J-type Shocks Research Articles

AKARINEAR-INFRARED SPECTRAL OBSERVATIONS OF SHOCKED H2GAS OF THE SUPERNOVA REMNANT IC 443

We present near-infrared (2.5 - 5.0 um) spectra of shocked H2 gas in the supernova remnant IC 443, obtained with the satellite AKARI. Three shocked clumps-known as B, C, and G-and one background region were observed, and only H2 emission lines were detected. Except the clump B, the extinctioncorrected level population shows the ortho-to-para ratio of ~ 3.0. From the level population of the clumps C and G-both AKARI's only and the one extended with previous mid-infrared observations-we found that the v = 0 levels are more populated than the v > 0 levels at a fixed level energy, which cannot be reproduced by any combination of H2 gas in Local Thermodynamic Equilibrium. The populations are described by the two-density power-law thermal admixture model, revised to include the collisions with H atoms. We attributed the lower (n(H2)=10^(2.8-3.8) cm-3) and higher (n(H2)=10^(5.4-5.8) cm-3) density gases to the shocked H2 gas behind C-type and J-type shocks, respectively, based on several arguments including the obtained high H I abundance n(H I)/n(H2)=0.01. Under the hierarchical picture of molecular clouds, the C-type and J-type shocks likely propagate into "clumps" and "clouds" (interclump media), respectively. The power-law index b of 1.6 and 3.5, mainly determined by the lower density gas, is attributed to the shock-velocity diversity, which may be a natural result during shock-cloud interactions. According to our results, H2 v = 1 - 0 S(1) emission is mainly from J-shocks propagating into interclump media. The H2 emission was also detected at the background region, and this diffuse H2 emission may originate from collisional process in addition to the ultraviolet photon pumping.

Origin of the hot gas in low-mass protostars

'Water in Star-forming regions with Herschel' (WISH) is a Herschel Key Programme aimed at understanding the physical and chemical structure of young stellar objects (YSOs) with a focus on water and related species. The low-mass protostar HH 46 was observed with the Photodetector Array Camera and Spectrometer (PACS) on the Herschel Space Observatory to measure emission in H2O, CO, OH, [OI], and [CII] lines located between 63 and 186 um. The excitation and spatial distribution of emission can disentangle the different heating mechanisms of YSOs, with better spatial resolution and sensitivity than previously possible. Far-IR line emission is detected at the position of the protostar and along the outflow axis. The OH emission is concentrated at the central position, CO emission is bright at the central position and along the outflow, and H2O emission is concentrated in the outflow. In addition, [OI] emission is seen in low-velocity gas, assumed to be related to the envelope, and is also seen shifted up to 170 km/s in both the red- and blue-shifted jets. Envelope models are constructed based on previous observational constraints. They indicate that passive heating of a spherical envelope by the protostellar luminosity cannot explain the high-excitation molecular gas detected with PACS, including CO lines with upper levels at >2500 K above the ground state. Instead, warm CO and H2O emission is probably produced in the walls of an outflow-carved cavity in the envelope, which are heated by UV photons and non-dissociative C-type shocks. The bright OH and [OI] emission is attributed to J-type shocks in dense gas close to the protostar. In the scenario described here, the combined cooling by far-IR lines within the central spatial pixel is estimated to be 2 \times 10-2 L_sun, with 60-80% attributed to J- and C-type shocks produced by interactions between the jet and the envelope.

Excitation and emission of H, CO and HO molecules in interstellar shock waves

The dissipation of kinetic energy that occurs in interstellar shock waves is accompanied by the emission of radiation. In the case of shocks that are propagating into mainly molecular gas, the emission occurs principally in lines of the species H 2 , H, O, CO and H 2 O The relative intensities of these emission lines are indicative of the type and speed of the shock wave and of the physical conditions in the ambient gas. We present the results of computations of the intensities of these lines, for small grids of models of C- and J-type shock waves, and compare with the results of previous calculations. Our results should serve to aid the interpretation of observations made with the Herschel and other satellites.

