Protostellar Environment Research Articles

Protostellar Interferometric Line Survey of the Cygnus-X region (PILS-Cygnus)

Context. Molecular lines are commonly detected towards protostellar sources. However, to get a better understanding of the chemistry of these sources we need unbiased molecular surveys over a wide frequency range for as many sources as possible to shed light on the origin of this chemistry, particularly any influence from the external environment. Aims. We present results from the PILS-Cygnus survey of ten intermediate- to high-mass protostellar sources in the nearby Cygnus-X complex, through high angular resolution interferometric observations over a wide frequency range. Methods. Using the Submillimeter Array (SMA), a spectral line survey of ten sources was performed in the frequency range 329–361 GHz, with an angular resolution of ~1″.5, or ~2000 AU at a source distance of 1.3 kpc from the Sun. Spectral modelling was performed to identify molecular emission and determine column densities and excitation temperatures for each source. Emission maps were made to study the morphology of emission. Finally, emission properties were compared across the sample. Results. We detect CH3OH towards nine of the ten sources, with CH3OCH3 and CH3OCHO towards three sources. We further detect CH3CN towards four sources. Towards five sources the chemistry is spatially differentiated, meaning that different species peak at different positions and are offset from the peak continuum emission. Low levels of deuteration are detected towards four sources in HDO emission, whereas deuterated complex organic molecule emission is detected towards one source (CH2DOH towards N63). The chemical properties of each source do not correlate with their position in the Cygnus-X complex, nor do the distance or direction to the nearest OB associations. However, the five sources located in the DR21 filament do appear to show less line emission compared to the five sources outside the filament. Conclusions. This work shows how important wide frequency coverage observations are combined with high angular resolution observations for studying the protostellar environment. Furthermore, based on the ten sources observed here, the external environment appears to only play a minor role in setting the chemical environment on these small scales (<2000 AU).

Modeling the early mass ejection in jet-driven protostellar outflows: Lessons from Cep E

Context. Protostellar jets and outflows are an important agent of star formation as they carry away a fraction of momentum and energy, which is needed for gravitational collapse and protostellar mass accretion to occur. Aims. Our goal is to provide constraints on the dynamics of the inner protostellar environment from the study of the outflow-jet propagation away from the launch region. Methods. We have used the axisymmetric chemo-hydrodynamical code WALKIMYA-2D to numerically model and reproduce the physical and CO emission properties of the jet-driven outflow from the intermediate-mass protostar CepE-mm, which was observed at ~800 au resolution in the CO J = 2−1 line with the IRAM interferometer. Our simulations take into account the observational constraints available on the physical structure of the protostellar envelope. Results. WALKIMYA-2D successfully reproduces the main qualitative and quantitative features of the Cep E outflow and the jet kinematics, naturally accounting for their time variability. Signatures of internal shocks are detected as knots along the jet. In the early times of the ejection process, the young emitted knots interact with the dense circumstellar envelope through high-velocity, dissociative shocks, which strongly decrease the CO gas abundance in the jet. As time proceeds, the knots propagate more smoothly through the envelope and dissociative shocks disappear after ~103 yr. The distribution of CO abundance along the jet shows that the latter bears memory of the early dissociative phase in the course of its propagation. Analysis of the velocity field shows that the jet material mainly consists of gas entrained from the circumstellar envelope and accelerated away from the protostar at 700 au scale. As a result, the overall jet mass-loss rate appears higher than the actual mass-ejection rate by a factor ~3. Conclusions. Numerical modeling of the Cep E jet-driven outflow and comparison with the CO observations have allowed us to peer into the outflow formation mechanism with unprecedented detail and to retrieve the history of the mass-loss events that have shaped the outflow.

Maser Tracers of Gas Dynamics near Young Stars New Perspectives

AbstractThe protostellar environment where young stars form has physical conditions suitable to excite a number of molecular maser lines that have traditionally provided an unique probe of star formation kinematics, at the highest angular resolution of radio very long baseline interferometry (VLBI) observations. In the following, we will discuss a number of recent results on our understanding of the gas dynamics traced by masers in the vicinity of young forming stars. These findings provide direct clues on how our community can substantially contribute to the field of star formation in the next decade.

