Shock-compressed Layer Research Articles

Unveiling the Dynamics of Dense Cores in Cluster-forming Clumps: A 3D Magnetohydrodynamics Simulation Study of Angular Momentum and Magnetic Field Properties

We conducted isothermal magnetohydrodynamics simulations with self-gravity to investigate the properties of dense cores in cluster-forming clumps. Two different setups were explored: a single rotating clump and colliding clumps. We focused on determining the extent to which the rotation and magnetic field of the parental clump are inherited by the formed dense cores. Our statistical analysis revealed that the alignment between the angular momentum of dense cores, , and the rotational axis of the clump is influenced by the strength of turbulence and the simulation setup. In single rotating clumps, we found that tends to align with the clump’s rotational axis if the initial turbulence is weak. In colliding clumps, however, this alignment does not occur, regardless of the initial turbulence strength. This misalignment in colliding clumps is due to the induced turbulence from the collision and the isotropic gas inflow into dense cores. Our analysis of colliding clumps also revealed that the magnetic field globally bends along the shock-compressed layer, and the mean magnetic field of dense cores, , aligns with it. Both in single rotating clumps and colliding clumps, we found that the angle between and is generally random, regardless of the clump properties. We also analyzed the dynamical states of the formed cores and found a higher proportion of unbound cores in colliding clumps. In addition, the contribution of rotational energy was only approximately 5% of the gravitational energy, regardless of the model parameters for both single and colliding cases.

The Effect of Shock-wave Duration on Star Formation and the Initial Condition of Massive Cluster Formation

Stars are born in dense molecular filaments irrespective of their mass. Compression of the interstellar medium by shocks causes filament formation in molecular clouds. Observations show that a massive star cluster formation occurs where the peak of gas column density in a cloud exceeds 1023 cm−2. In this study, we investigate the effect of the shock-compressed layer duration on filament/star formation and how the initial conditions of massive star formation are realized by performing three-dimensional isothermal magnetohydrodynamics simulations with gas inflow duration from the boundaries (i.e., shock-wave duration) as a controlling parameter. Filaments formed behind the shock expand after the duration time for short-shock-duration models, whereas long-duration models lead to star formation by forming massive supercritical filaments. Moreover, when the shock duration is longer than two postshock freefall times, the peak column density of the compressed layer exceeds 1023 cm−2, and the gravitational collapse of the layer causes the number of OB stars expected to be formed in the shock-compressed layer to reach the order of 10 (i.e., massive cluster formation).

Ionizing feedback from an O star formed in a shock-compressed layer

ABSTRACT We develop a simple analytical model for what happens when an O star (or compact cluster of OB stars) forms in a shock-compressed layer and carves out an approximately circular hole in the layer, at the waist of a bipolar H ii region (H iiR). The model is characterized by three parameters: the half-thickness of the undisturbed layer, $Z_{_{\rm LAYER}}$, the mean number density of hydrogen molecules in the undisturbed layer, $n_{_{\rm LAYER}}$, and the (collective) ionizing output of the star(s), $\dot{\cal N}_{_{\rm LyC}}$. The radius of the circular hole is given by $W_{_{\rm IF}}(t)\sim 3.8\, {\rm pc}\, [Z_{_{\rm LAYER}}/0.1\,{\rm pc}]^{-1/6}[n_{_{\rm LAYER}}/10^4\,{\rm cm^{-3}}]^{-1/3}[\dot{\cal N}_{_{\rm LyC}}/10^{49}\,{\rm s^{-1}}]^{1/6}[t/{\rm Myr}]^{2/3}$. Similar power-law expressions are obtained for the rate at which ionized gas is fed into the bipolar lobes, the rate at which molecular gas is swept up into a dense ring by the shock front that precedes the ionization front, and the density in this dense ring. We suggest that our model might be a useful zeroth-order representation of many observed H iiRs. From viewing directions close to the mid-plane of the layer, the H iiR will appear bipolar. From viewing directions approximately normal to the layer, it will appear to be a limb-brightened shell but too faint through the centre to be a spherically symmetric bubble. From intermediate viewing angles, more complicated morphologies can be expected.