Transient evolution of C-type shocks in dusty regions of varying density

Outflows of young stars drive shocks into dusty, molecular regions. Most models of such shocks assume that they are steady and propagating perpendicular to the magnetic field. Real shocks often violate both of these assumptions and the media through which they propagate are inhomogeneous. We use the code employed previously to produce the first time-dependent simulations of fast-mode, oblique C-type shocks interacting with density perturbations. We include a self-consistent calculation of the thermal and ionisation balances and a fluid treatment of grains. We identify features that develop when a multifluid shock encounters a density inhomogeneity to investigate whether any part of the precursor region ever behaves in a quasi-steady fashion. If it does the shock may be modelled approximately without solving the time-dependent hydromagnetic equations. Simulations were made for initially steady oblique C-type shocks encountering density inhomogeneities. For a semi-finite inhomogeneity with a density larger than the surrounding medium, a transmitted shock evolves from being J-type to a steady C-type shock on a timescale comparable to the ion-flow time through it. A sufficiently upstream part of the precursor of an evolving J-type shock is quasi-steady. The ion-flow timescale is also relevant for the evolution of a shock moving into a region of decreasing density. The models for shocks propagating into regions in which the density increases and then decreases to its initial value cannot be entirely described in terms of the results obtained for monotonically increasing and decreasing densities. For the latter model, the long-term evolution to a C-type shock cannot be approximated by quasi-steady models.

An outflow origin of the [Ne II] emission in the T Tauri triplet

Context. The 12.81 mu m [Ne II] line has recently gained interest as a potential tracer of gas in the tenuous surface layers of circumstellar disks and in outflow-related shocks. Evidence has been found for a proportionality between [Ne II] emission and X-ray luminosity, supporting the hypothesis that X-rays are responsible for the required ionization and heating of the gas. Alternatively, ionization and heating by EUV photons and in J-type (dissociative) shocks has been proposed. Aims. The T Tau multiple system harbors three stars with circumstellar disks, at least one strong X-ray source (T Tau N), and regions of shocked gas in the immediate vicinity. ISO and Spitzer spectra revealed remarkably strong [Ne II] emission, but because of insuficient spatial and spectral resolution those observations could neither pinpoint where in the system the [Ne II] emission arises, nor identify the emission mechanism. We aim to clarify this by observing the system with enough resolution to spatially separate the various components and spectrally resolve the line emission. Methods. We performed high-resolution (R = 30 000) spectroscopy of the T Tau triplet at similar to 0 ''.4 spatial resolution with VISIR at the VLT early February 2008. We spatially separated T Tau N from the southern close binary T Tau S, as well as the structures of shocked gas surrounding the stars. The individual southern components Sa and Sb remained spatially unresolved in our observations. Results. The dominant component of [Ne II] emission is centered on T Tau S and has a spatial extent of FWHM similar to 1 ''.1 in a Gaussian fit. We detect spatially extended red-shifted emission NW of the system and fainter blue-shifted emission to the SE, which we associate with the N-S outflow from T Tau S. Only a small fraction of the [Ne II] emission appears directly related to the X-ray bright northern component. Shocks may account for a substantial and possibly dominant fraction of the observed [Ne II] emission. We estimate the total [Ne II] flux to be 23 +/- 6 x 10(-16) Wm(-2), in good agreement with the values measured by ISO in late 1997 and Spitzer in early 2004. Conclusions. Our observations show that outflows rather than the disk surface may dominate the observed [Ne II] emission in stars with strong outflow activity. We propose [Ne II] emission in jets as a major factor causing the observed large scatter in the L(X) vs. L[Ne II] relation. We argue that T Tau S is the driving source of the T Tau NW-blob.

Shocks in dense clouds

<i>Context. <i/>Observations of SiO line emission in shocks in star-forming regions indicate that silicate dust destruction must be occurring in these dense regions. Current models rely on predictions for dust destruction by sputtering in C-type shock waves. However, J-type shocks may also be relevant for interpreting the widely-observed optical line emission from species such as O I and Fe II. <i>Aims. <i/>In this work we explore, for the first time, dust destruction in J-type shocks slower than 50 km s<sup>-1<sup/>. <i>Methods. <i/>We follow the dust trajectories throughout the shock using a model for the dust dynamics that allows us to solve the shock structure and at the same time calculate the degree of dust processing. We include the effects of sputtering in gas-grain collisions, and vaporisation and shattering in grain-grain collisions.<i>Results. <i/>We find that the amount of silicon released into the gas phase is a few percent. The dominant destructive process is vaporisation, not sputtering. The degree of dust destruction increases with the shock velocity but decreases as the preshock density increases. <i>Conclusions. <i/>Our results compare well with that of C-type shock models. J-type shocks are therefore reasonable candidates for an interpretation of SiO line emission in molecular outflows and jets.