Accretion Burst Echoes as Probes of Protostellar Environments and Episodic Mass Assembly

Protostars likely accrete material at a highly time-variable rate, but measurements of accretion variability from the youngest protostars are rare, as they are still deeply embedded within their envelopes. Submillimeter/millimeter observations can trace the thermal response of dust in the envelope to accretion luminosity changes, allowing variations in the accretion rate to be quantified. In this paper, we present contemporaneous submillimeter/millimeter light curves of variable protostars in Serpens Main, as observed by the ALMA Atacama Compact Array (ACA), the Submillimeter Array (SMA), and the James Clerk Maxwell Telescope (JCMT). The most recent outburst of EC 53 (V371 Ser), an ∼18 month periodic variable, is well sampled in the SMA and JCMT observations. The SMA light curve of EC 53 is observed to peak weeks earlier and exhibit a stronger amplitude than at the JCMT. Stochastic variations in the ACA observations are detected for SMM 10 IR, with an amplitude a factor of ∼2 greater than that seen by the JCMT. We develop a toy model of the envelope response to accretion outbursts to show that EC 53's light curves are plausibly explained by the delay associated with the light travel time across the envelope and the additional dilution of the JCMT response, due to the incorporation of cold envelope material in the beam. The larger JCMT beam can also wash out the response to rapid variations, which may be occurring for SMM 10 IR. Our work thus provides a valuable proof of concept for the use of submillimeter/millimeter observations as a means of probing both the underlying accretion luminosity variations and the protostellar environment.

Grain growth during protostellar disc formation

ABSTRACT Recent observations indicate that mm/cm-sized grains may exist in the embedded protostellar discs. How such large grains grow from the micron size (or less) in the earliest phase of star formation remains relatively unexplored. In this study, we take a first step to model the grain growth in the protostellar environment, using 2D (axisymmetric) radiation hydrodynamic and grain growth simulations. We show that the grain growth calculations can be greatly simplified by the ‘terminal velocity approximation’, where the dust drift velocity relative to the gas is proportional to its stopping time, which is proportional to the grain size. We find that the grain–grain collision from size-dependent terminal velocity alone is too slow to convert a significant fraction of the initially micron-sized grains into mm/cm sizes during the deeply embedded Class 0 phase. Substantial grain growth is achieved when the grain–grain collision speed is enhanced by a factor of 4. The dust growth above and below the disc midplane enables the grains to settle faster towards the midplane, which increases the local dust-to-gas ratio, which, in turn, speeds up further growth there. How this needed enhancement can be achieved is unclear, although turbulence is a strong possibility that deserves further exploration.

Ultraviolet Photoprocessing of Glycine Adsorbed on Various Space-Relevant Minerals

Mineral surfaces are energetic environments that can assist prebiotic organization by adsorbing selected molecules and allowing their concentration and chemical evolution, possibly toward complex (pre)biological systems. This dynamic rocky environment can also play a crucial role as promoters of chemical reactions towards increasing molecular complexity. Moreover, presence of minerals can mediate the effects of electromagnetic radiation and influence the photostability of biomolecules, catalyzing photoreaction or protecting molecules against degradation. Such interactions are responsible for the preservation/degradation mechanisms of organic molecules in space. In particular, the discovery of organic molecules on asteroids and comets confirms their role as transport and delivery vehicles of building blocks of life on Earth and possibly on other bodies of the Solar System, and the study of this kind of processes can improve our knowledge in astrobiology.Many different prebiotic and bio-molecules were detected in space and in meteorites, but among them amino acid glycine (H2NCH2COOH), is one of the most important. It is the simplest of the 20 biogenic amino acids and it is conceivably the link between complex organic molecules detected in our evolved Solar System and prebiotic molecules observed in the interstellar medium and protostellar environment. Laboratory simulations of UV processing may provide key insights into the survival of glycine and other biomarker in space environment, which is of particular relevance for upcoming space exploration missions on planetary surfaces, like ESA-Roscosmos ExoMars 2020, and sample return missions, like NASA OSIRIS-REx.We report a laboratory study of UV irradiation of glycine adsorbed on various space relevant minerals, i.e. forsterite, antigorite, spinel and pyrite, using a calibrated enhanced UV Xenon lamp, which demonstrated good performance as solar simulator. Comparing protective/catalytic effects of various minerals helps to unravel the role of rocky surfaces in the origin of life and provides hints for the search of organics in present and future robotic space exploration missions.

