussion of all the possible grain acceleration mechanisms in interstellar medium. We show that supersonic velocities are attainable for Galactic dust grains. We discuss the consequence of the acceleration. The implications for extinction curve, grain alignment, chemical abundance etc. are provided.  相似文献   

7.
We present the results of modelling of the H2 emission from molecular outflow sources, induced by shock waves propagating in the gas. We emphasize the importance of proper allowance for departures from equilibrium owing to the finite flow velocity of the hot, compressed gas, with special reference to the excitation, dissociation and reformation of H2. The salient features of our computer code are described. The code is applied to interpreting the spectra of the outflow sources Cepheus A West and HH43. Particular attention is paid to determining the cooling times in shocks whose speeds are sufficient for collisional dissociation of H2 to take place; the possible observational consequences of the subsequent reformation of H2 are also examined. Because molecular outflow sources are intrinsically young objects, J-type shocks may be present in conjunction with magnetic precursors, which have a C-type structure. We note that very different physical and dynamical conditions are implied by models of C- and J-type shocks which may appear to fit the same H2 excitation diagram.  相似文献   

8.
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.  相似文献   

9.
We investigate shattering and coagulation of dust grains in turbulent interstellar medium (ISM). The typical velocity of dust grain as a function of grain size has been calculated for various ISM phases based on a theory of grain dynamics in compressible magnetohydrodynamic turbulence. In this paper, we develop a scheme of grain shattering and coagulation and apply it to turbulent ISM by using the grain velocities predicted by the above turbulence theory. Since large grains tend to acquire large velocity dispersions as shown by earlier studies, large grains tend to be shattered. Large shattering effects are indeed seen in warm ionized medium within a few Myr for grains with radius   a ≳ 10−6  cm. We also show that shattering in warm neutral medium can limit the largest grain size in ISM  ( a ∼ 2 × 10−5 cm)  . On the other hand, coagulation tends to modify small grains since it only occurs when the grain velocity is small enough. Coagulation significantly modifies the grain size distribution in dense clouds (DC), where a large fraction of the grains with   a < 10−6 cm  coagulate in 10 Myr. In fact, the correlation among   RV   , the carbon bump strength and the ultraviolet slope in the observed Milky Way extinction curves can be explained by the coagulation in DC. It is possible that the grain size distribution in the Milky Way is determined by a combination of all the above effects of shattering and coagulation. Considering that shattering and coagulation in turbulence are effective if dust-to-gas ratio is typically more than ∼1/10 of the Galactic value, the regulation mechanism of grain size distribution should be different between metal-poor and metal-rich environments.  相似文献   

10.
We present ISOPHOT observations of eight interstellar regions in the 60–200 μm wavelength range. The regions belong to mostly quiescent high-latitude clouds and have optical extinction peaks from   AV ∼1–6 mag  . From the 150- and 200-μm emission, we derived colour temperatures for the classical big grain component which show a clear trend of decreasing temperature with increasing 200-μm emission. The 200-μm emission per unit   AV   , however, does not drop at lower temperatures. This fact can be interpreted in terms of an increased far-infrared (FIR) emissivity of the big grains. We developed a two-component model including warm dust with the temperature of the diffuse interstellar medium (ISM) of   T = 17.5 K  , and cold dust with   T = 13.5 K  and FIR emissivity increased by a factor of >4. A mixture of the two components can reproduce the observed colour variations and the ratios   I 200/ AV   and  τ200/ AV   . The relative abundance of small grains with respect to the big grains shows significant variations from region to region at low column densities. However, in lines of sight of higher column density, our data indicate the disappearance of small grains, perhaps a signature of adsorption/coagulation of dust. The larger size and porous structure could also explain the increased FIR emissivity. Our results from eight independent regions suggest that these grains might be ubiquitous in the galactic ISM.  相似文献   