SiO line emission from interstellar jets and outflows: silicon-containing mantles and non-stationary shock waves

Context. We study the production and emission of SiO and H 2 in the gas phase of molecular outflows, extending previous work in which we considered steady-state C-type shock waves and assumed the silicon to be present only in the cores of silicate grains. Aims. We place constraints on the physical parameters of the pre-shock region, using recent observations of SiO and observations of molecular hydrogen. We show the effects of introducing SiO-containing mantles and of varying the age of the shock wave. We consider simultaneously the emission of SiO and H 2 from the young L1157 outflow. Methods. The molecular outflows are studied by means of a code that can generate stationary C- and J-type shock models and approximate non-stationary solutions, which combine these two types of shock wave. The emission of molecular hydrogen is computed by this code, whereas the SiO emission is computed by means of a separate LVG model, which uses the calculated physical and chemical profiles. A grid of models has been computed, with shock speeds in the range 10 ≤ u s < 35 km s -1 and pre-shock gas densities 10 4 ≤ n H ≤ 10 6 cm -3 . A wide range of magnetic field strengths has been investigated, from 45 pG to about 600 pG. Results. We illustrate our results by means of observational data obtained on the blue lobe of the L1157 outflow. Given the combinations of pre-shock densities and shock velocities necessary to fit the H 2 observations, we find that the erosion only of the silicate material in the grains cores cannot account for the observed SiO line intensities. We investigate the possiblity that a fraction of the SiO is present initially in the grain mantles, and we succeed in constraining this fraction. Introducing even a few percent of the silicon (as SiO) into the mantles is sufficient to increase the SiO line widths and fluxes by an order of magnitude. With this assumption, it is possible to find a non-stationary shock model that provides a reasonable fit of the observations of both H 2 and SiO. Conclusions. With a few percent of the silicon present initially in the grain mantles, good agreement is obtained with recent observations of SiO line integrated line intensities for a pre-shock density n H = 10 4 cm - and a shock speed v s = 20 km s -1 . The magnetic field strength and the shock age are not well constrained by the observations of either H 2 or SiO. We show that CO observations (in particular, with the Herschel satellite) could provide further discrimination between the models.

Shocks in dense clouds

Aims. A new multi-fluid approach to the dust dynamics in transverse shocks in dense clouds is presented with the aim of modelling the dust processing in C- and J-type shocks. Methods. We have augmented an existing steady-state shock code to include the effects of an MRN size distribution of grain cores with icy mantles. The dust charge distribution and its evolution is considered in detail and included in the ionization balance. The 2-D grain dynamics are determined, including the effects of grain inertia and charge fluctuations, paying particular attention to the gyration of the charged grains around the magnetic field lines and the feedback of the ionization state on grain dynamics. Results. We find that the critical velocity for C shocks increases with the gas density but that it is only weakly dependent on a high abundance of PAHs and on the photodetachment of electrons by secondary photons induced by cosmic-rays. The detailed dust dynamics in C shocks is shown to comprise two distinct phases: 1) a short gyration phase followed by 2) a long term drift phase. In J shocks only the first gyration phase is present. In C shocks propagating through molecular clouds (n H = 10 4 cm -3 ), large grains (»100 A) remain coupled to the magnetic field during the second phase. However, a high abundance of PAHs can lead to a shortage of electrons in the gas and the decoupling of large grains in the shock tail. Large grains are decoupled from the magnetic field all through the C shock in high density clouds (n H = 10 6 cm -3 ). In all C shocks small grains (≃100 A) remain strongly coupled to the magnetic field, whereas very small grains («100 A) are subject to stochastic dynamics. As long as they are charged very small grains remain strongly coupled to the magnetic field but tend to couple to the neutral gas everytime they become neutral. We have investigated the effects of an electric field along the shock direction in C shocks and find that it does not significantly modify the relative velocities between grains. The derived grain dynamics can be used to study dust processing in C and J shocks in dense clouds through the effects of gas-grain and grain-grain collisions.