MHD Simulations of Dense Core Collision

We investigated the effect of magnetic fields on the collision process between dense molecular cores. We performed three-dimensional magnetohydrodynamic simulations of collisions between two self-gravitating cores using the Enzo adaptive mesh refinement code. The core was modeled as a stable isothermal Bonnor–Ebert (BE) sphere immersed in uniform magnetic fields. Collisions were characterized by the offset parameter b, Mach number of the initial core , magnetic field strength B 0, and angle θ between the initial magnetic field and collision axis. For head-on (b = 0) collisions, one protostar was formed in the compressed layer. The higher the magnetic field strength, the lower the accretion rate. For models with b = 0 and θ = 90°, the accretion rate was more dependent on the initial magnetic field strength compared with b = 0 and θ = 0° models. For off-center (b = 1) collisions, the higher specific angular momentum increased; therefore, the gas motion was complicated. In models with b = 1 and , the number of protostars and gas motion highly depended on B 0 and θ. For models with b = 1 and , no significant shock-compressed layer was formed and star formation was not triggered.

Bimodal Behavior and Convergence Requirement in Macroscopic Properties of the Multiphase Interstellar Medium Formed by Atomic Converging Flows

Abstract We systematically perform hydrodynamics simulations of 20 converging flows of the warm neutral medium (WNM) to calculate the formation of the cold neutral medium (CNM), focusing especially on the mean properties of the multiphase interstellar medium (ISM), such as the mean density on a 10 pc scale. Our results show that convergence in those mean properties requires a 0.02 pc spatial resolution that resolves the cooling length of the thermally unstable neutral medium (UNM) to follow the dynamical condensation from the WNM to the CNM. We also find that two distinct postshock states appear in the mean properties depending on the amplitude of the upstream WNM density fluctuation ( ). When , the interaction between shocks and density inhomogeneity leads to a strong driving of the postshock turbulence of >3 km s−1, which dominates the energy budget in the shock-compressed layer. The turbulence prevents dynamical condensation by cooling, and the CNM mass fraction remains at ∼45%. In contrast, when , the CNM formation proceeds efficiently, resulting in the CNM mass fraction of ∼70%. The velocity dispersion is limited to the thermal-instability-mediated level of ∼2–3 km s−1, and the layer is supported by both turbulent and thermal energy equally. We also propose an effective equation of state that models the multiphase ISM formed by the WNM converging flow as a one-phase ISM in the form of P ∝ ργ eff , where γ eff varies from 0.9 (for a large pre-shock Δρ 0) to 0.7 (for a small pre-shock Δρ 0).

Rapid and efficient mass collection by a supersonic cloud–cloud collision as a major mechanism of high-mass star formation

Abstract A supersonic cloud–cloud collision produces a shock-compressed layer which leads to formation of high-mass stars via gravitational instability. We carried out a detailed analysis of the layer by using the numerical simulations of magneto-hydrodynamics which deal with colliding molecular flows at a relative velocity of 20 km s−1 (Inoue & Fukui 2013, ApJ, 774, L31). Maximum density in the layer increases from 1000 cm−3 to more than 105 cm−3 within 0.3 Myr by compression, and the turbulence and the magnetic field in the layer are amplified by a factor of ∼5, increasing the mass accretion rate by two orders of magnitude to more than 10−4 $ M_{\odot } $ yr−1. The layer becomes highly filamentary due to gas flows along the magnetic field lines, and dense cores are formed in the filaments. The massive dense cores have size and mass of 0.03–0.08 pc and 8–$ 50\, M_{\odot } $ and they are usually gravitationally unstable. The mass function of the dense cores is significantly top-heavy as compared with the universal initial mass function, indicating that the cloud–cloud collision preferentially triggers the formation of O and early B stars. We argue that the cloud–cloud collision is a versatile mechanism which creates a variety of stellar clusters from a single O star like RCW 120 and M 20 to tens of O stars of a super star cluster like RCW 38 and a mini-starburst W 43. The core mass function predicted by the present model is consistent with the massive dense cores obtained by recent ALMA observations in RCW 38 (Torii et al. 2021, PASJ, in press) and W 43 (Motte et al. 2018, Nature Astron., 2, 478). Considering the increasing evidence for collision-triggered high-mass star formation, we argue that cloud–cloud collision is a major mechanism of high-mass star formation.