11.
We present consistent modelling of line and continuum infrared (IR) spectra in the region close to the Galactic Centre. The models account for the coupled effect of shocks and photoionization from an external source. The results show that the shock velocities range between ∼65 and 80 km s−1 and the pre-shock densities between 1 cm−3 in the interstellar medium (ISM) to 200 cm−3 in the filamentary structures. The pre-shock magnetic field increases from 5 × 10−6 G in the surrounding ISM to ∼8 × 10−5 G in the arched filaments. The stellar temperatures are ∼38 000 K in the Quintuplet cluster and ∼27 000 K in the Arches Cluster. The ionization parameter is relatively low (<0.01) with the highest values near the clusters, reaching a maximum >0.01 near the Arches Cluster. Depletion from the gaseous phase of Si is found throughout the whole observed region, indicating the presence of silicate dust. Grains including iron are concentrated throughout the arched filaments. The modelling of the continuum spectral energy distribution in the IR range indicates that a component of dust at temperatures of ∼100–200 K is present in the central region of the Galaxy. Radio emission appears to be thermal bremsstrahlung in the E2–W1 filaments crossing strip; however, a synchrotron component is not excluded. More data are necessary to resolve these questions.  相似文献   

12.
We present isophot spectrophotometry of three positions within the isolated high-latitude cirrus cloud G 300.2−16.8, spanning from the near- to far-infrared (NIR to FIR). The positions exhibit contrasting emission spectrum contributions from the unidentified infrared bands (UIBs), very small grains (VSGs) and large classical grains, and both semi-empirical and numerical models are presented. At all three positions, the UIB spectrum shapes are found to be similar and the large grain emission may be fitted by an equilibrium temperature of  ∼17.5 K  . The energy requirements of both the observed emission spectrum and optical scattered light are shown to be satisfied by the incident local interstellar radiation field (ISRF). The FIR emissivity of dust in G 300.2−16.8 is found to be lower than in globules or dense clouds and is even lower than model predictions for dust in the diffuse interstellar medium (ISM). The results suggest physical differences in the ISM mixtures between positions within the cloud, possibly arising from grain coagulation processes.  相似文献   

13.
We investigate the dynamics of magnetic fields in spiral galaxies by performing 3D magnetohydrodynamics simulations of galactic discs subject to a spiral potential using cold gas, warm gas and a two-phase mixture of both. Recent hydrodynamic simulations have demonstrated the formation of interarm spurs as well as spiral arm molecular clouds, provided the interstellar medium model includes a cold H  i phase. We find that the main effect of adding a magnetic field to these calculations is to inhibit the formation of structure in the disc. However, provided a cold phase is included, spurs and spiral arm clumps are still present if β≳ 0.1 in the cold gas. A caveat to the two-phase calculations though is that by assuming a uniform initial distribution, β≳ 10 in the warm gas, emphasizing that models with more consistent initial conditions and thermodynamics are required. Our simulations with only warm gas do not show such structure, irrespective of the magnetic field strength.
Furthermore, we find that the introduction of a cold H  i phase naturally produces the observed degree of disorder in the magnetic field, which is again absent from simulations using only warm gas. Whilst the global magnetic field follows the large-scale gas flow, the magnetic field also contains a substantial random component that is produced by the velocity dispersion induced in the cold gas during the passage through a spiral shock. Without any cold gas, the magnetic field in the warm phase remains relatively well ordered apart from becoming compressed in the spiral shocks. Our results provide a natural explanation for the observed high proportions of disordered magnetic field in spiral galaxies and we thus predict that the relative strengths of the random and ordered components of the magnetic field observed in spiral galaxies will depend on the dynamics of spiral shocks.  相似文献   