Molecular line emission in HH54: a coherent view from near to far infrared

Aims. We present a detailed study of the infrared line emission (1–200 µm) in the Herbig-Haro object HH54. Our database comprises: high- (R ∼ 9000) and low- (R ∼ 600) resolution spectroscopic data in the near-infrared band (1–2.5 µm); mid-infrared spectrophotometric images (5–12 µm); and, far-IR (45–200 µm, R ∼ 200) spectra acquired with the ISO satellite. As a result, we provide the detection of and the absolute fluxes for more than 60 molecular features (mainly from H2 in the near- and mid-infrared and from H2O, CO and OH in the far-infrared) and 23 ionic lines. Methods. The H2 lines, coming from levels from v = 0t ov = 4 have been interpreted in the context of a state-of-the-art shock code, whose output parameters are adopted as input to a Large Velocity Gradient computation in order to interpret the FIR emission of CO, H2O and OH. Results. The H2 emission can be interpreted as originating in either steady-state J-type shocks or in quasi-steady J-type shocks with magnetic precursor. However, our multi-species analysis shows that only a model of a J-type shock with magnetic precursor (vshock = 18 km s −1 , nH = 10 4 cm −3 , B = 100 µG, age = 400 yr) can account for both the observed H2 emission and the CO and H2O lines. Such a model predicts a H2O abundance of ∼7 × 10 −5 , in agreement with estimations from other shock models of outflows associated with low mass protostars. We can exclude the possibility that the observed atomic lines arise in the same shock as the molecular lines, and give arguments in favour of the presence of a further high-velocity, fully dissociative shock component in the region. Finally, in view of the forthcoming spectroscopic facilities on board of the Herschel satellite, we provide predictions for H2O lines considered to be the most suitable for diagnostic purposes.

Dissociative Shocks in the Neighborhood of Orion IRc2 Traced with Atomic Carbon

We present Caltech Submillimeter Observatory 10'' resolution maps of the 3P2 → 3P1 line of neutral atomic carbon (809.3435 GHz) and the J = 7-6 line of CO (806.6518 GHz) covering a region of 2' × 45 around Orion IRc2 in the BN/KL Nebula. The two lines were observed with the same receiver simultaneously, and therefore the two maps have zero relative pointing error. The atomic carbon (C I) emission does not peak toward the IRc2 position and displays a very different spatial distribution with respect to most molecular species. Moderately strong red wings are detected in C I around the region defined by the high-velocity CO gas with a spatial distribution that delineates a shell of ~20'' radius. We propose that CO dissociation in the J-type shocks produced by the interaction of the IRc2 outflows with the surrounding quiescent gas could play a significant role in this enhancement of atomic carbon in that shell.

Spatially Resolved Low-Frequency Very Large Array Observations of the Supernova Remnant 3C 391

We present VLA images of the supernova remnant (SNR) 3C 391 at 74, 330, and 1465 MHz. This remnant has been known for some time to exhibit a turnover in its integrated radio continuum spectrum at frequencies <100 MHz, indicative of free-free absorption from thermal ionized gas along the line of sight. For the first time, our data reveal the spatially resolved morphology of the low-frequency free-free absorption with a resolution of ~70''. Contrary to the expectation that such absorption arises from unrelated low-density H II regions (or their envelopes) along the line of sight, these data suggest that in this case the absorbing medium is directly linked to the SNR itself. 3C 391 has been shown in a number of recent papers to be interacting with a molecular cloud. Indeed, it exhibits a number of signposts of SNR/molecular cloud shocks, including OH (1720 MHz) masers and broad molecular emission lines. Comparison of the regions of strongest 74 MHz absorption with existing X-ray, IR, and molecular data suggests that the free-free absorption originates from the SNR/molecular cloud shock boundaries due to ionized gas created from the passage of a J-type shock with a speed of ~100 km s-1. This makes it only the second SNR for which such (extrinsic) spatially resolved absorption has been measured and the only one for which the absorption is thought to arise from a SNR/molecular cloud interface region.