Globular Cluster Formation from Colliding Substructure

Abstract We investigate a scenario where the formation of globular clusters (GCs) is triggered by high-speed collisions between infalling atomic-cooling subhalos during the assembly of the main galaxy host, a special dynamical mode of star formation that operates at high gas pressures and is intimately tied to ΛCDM hierarchical galaxy assembly. The proposed mechanism would give origin to “naked” globular clusters as colliding dark matter subhalos and their stars will simply pass through one another while the warm gas within them clashes at highly supersonic speed and decouples from the collisionless component, in a process reminiscent of the Bullet galaxy cluster. We find that the resulting shock-compressed layer cools on a timescale that is typically shorter than the crossing time, first by atomic line emission and then via fine-structure metal-line emission, and is subject to gravitational instability and fragmentation. Through a combination of kinetic theory approximation and high-resolution N-body simulations, we show that this model may produce: (a) a GC number–halo mass relation that is linear down to dwarf galaxy scales and agrees with the trend observed over five orders of magnitude in galaxy mass; (b) a population of old globular clusters with a median age of 12 Gyr and an age spread similar to that observed; (c) a spatial distribution that is biased relative to the overall mass profile of the host; and (d) a bimodal metallicity distribution with a spread similar to that observed in massive galaxies.

An ALMA View of Molecular Filaments in the Large Magellanic Cloud. II. An Early Stage of High-mass Star Formation Embedded at Colliding Clouds in N159W-South

Abstract We have conducted ALMA CO isotopes and 1.3 mm continuum observations toward filamentary molecular clouds of the N159W-South region in the Large Magellanic Cloud with an angular resolution of ∼0.″25 (∼0.07 pc). Although the previous lower-resolution (∼1″) ALMA observations revealed that there is a high-mass protostellar object at an intersection of two line-shaped filaments in 13CO with the length scale of ∼10 pc, the spatially resolved observations, in particular, toward the highest column density part traced by the 1.3 mm continuum emission, the N159W-South clump, show complicated hub-filamentary structures. We also discovered that there are multiple protostellar sources with bipolar outflows along the massive filament. The redshifted/blueshifted components of the 13CO emission around the massive filaments/protostars have complementary distributions, which is considered to be possible evidence for a cloud–cloud collision. We propose a new scenario in which the supersonically colliding gas flow triggers the formation of both the massive filament and protostars. This is a modification of the earlier scenario of cloud–cloud collision, by Fukui et al., that postulated the two filamentary clouds occur prior to the high-mass star formation. A recent theoretical study of the shock compression in colliding molecular flows by Inoue et al. demonstrates that the formation of filaments with hub structure is a usual outcome of the collision, lending support for the present scenario. The theory argues that the filaments are formed as dense parts in a shock compressed sheet-like layer, which resembles “an umbrella with pokes.”

A Probe Method for Microwave Diagnostics of Pulsed Plasma

Abstract—An original method for microwave diagnostics of pulsed plasma is proposed and tested. The object of the diagnostics is a shock-compressed plasma layer that is formed when a high-speed plasma jet created by a magnetic-plasma compressor is decelerated by a barrier. The essence of this method consists in placing a small resonant cavity on the plasma-jet path. This cavity is simultaneously a barrier and a diagnostic tool. The plasma parameters (the electron concentration and the electron-collision frequency) are calculated from the measured values of the probing microwave signal power transmitted through the resonant cavity and reflected from it.

The role of magnetic field in the formation and evolution of filamentary molecular clouds

AbstractRecent observations have emphasized the importance of the formation and evolution of magnetized filamentary molecular clouds in the process of star formation. Theoretical and observational investigations have provided convincing evidence for the formation of molecular cloud cores by the gravitational fragmentation of filamentary molecular clouds. In this review we summarize our current understanding of various processes that are required in describing the filamentary molecular clouds. Especially we can explain a robust formation mechanism of filamentary molecular clouds in a shock compressed layer, which is in analogy to the making of “Sushi.” We also discuss the origin of the mass function of cores.