A multi-scale exploration of a massive young stellar object

Context.The rarity of young massive stars combined with the fact that they are often deeply embedded has limited the understanding of the formation of stars larger than 8M⊙. Ground based mid-infrared (IR) interferometry is one way of securing the spatial resolution required to probe the circumstellar environments of massive young stellar objects (MYSOs). Given that the spatial-frequency coverage of such observations is often incomplete, direct-imaging can be supplementary to such a dataset. By consolidating these observations with modelling, the features of a massive protostellar environment can be constrained.Aims.This paper aims to detail the physical characteristics of the protostellar environment of the MYSO G305.20+0.21 at three size-scales by fitting one 2.5D radiative transfer model to three different types of observations simultaneously, providing an extensive view of the accreting regions of the MYSO.Methods.Interferometry, imaging and a multi-wavelength spectral energy distribution (SED) are combined to study G305.20+0.21. The high-resolution observations were obtained using the Very Large Telescope’s MIDI and VISIR instruments, producing visibilities in theN-band and near-diffraction-limited imaging in theQ-band respectively. By fitting simulated observables, derived from the radiative transfer model, to our observations the properties of the MYSO are constrained.Results.The VISIR image shows elongation at 100 mas scales and also displays a degree of asymmetry. From the simulated observables derived from the radiative transfer model output we find that a central protostar with a luminosity of ~5 × 104L⊙surrounded by a low-density bipolar cavity, a flared 1M⊙disk and an envelope is sufficient to fit all three types of observational data for G305.20+0.21. The weak silicate absorption feature within the SED requires low-density envelope cavities to be successfully fit and is an atypical characteristic in comparison to previously studied MYSOs.Conclusions.The fact that the presence of a dusty disk provides the best fit to the MIDI visibilities implies that this MYSO is following a scaled-up version of the low-mass star formation process. The low density, low extinction environment implies the object is a more evolved MYSO and this combined with large inner radius of the disk suggests that it could be an example of a transitional disk around an MYSO.

The chemistry of episodic accretion

AbstractEpisodic accretion is an important process in the evolution of young stars and their surroundings. A consequence of an episodic accretion event is a luminosity burst, which heats the protostellar environment and has a long lasting impact on the chemical evolution of the disk and envelope of young stars. We present a new model for the chemistry of episodic accretion based on the 2D radiation thermo-chemical disk code ProDiMo. We discuss the impact of an episodic accretion burst on the chemical evolution of CO and its observables. Furthermore we present a model for the outbursting source V883 Ori where we fitted available observational data to get an accurate physical structure that allows for a detailed study of the chemistry.

Masers as probes of the gas dynamics close to forming high-mass stars

AbstractImaging the inner few 1000 AU around massive forming stars, at typical distances of several kpc, requires angular resolutions of better than 0″.1. Very Long Baseline Interferometry (VLBI) observations of interstellar molecular masers probe scales as small as a few AU, whereas (new-generation) centimeter and millimeter interferometers allow us to map scales of the order of a few 100 AU. Combining these informations all together, it presently provides the most powerful technique to trace the complex gas motions in the proto-stellar environment. In this work, we review a few compelling examples of this technique and summarize our findings.

OUTFLOW-CONFINED H ii REGIONS. I. FIRST SIGNPOSTS OF MASSIVE STAR FORMATION

ABSTRACT We present an evolutionary sequence of models of the photoionized disk-wind outflow around forming massive stars based on the Core Accretion model. The outflow is expected to be the first structure to be ionized by the protostar and can confine the expansion of the H ii region, especially in lateral directions in the plane of the accretion disk. The ionizing luminosity increases as Kelvin–Helmholz contraction proceeds, and the H ii region is formed when the stellar mass reaches ∼10– 20 M ⊙ ?> depending on the initial cloud core properties. Although some part of the outer disk surface remains neutral due to shielding by the inner disk and the disk wind, almost the whole of the outflow is ionized in 103– 10 4 y r ?> after initial H ii region formation. Having calculated the extent and temperature structure of the H ii region within the immediate protostellar environment, we then make predictions for the strength of its free–free continuum and recombination line emission. The free–free radio emission from the ionized outflow has a flux density of ∼(20–200) × ( &ngr; / 10 GHz ) p mJy ?> for a source at a distance of 1 kpc with a spectral index p ≃ 0.4–0.7, and the apparent size is typically ∼500 AU at 10 GHz. The H 40 &agr; ?> line profile has a width of about 100 km s - 1 ?> . These properties of our model are consistent with observed radio winds and jets around forming massive protostars.