Detection of Water in the Shocked Gas Associated with IC 443: Constraints on Shock Models

We have used the Submillimeter Wave Astronomy Satellite (SWAS) to observe the ground-state 110 → 101 transition of ortho-H2O at 557 GHz in three of the shocked molecular clumps associated with the supernova remnant IC 443. We also observed simultaneously the 487 GHz line (3,1 → 3,2) of O2, the 492 GHz line (3P1 → 3P0) of C I, and the 550 GHz line (J = 5 → 4) of 13CO. We detected the H2O, C I, and 13CO lines toward the shocked clumps B, C, and G. In addition, ground-based observations of the J = 1 → 0 transitions of CO and HCO+ were obtained. Assuming that the shocked gas has a temperature of 100 K and a density of 5 × 105 cm-3, we derive SWAS beam-averaged ortho-H2O column densities of 3.2 × 1013, 1.8 × 1013, and 3.9 × 1013 cm-2 in clumps B, C, and G, respectively. Combining the SWAS results with our ground-based observations, we derive a relative abundance of ortho-H2O to CO in the postshock gas of between 2 × 10-4 and 3 × 10-3. On the basis of our results for H2O, published results of numerous atomic and molecular shock tracers, and archival Infrared Space Observatory (ISO) observations, we conclude that no single shock type can explain these observations. However, a combination of fast J-type shocks (~100 km s-1) and slow C-type shocks (~12 km s-1) or, more likely, slow J-type shocks (~12-25 km s-1) can most naturally explain the postshock velocities and the emission seen in various atomic and molecular tracers. Such a superposition of shocks might be expected as the supernova remnant overtakes a clumpy interstellar medium. The fast J-type shocks provide a strong source of ultraviolet radiation, which photodissociates the H2O in the cooling (T ≤ 300 K) gas behind the slow shocks and strongly affects the slow C-type shock structure by enhancing the fractional ionization. At these high ionization fractions, C-type shocks break down at speeds ~10-12 km s-1, while faster flows will produce J-type shocks. Our model favors a preshock gas-phase abundance of oxygen not in CO that is depleted by a least a factor of 2, presumably as water ice on grain surfaces. Both freezeout of H2O and photodissociation of H2O in the postshock gas must be significant to explain the weak H2O emission seen by SWAS and ISO from the shocked and postshock gas.

On the excitation of the infrared knots in the HH99 outflow

We present near infrared (IR) spectra (0.98--2.5 mu m) from the group of Herbig-Haro (HH) objects comprising HH99: a series of knots forming a bow, HH99B, and a separate knot, HH99A. Observations of H2, [Fe II] and [C I] are used to constrain shock model parameters and determine the origin of the emission. Previous work has shown that it is likely that the atomic and ionic emission arises in regions of higher ionization than the molecular emission. On the basis of observations, it has often been suggested that the [Fe II] and [C I] emission could arise, for example, at the apex of a bow shock, with the H2 emission produced in the wings. Accordingly, we have combined models of C-component and J-type shocks in order to reproduce the observed H2, [Fe II] and [C I] line spectra. We can account for the H2 emission towards the HH99B complex by means of J-type shocks with magnetic precursors. We derive shock velocities of about 30 km s-1 and ages of order 100 yr; the pre-shock gas has a density of approximately 10^4 cm-3 and the magnetic field is approximately 100 mu G. The J-type shocks required to reproduce the intensities of the observed [Fe II] and [C I] lines have velocities of 50 km s-1. It is necessary to assume that Fe has been previously eroded from grains, probably by the earlier passage of a C-type shock wave. Thus, our analysis supports the view that molecular outflows are episodic phenomena whose observed emission arises in shock waves.

The contributions of J-type shocks to the H2emission from molecular outflow sources

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.

Clustered Star Formation in W75 N

We present 2 to 7 resolution 3 mm continuum and CO(J=1-0) line emission and near infrared Ks, H2, and [FeII] images toward the massive star forming region W75 N. The CO emission uncovers a complex morphology of multiple, overlapping outflows. A total flow mass of greater than 255 Msun extends 3 pc from end-to-end and is being driven by at least four late to early-B protostars. More than 10% of the molecular cloud has been accelerated to high velocities by the molecular flows (> 5.2 km/s relative to v{LSR}) and the mechanical energy in the outflowing gas is roughly half the gravitational binding energy of the cloud. The W75 N cluster members represent a range of evolutionary stages, from stars with no apparent circumstellar material to deeply embedded protostars that are actively powering massive outflows. Nine cores of millimeter-wavelength emission highlight the locations of embedded protostars in W75 N. The total mass of gas & dust associated with the millimeter cores ranges from 340 Msun to 11 Msun. The infrared reflection nebula and shocked H2 emission have multiple peaks and extensions which, again, suggests the presence of several outflows. Diffuse H2 emission extends about 0.6 parsecs beyond the outer boundaries of the CO emission while the [FeII] emission is only detected close to the protostars. The infrared line emission morphology suggests that only slow, non-dissociative J-type shocks exist throughout the pc-scale outflows. Fast, dissociative shocks, common in jet-driven low-mass outflows, are absent in W75 N. Thus, the energetics of the outflows from the late to early B protostars in W75 N differ from their low-mass counterparts -- they do not appear to be simply scaled-up versions of low-mass outflows.