Bipolar H ii regions produced by cloud–cloud collisions

Abstract We suggest that bipolar H ii regions may be the aftermath of collisions between clouds. Such a collision will produce a shock-compressed layer, and a star cluster can then condense out of the dense gas near the center of the layer. If the clouds are sufficiently massive, the star cluster is likely to contain at least one massive star, which emits ionizing radiation, and excites an H ii region, which then expands, sweeping up the surrounding neutral gas. Once most of the matter in the clouds has accreted onto the layer, expansion of the H ii region meets little resistance in directions perpendicular to the midplane of the layer, and so it expands rapidly to produce two lobes of ionized gas, one on each side of the layer. Conversely, in directions parallel to the midplane of the layer, expansion of the H ii region stalls due to the ram pressure of the gas that continues to fall towards the star cluster from the outer parts of the layer; a ring of dense neutral gas builds up around the waist of the bipolar H ii region, and may spawn a second generation of star formation. We present a dimensionless model for the flow of ionized gas in a bipolar H ii region created according to the above scenario, and predict the characteristics of the resulting free–free continuum and recombination-line emission. This dimensionless model can be scaled to the physical parameters of any particular system. Our intention is that these predictions will be useful in testing the scenario outlined above, and thereby providing indirect support for the role of cloud–cloud collisions in triggering star formation.

The impact of a shock-compressed layer on the mass transfer of target material during processing compression plasma flows

The paper describes the results of experimental and numerical research of the thickness of the molten bath and the surface erosion of a substance under the influence of compression plasma flows. It has been demonstrated that the formation of the shock-compressed layer affects the melted depth and the mass erosion from the surface of the treated target.

The impact of a shock-compressed layer on the mass transfer of target material during processing compression plasma flows

The paper describes the results of experimental and numerical research of the thickness of the molten bath and the surface erosion of a substance under the influence of compression plasma flows. It has been demonstrated that the formation of the shock-compressed layer affects the melted depth and the mass erosion from the surface of the treated target.

Numerical study of mass transfer of a material under intense flows of high-speed electrons and plasma

The paper presents the mechanisms of mass transfer in a material under compressive plasma flows. The work contains numerical studies on mass loss from the surface of a sample treated by Compressive Plasma Flow, in the first case, and by High-Current Electron Beam, in the second case. The mass ablated from the material surface was computed with BETAIN software. The effects of plasma flow pressure on mass transfer of the material and processes of formation of the molten layer and erosion of mass from the material surface were investigated. It was revealed that plasma flow pressure formed a shock-compressed layer on the sample, and molten liquid was displaced from the center to peripheral regions. If sample size is less than the diameter of plasma spot, the melt is displaced out of the sample. This approach can explain experimental values of molten thickness and mass loss.

Passage of the discharge current through the plasma–electrode interface in the electromagnetic rail accelerator channel

We investigate the phenomena that accompany the acceleration of a free plasma piston (without a striker) in the electromagnetic rail accelerator channel filled with different gases (argon, helium). An intense glow appears in the shock-compressed layer (SCL) in the case of strong shock waves that produce a high electron concentration (~1017–1018 cm–3) behind the front. We have proposed that explosive electron emission (EEE) ensures the high-intensity emission of electrons, the passage of a part of the discharge current through the SCL, and the glow of the SCL. The velocity of a shock wave for which the strong electric field in the Debye layer at the cathode causes EEE from its surface and the passage of the current in the SCL has been determined. It has been concluded that, for high velocities of the plasma, the EEE is a universal mechanism that ensure the passage of a strong current through the interface between the cold electrode and the plasma.

A ram-pressure threshold for star formation

In turbulent fragmentation, star formation occurs in condensations created by converging flows. The condensations must be sufficiently massive, dense and cool to be gravitationally unstable, so that they start to contract; {\it and} they must then radiate away thermal energy fast enough for self-gravity to remain dominant, so that they continue to contract. For the metallicities and temperatures in local star forming clouds, this second requirement is only met robustly when the gas couples thermally to the dust, because this delivers the capacity to radiate across the full bandwidth of the continuum, rather than just in a few discrete spectral lines. This translates into a threshold for vigorous star formation, which can be written as a minimum ram-pressure Pcrit ~ 4 10^-11 dyne. Pcrit is independent of temperature, and corresponds to flows with molecular hydrogen number-density nH2 and velocity v satisfying nH2 v^2 > 800 cm^-3 (km/s)^2. This in turn corresponds to a minimum molecular hydrogen column-density for vigorous star formation, NH2crit ~ 4 10^21 cm^-2 (SIGMAcrit ~ 100 MSun pc^-2), and a minimum visual extinction AVcrit ~ 9. The characteristic diameter and line-density for a star-forming filament when this threshold is just exceeded -- a sweet spot for local star formation regions -- are 2Rfil ~ 0.1 pc and mufil ~ 13 MSun pc^-2. The characteristic diameter and mass for a prestellar core condensing out of such a filament are 2Rcore ~ 0.1 pc, and Mcore ~ MSun. We also show that fragmentation of a shock-compressed layer is likely to commence while the convergent flows creating the layer are still ongoing, and we stress that, under this circumstance, the phenomenology and characteristic scales for fragmentation of the layer are fundamentally different from those derived traditionally for pre-existing layers.