Shockingly low water abundances inHerschel/PACS observations of low-mass protostars in Perseus

Protostars interact with their surroundings through jets and winds impacting on the envelope and creating shocks, but the nature of these shocks is still poorly understood. Our aim is to survey far-infrared molecular line emission from a uniform and significant sample of deeply-embedded low-mass young stellar objects in order to characterize shocks and the possible role of ultraviolet radiation in the immediate protostellar environment. Herschel/PACS spectral maps of 22 objects in the Perseus molecular cloud were obtained as part of the `William Herschel Line Legacy' survey. Line emission from H$_\mathrm{2}$O, CO, and OH is tested against shock models from the literature. Observed line ratios are remarkably similar and do not show variations with source physical parameters. Observations show good agreement with the shock models when line ratios of the same species are compared. Ratios of various H$_\mathrm{2}$O lines provide a particularly good diagnostic of pre-shock gas densities, $n_\mathrm{H}\sim10^{5}$ cm$^{-3}$, in agreement with typical densities obtained from observations of the post-shock gas. The corresponding shock velocities, obtained from comparison with CO line ratios, are above 20 km\,s$^{-1}$. However, the observations consistently show one-to-two orders of magnitude lower H$_\mathrm{2}$O-to-CO and H$_\mathrm{2}$O-to-OH line ratios than predicted by the existing shock models. The overestimated model H$_\mathrm{2}$O fluxes are most likely caused by an overabundance of H$_\mathrm{2}$O in the models since the excitation is well-reproduced. Illumination of the shocked material by ultraviolet photons produced either in the star-disk system or, more locally, in the shock, would decrease the H$_\mathrm{2}$O abundances and reconcile the models with observations. Detections of hot H$_\mathrm{2}$O and strong OH lines support this scenario.

Magnetic field regulated infall on the disc around the massive protostar Cepheus A HW2

We present polarization observations of the 6.7-GHz methanol masers around the massive protostar Cepheus A HW2 and its associated disc. The data were taken with the Multi-Element Radio Linked Interferometer Network. The maser polarization is used to determine the full three-dimensional magnetic field structure around Cepheus A HW2. The observations suggest that the masers probe the large scale magnetic field and not isolated pockets of a compressed field. We find that the magnetic field is predominantly aligned along the protostellar outflow and perpendicular to the molecular and dust disc. From the three-dimensional magnetic field orientation and measurements of the magnetic field strength along the line of sight, we are able to determine that the high density material, in which the masers occurs, is threaded by a large scale magnetic field of ~23 mG. This indicates that the protostellar environment at ~1000 AU from Cepheus A HW2 is slightly supercritical (lambda approximately 1.7) and the relation between density and magnetic field is consistent with collapse along the magnetic field lines. Thus, the observations indicate that the magnetic field likely regulates accretion onto the disc. The magnetic field dominates the turbulent energies by approximately a factor of three and is sufficiently strong to be the crucial component stabilizing the massive accretion disc and sustaining the high accretion rates needed during massive star-formation.

Methylammonium methylcarbamate thermal formation in interstellar ice analogs: a glycine salt precursor in protostellar environments

<i>Context. <i/>Analyses of dust cometary grains collected by the Stardust spacecraft have shown the presence of amines and amino acids molecules, and among them glycine (NH<sub>2<sub/>CH<sub>2<sub/>COOH). We show how the glycine molecule could be produced in the protostellar environments before its introduction into comets.<i>Aims. <i/>We study the evolution of the interstellar ice analogues affected by both thermal heating and vacuum ultraviolet (VUV) photons, in addition to the nature of the formed molecules and the confrontation of our experimental results with astronomical observations.<i>Methods. <i/>Infrared spectroscopy and mass spectrometry are used to monitor the evolution of the H<sub>2<sub/>O:CO<sub>2<sub/>:CH<sub>3<sub/>NH<sub>2<sub/> and CO<sub>2<sub/>:CH<sub>3<sub/>NH<sub>2<sub/> ice mixtures during both warming processes and VUV photolysis.<i>Results. <i/>We first show how carbon dioxide (CO<sub>2<sub/>) and methylamine (CH<sub>3<sub/>NH<sub>2<sub/>) thermally react in water-dominated ice to form methylammonium methylcarbamate [ CH<sub>3<sub/>NH] [ CH<sub>3<sub/>NHCOO<sup>-<sup/>] noted C. We then determine the reaction rate and activation energy. We show that C thermal formation can occurs in the 50–70 K temperature range of a protostellar environment. Secondly, we report that a VUV photolysis of a pure C sample produces a glycine salt, methylammonium glycinate [ CH<sub>3<sub/>NH] [ NH<sub>2<sub/>CH<sub>2<sub/>COO<sup>-<sup/>] noted G. We propose a scenario explaining how C and subsequently G can be synthesized in interstellar ices and precometary grains.<i>Conclusions. <i/>[ CH<sub>3<sub/>NH] [ CH<sub>3<sub/>NHCOO<sup>-<sup/>] could be readily formed and would act as a glycine salt precursor in protostellar environments dominated by thermal and UV processing. We propose a new pathway leading to a glycine salt, which is consistent with the detection of glycine and methylamine within the returned samples of comet 81P/Wild 2 from the Stardust mission.