J-type shock chemistry for the W3 IRS5 region

A chemical evolution of W3 IRS5 have been studied during J-type shock. We modified our code of for one-dimensional J-type shock to satisfy the physical conditions of W3 IRS5. The chemical rate equations and the hydrodynamics of shocks are integrated simultaneously. We find that, a shock of speed Vs = 10 km s-1 and magnetic induction B=4.1×10-6 Gauss, propagate into molecular gas density of n=104cm-3, and with initial temperature of 30 K (pre shock) satisfy the physical conditions of W3 IRS5. The results using the extensive chemical network with the most updating reaction rates show that the calculated fractional abundances are in agreement with observation.

The acceleration of molecular hydrogen to high velocities in time-dependent fast J-shocks

It is a well known but puzzling result that zones within star formation regions sometimes show molecular hydrogen emission at very high (∼100 km s - 1 ) velocities. These kinds of observations are somewhat difficult to explain because non-magnetized, J-type shock waves of velocities above ∼20 km s - 1 mostly dissociate the molecules present in the preshock medium, and therefore produce almost no H 2 emission. We quantify this result by presenting models of steady shock waves moving into a molecular environment, which show that the H 2 molecules are indeed dissociated in the immediate postshock region for higher shock velocities. We argue that the total destruction of molecules by high-velocity shocks is a direct result of the assumption of an instantaneous 'turning on' of the flow that is generally done in computing shock models. We present models in which a shock wave gradually accelerates over a period of ∼1000 yr as would be expected, for example, from the 'turning on' of an outflow from a young star. We find that such shock waves are indeed able to accelerate significant masses of molecular material to velocities of ∼100 km s - 1 , and are a plausible explanation for widely observed high-velocity H 2 emission.

New determinations of the critical velocities of C-type shock waves in dense molecular clouds: application to the outflow source in Orion

We report calculations of the intensities of rovibrational transitions of H2 emitted from C-type shock waves propagating in molecular gas. Attention was paid to the thermal balance of the gas and to the rates of collisional dissociation and ionization of H2. We found that the maximum shock speeds which can be attained, prior to the collisional dissociation of H2 (which results in a sonic point in the flow and hence a J-type shock wave), can be much higher than had previously been believed. Thus, adopting the `standard' scaling of the transverse magnetic induction with the gas density, B(muG)=[nH(cm-3)]1/2, we established that the maximum shock speed increased from 20-30kms-1 at high pre-shock densities (nH>=106cm-3) to 70-80kms-1 at low densities (nH<=104cm-3). The critical shock speed, vcrit, also increases significantly with the transverse magnetic induction, B, at a given pre-shock gas density, nH. By way of an application of these results, we demonstrate that a two-component model, comprising shock waves with velocities vs=60 and 40kms-1, reproduces the column densities of H2 observed by ISO-SWS up to the highest level (possibly) detected, v=0, J=27, which lies 42515K above the ground state. We found no necessity to invoke mechanisms other than thermal collisional excitation in the gas phase; but the v=1 vibrational band remains less completely thermalized than is indicated by the observations. Fine structure transitions of atoms and ions were also considered. The intensity of the [SiI] 68.5mum transition, observed by Gry et al. using ISO-LWS, is satisfactorily reproduced by the same model and may also originate in OMC-1, rather than Orion-KL as originally believed. The transitions of [FeII] and [SI] observed by Rosenthal et al. may also arise in the shock-heated gas.

Understanding the Nature of Protostellar Jets through Near-Infrared Spectroscopy: The Key Role of the [ITAL]J[/ITAL] Band Proved on HH 43

Spectroscopy in the J band (even at low resolutions of R 103) is a mode commonly available in most near-IR spectrographs but is rarely exploited in star formation studies. We show how it is a crucial tool to tracing the physics (temperature, density, excitation mechanisms) of protostellar jets and Herbig-Haro (HH) objects. In this framework the spectrum of HH 43 is revisited as both a clear demonstration of our statement and a case of slow J-type shock excitation.

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.