Star formation triggered by cloud–cloud collisions

We present the results of SPH simulations in which two clouds, each having mass $M_{_{\rm{o}}}\!=\!500\,{\rm M}_{_\odot}$ and radius $R_{_{\rm{o}}}\!=\!2\,{\rm pc}$, collide head-on at relative velocities of $\Delta v_{_{\rm{o}}} =2.4,\;2.8,\;3.2,\;3.6\;{\rm and}\;4.0\,{\rm km}\,{\rm s}^{-1}$. There is a clear trend with increasing $\Delta v_{_{\rm{o}}}$. At low $\Delta v_{_{\rm{o}}}$, star formation starts later, and the shock-compressed layer breaks up into an array of predominantly radial filaments; stars condense out of these filaments and fall, together with residual gas, towards the centre of the layer, to form a single large-$N$ cluster, which then evolves by competitive accretion, producing one or two very massive protostars and a diaspora of ejected (mainly low-mass) protostars; the pattern of filaments is reminiscent of the hub and spokes systems identified recently by observers. At high $\Delta v_{_{\rm{o}}}$, star formation occurs sooner and the shock-compressed layer breaks up into a network of filaments; the pattern of filaments here is more like a spider's web, with several small-$N$ clusters forming independently of one another, in cores at the intersections of filaments, and since each core only spawns a small number of protostars, there are fewer ejections of protostars. As the relative velocity is increased, the {\it mean} protostellar mass increases, but the {\it maximum} protostellar mass and the width of the mass function both decrease. We use a Minimal Spanning Tree to analyse the spatial distributions of protostars formed at different relative velocities.

Intense shock waves and shock-compressed gas flows in the channels of rail accelerators

Shock wave generation and shock-compressed gas flows attendant on the acceleration of an striker-free plasma piston in the channels of electromagnetic rail accelerators (railguns) are studied. Experiments are carried out in channels filled with helium or argon to an initial pressure of 25–500 Torr. At a pressure of 25 Torr, Mach numbers equal 32 in argon and 16 in helium. It is found that with the initial currents and gas initial densities in the channels being the same, the shock wave velocities in both gases almost coincide. Unlike standard shock tubes, a high electric field (up to 300 V/cm) present in the channel governs the motion of a shock-compressed layer. Once the charged particle concentration behind the shock wave becomes sufficiently high, the field causes part of the discharge current to pass through the shock-compressed layer. As a result, the glow of the layer becomes much more intense.

Nitriding of steel and titanium surface layers under the action of compression plasma flows

The elemental and phase compositions of St3 steel and VT1-0 titanium surface layers nitrided by the action of compression plasma flows (CPFs) have been investigated. The plasma flow parameters are shown to be correlated with the modified-layer nitrogen content. The basic mechanism by which the steel and titanium surface layers are saturated with nitrogen has been revealed. The performed experiments indicate that an increase in the absorbed energy density leads to a decrease in the nitrogen concentration because a shock-compressed layer is formed in the near-surface region, impeding nitrogen diffusion into the sample. The higher nitrogen concentration of surface layers treated by CPFs is achieved by increasing the pressure of the residual nitrogen atmosphere. It has been established that γN-Fe nitrous austenite, α″-Fe(N) and α′-Ti(N) martensitic phases, and γ′-Fe4N and δ-TiNx nitrides can be produced by nitriding the surface layers of St3 steel and VT1-0 titanium.

Dynamics of the interaction between plasma flows generated by a miniature magnetoplasma compressor and a target

Spectroscopic studies of compression plasma flows generated by a miniature magnetoplasma compressor and of the shock compressed plasma layer formed near a target surface exposed to these flows are reported. The peak electron temperature and density are found to be 3 eV and 1.2⋅1016 cm−3, respectively, in the compressor flow and 4.5 eV and 6.7⋅1016 cm−3 in the shock compressed layer.