L1157: Interaction of the molecular outflow with the Class 0 environment

We present high angular resolution interferometric observations of the dust continuum at 2.7 and 1.3 mm, and of the HC3N () and C18O () emission around L1157-mm, a Class 0 object that drives a spectacular molecular outflow. The millimeter dust emission is clearly resolved into two components, a flattened compact source of ~ AU at 1.3 mm, and mass ~0.1 , plus an extended envelope of ~3000 AU at 1.3 mm, and mass ~1.1 . The millimeter spectral index varies throughout the region, with the lower value found toward the compact protostar, possibly indicating grain growth in the denser regions. A strong interaction between the molecular outflow and the close protostellar environment is taking place and affects the structure of the innermost parts of the envelope. This is shown by the spatial coincidence between the molecular outflow and the dust (1.3 mm continuum) and HC3N emission: both tracers show structures associated to the edges of the outflow lobes. Basically, the global picture sketched for the Class 0 object L1157-mm by Gueth et al. ([CITE]) is supported. We find possible evidence of infall, but we do not detect any velocity gradient indicative of a rotating circumstellar disk.

Does IRAS 16293–2422 have a hot core? Chemical inventory and abundance changes in its protostellar environment

A detailed radiative transfer analysis of the observed continuum and molecular line emission toward the deeply embedded young stellar object IRAS 16293-2422 is performed. Accurate molecular abundances and abundance changes with radius are presented. The continuum modelling is used to constrain the temperature and density distributions in the envelope, enabling quantitative estimates of various molecular abundances. The density structure is well described by a single power-law falling off as r^(-1.7), i.e., in the range of values predicted by infall models. A detailed analysis of the molecular line emission strengthens the adopted physical model and lends further support that parts of the circumstellar surroundings of IRAS are in a state of collapse. The molecular excitation analysis reveals that the emission from some molecular species is well reproduced assuming a constant fractional abundance throughout the envelope. The abundances and isotope ratios are generally close to typical values found in cold molecular clouds in these cases, and there is a high degree of deuterium fractionation. There are, however, a number of notable exceptions. Lines covering a wide range of excitation conditions indicate for some molecules, e.g., H_2CO, CH_3OH, SO, SO_2 and OCS, a drastic increase in their abundances in the warm and dense inner region of the circumstellar envelope. The location at which this increase occurs is consistent with the radius at which ices are expected to thermally evaporate off the grains. In all, there is strong evidence for the presence of a core close to the protostar, whose physical properties are similar to those detected towards most high mass protostars except for a scaling factor. However, the small scale of the hot gas and the infalling nature of the envelope lead to very different chemical time scales between low mass and high mass hot cores, such that only very rapidly produced second-generation complex molecules can be formed in IRAS 16293-2422 . Alternatively, the ices may be liberated due to grain-grain collisions in turbulent shear zones where the outflow interacts with the envelope. Higher angular resolution observations are needed to pinpoint the origin of the abundance enhancements and distinguish these two scenarios. The accurate molecular abundances presented for this low-mass protostar serve as a reference for comparison with other objects, in particular circumstellar disks and comets.