14.
We undertake calculations of the time-dependent structure of shock waves propagating in dark and diffuse interstellar clouds. The results of the time-dependent model are compared with those obtained by means of an independent steady-state code and found to agree well at sufficiently late times. Discontinuities in the flow of the neutral fluid are handled by introducing a pseudo-viscosity. Special procedures are adopted to correct for the associated widening of the discontinuity, in order not to distort the role of inelastic collision processes. We find that, in dark clouds, C shocks will tend to predominate, but are unlikely to have attained steady state in the cloud lifetime. On the other hand, in diffuse clouds, steady state may be reached but the discontinuity in the flow of the neutral fluid remains. We find no evidence for the existence of C* shocks, in which the neutral fluid undergoes a continuous transition from supersonic to subsonic flow (in the reference frame of the shock wave). Attention is drawn to the possible importance of these results for the interpretation of H2 rovibrational line intensities.  相似文献   

15.
The Cepheus A star-forming region has been investigated through a multiline H2S and SO2 survey at millimetre wavelengths. Large-scale maps and high-resolution line profiles reveal the occurrence of several outflows. Cep A East is associated with multiple mass-loss processes: in particular, we detect a 0.6-pc jet-like structure which shows for the first time that the Cep A East young stellar objects are driving a collimated outflow moving towards the south.
The observed outflows show different clumps associated with definitely different H2S/SO2 integrated emission ratios, indicating that the gas chemistry in Cepheus A has been altered by the passage of shocks. H2S appears to be more abundant than SO2 in high-velocity clumps, in agreement with chemical models. However, we also find quite small H2S linewidths, suggestive of regions where the evaporated H2S molecules had enough time to slow down but not to freeze out on to dust grains. Finally, comparison between the line profiles indicates that the excitation conditions increase with the velocity, as expected for a propagation of collimated bow shocks.  相似文献   

16.
We present models for the chemistry in gas moving towards the ionization front of an HII region. When it is far from the ionization front, the gas is highly depleted of elements more massive than helium. However, as it approaches the ionization front, ices are destroyed and species formed on the grain surfaces are injected into the gas phase. Photodissociation removes gas phase molecular species as the gas flows towards the ionization front. We identify models for which the OH column densities are comparable to those measured in observations undertaken to study the magnetic fields in star forming regions and give results for the column densities of other species that should be abundant if the observed OH arises through a combination of the liberation of H2O from surfaces and photodissociation. They include CH3OH, H2CO, and H2S. Observations of these other species may help establish the nature of the OH spatial distribution in the clouds, which is important for the interpretation of the magnetic field results.  相似文献   

17.
We present calculations of the internal structures of stationary ionization fronts with oblique upstream magnetic fields. Solutions are found only within the range of upstream parameters allowed by the evolutionary conditions derived by us in a previous paper. As is the case for hydrodynamic ionization fronts, overheating of the gas within the front can mean that not all the solutions allowed by the evolutionary conditions correspond to resolved internal structures. Some, but not all, of these jumps can be realized by structures with included subshocks: we present a full range of internal structures for fronts close to criticality, and discuss how to avoid spurious 'strong-R' type solutions.
The solutions we study suggest that significant transverse velocities and modulations of the perpendicular magnetic field may characterize obliquely magnetized ionization fronts. We briefly discuss the observability of the internal structures.  相似文献   

18.
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   

19.
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 magnetic field strength and the degree of ionisation. Intermediate situations also exist, in which a J-discontinuity terminates or is embedded in a C-type flow. We show that proper allowance for the departure of the chemistry from equilibrium (particularly the dissociation/reformation of H2) and for the inertia of charged dust grains, is crucial for an accurate treatment of the C to J transition. We illustrate the use of H2 population diagrams and H2 line profiles, in conjunction with our detailed shock model, to constrain conditions in shocks propagating in molecular outflows. We show that H2 pure rotational lines yield evidence for C-type precursors in bipolar outflows from young stars, with transverse magnetic field strengths B (μG) ? 1–10 × $\sqrt {n_{H/{\text{cm}}^{ - 3} } } $ similar to those inferred from Zeeman splitting and from the dispersion of dust polarization vectors in dense clouds.  相似文献   

20.
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   

设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号