Spectroscopic diagnostics of organic chemistry in the protostellar environment

A combination of astronomical observations, laboratory studies, and theoretical modelling is necessary to determine the organic chemistry of dense molecular clouds. We present spectroscopic evidence for the composition and evolution of organic molecules in protostellar environments. The principal reaction pathways to complex molecule formation by catalysis on dust grains and by reactions in the interstellar gas are described. Protostellar cores, where warming of dust has induced evaporation of icy grain mantles, are excellent sites in which to study the interaction between gas phase and grain-surface chemistries. We investigate the link between organics that are observed as direct products of grain surface reactions and those which are formed by secondary gas phase reactions of evaporated surface products. Theory predicts observable correlations between specific interstellar molecules, and also which new organics are viable for detection. We discuss recent infrared observations obtained with the Infrared Space Observatory, laboratory studies of organic molecules, theories of molecule formation, and summarise recent radioastronomical searches for various complex molecules such as ethers, azaheterocyclic compounds, and amino acids.

First Image with the CfA Superconductive HEB Receiver: The Protostellar Outflow from IRAS 20126+4104 in CO (J = 7–6)

ABSTRACTWe present a spectral line image in CO (J = 7–6) of the spatially compact molecular outflow from the massive protostar IRAS 20126+4014, observed using a novel superconductive hot‐electron bolometer (HEB) heterodyne receiver we recently installed at the 10 m Heinrich Hertz Telescope. The bipolar outflow is clearly detected and resolved with a center of symmetry situated at the position of the massive protostar, previously identified as an IRAS source and a millimeter continuum source. The peaks of the emission from the red‐ and blueshifted CO lobes are separated by ≈14″ (0.1 pc), and the velocity of the CO emission extends ±30 km s−1 from the ambient cloud velocity. The total outflowing gas mass is approximately 4M⊙, while the outflow rate is at least 6 × 10-4 M⊙ yr−1. Both high‐velocity CO emission maxima trail the NH3 (3, 3) and SiO clumps located 3″–4″ downstream (farther from the IR source) in the flow. The high‐excitation CO line and the NH3 are likely tracing the heated gas entrained in different parts of the bow shocks. By interpreting our observations in terms of a jet‐driven model, we estimate the density in the underlying jet to be at least 105 cm−3. The outflow appears much more compact and in a different orientation than the arcminute‐scale north/south flow seen in low‐J transitions of CO. These observations represent the first successful operation of a superconductive HEB receiver on a telescope and demonstrate the importance of high‐frequency submillimeter lines to the understanding of the protostellar environment.

Magnetic Field Structure in Protostellar Envelopes

We have imaged the linearly polarized emission toward the young stellar objects NGC 1333/IRAS 4A and IRAS 16293-2422. We suggest that the polarized emission from the dense protostellar environment arises from magnetically aligned dust grains and use these observations to investigate the magnetic field structure. The observations were obtained using the new polarimetric capabilities at 3 mm of the six-element millimeter array of the Owens Valley Radio Observatory. Toward IRAS 4A, the average polarization observed is 4.4% ± 0.7%. The magnetic field direction implied by the observations is roughly parallel to the direction of the bipolar molecular outflow. Structure is seen at 3'' resolution in the linearly polarized image. The peak of the linear polarization is offset 15 from the peak of the total intensity and elongated perpendicular to the implied field direction. Models calculated with an hourglass magnetic field morphology in a spherically symmetric dusty envelope are consistent with the observations. Toward IRAS 16293 the polarized emission is located between the two binary components, and the polarization percentage at the peak of the polarized emission is 2.7% ± 0.7%. The magnetic field direction implied from our measurements is parallel to the major axis of the circumbinary disk. The polarized emission could be produced by a toroidal magnetic field in the circumbinary disk. The high resolution of the interferometer provides a probe of the polarized emission at the high densities (n ≥ 108 cm-3) characteristic of protostellar envelopes and disks. The detection of polarized emission at these densities provides a test of grain alignment mechanisms. We conclude that alignment by paramagnetic relaxation of thermally rotating grains is unlikely, because of the well-coupled gas and grain temperatures, but alignment of suprathermally rotating grains is not ruled out.

Modelling jet-driven molecular outflows

We have investigated the basic physical properties of the outflow that is created by a supersonic jet in a dense molecular cloud. We show that the dynamics of the interaction is strongly controlled by the rapid cooling of the post-shock gas at the head of the jet. The velocity of the gas is high in the vicinity of the jet head, but decreases rapidly as more material is swept-up. This type of outflow produces extremely high velocity clumps of post-shock gas which resemble the features seen in outflows. We also show that momentum transfer in bow shocks is more important than entrainment in high Mach number jets, as found in the protostellar environment.