Merging Of Filaments Research Articles

Observational Evidence of the Merging of Filaments and Hub Formation in G083.097+03.270

We uncover a hub–filament system correlated with massive young stellar associations in G083.097+03.270. Diagnosed with simultaneous 12CO, 13CO, and C18O line observations, the region is found to host two distinct and elongated filaments having separate velocity components, interacting spatially and kinematically, that appear to have seeded the formation of a dense hub at the intersection. A large velocity spread at the hub, in addition to a clear bridging feature connecting the filaments in velocity, indicate the merging of filaments. Along the filament axis, the velocity gradient reveals a global gas motion with an increasing velocity dispersion inward to the hub signifying turbulence. Altogether, the clustering of Class I sources, a high excitation temperature, a high column density, and the presence of a massive outflow at the central hub suggest enhanced star formation. We propose that the merging of large-scale filaments and velocity gradients along filaments are the driving factors in the mass accumulation process at the hub that have sequentially led to the massive star formation. With two giant filaments merging to coincide with a hub therein with ongoing star formation, this site serves as a benchmark for the “filaments to clusters” star-forming paradigm.

Kinetic simulations of the filamentation instability in pair plasmas

ABSTRACT The non-linear interaction between electromagnetic waves and plasmas attracts significant attention in astrophysics because it can affect the propagation of Fast Radio Bursts (FRBs) – luminous millisecond-duration pulses detected at radio frequency. The filamentation instability (FI) – a type of non-linear wave–plasma interaction – is considered to be dominant near FRB sources, and its non-linear development may also affect the inferred dispersion measure of FRBs. In this paper, we carry out fully kinetic particle-in-cell simulations of the FI in unmagnetized pair plasmas. Our simulations show that the FI generates transverse density filaments, and that the electromagnetic wave propagates in near vacuum between them, as in a waveguide. The density filaments keep merging until force balance between the wave ponderomotive force and the plasma pressure gradient is established. We estimate the merging time-scale and discuss the implications of filament merging for FRB observations.

Solute Front Shear and Coalescence Control Concentration Gradient Dynamics in Porous Micromodel

AbstractIn subsurface environments, incomplete mixing at the pore scale limits reaction rates, rendering their prediction by Darcy‐scale models challenging. Such pore scale concentration gradients are enhanced by the deformation of solute fronts and decay under the action of molecular diffusion and solute filament merging. It is currently unclear how these processes govern concentration gradient dynamics under different flow rates. We measure experimentally pore scale concentrations in solute fronts transported in a two‐dimensional porous micromodel over an extensive range of flow rates. We demonstrate that pore‐scale shear flow increases concentration gradients up to a time predicted by the lamellar mixing theory in shear flow. However, the flow rate‐dependency of the mean concentration gradient at this so‐called mixing time is weaker than predicted theoretically, a discrepancy which we explain quantitatively by accounting for lamellae aggregation. These findings shed new light on the pore‐scale mechanisms driving mixing dynamics in porous media.

Cooperative bundling by fascin generates actin structures with architectures that depend on filament length.

The assembly of actin-based structures with precisely defined architectures supports essential cellular functions, including motility, intracellular transport, and division. The geometric arrangements of the filaments within actin structures are stabilized via the association of crosslinking proteins, which bind two filaments simultaneously. Because actin polymerization and crosslinking occur concurrently within the dynamic environment of the cell, these processes likely play interdependent roles in shaping the architectures of actin-based structures. To dissect the contribution of polymerization to the construction of higher-order actin structures, we investigated how filament elongation affects the formation of simple, polarized actin bundles by the crosslinking protein fascin. Using populations of actin filaments to represent distinct stages of elongation, we found that the rate of bundle assembly increases with filament length. Fascin assembles short filaments into discrete bundles, whereas bundles of long filaments merge with one another to form interconnected networks. Although filament elongation promotes bundle coalescence, many connections formed between elongating bundles are short-lived and are followed by filament breakage. Our data suggest that initiation of crosslinking early in elongation aligns growing filaments, creating a template for continued bundle assembly as elongation proceeds. This initial alignment promotes the assembly of bundles that are resistant to large changes in curvature that are required for coalescence into interconnected networks. As a result, bundles of short filaments remain straighter and more topologically discrete as elongation proceeds than bundles assembled from long filaments. Thus, uncoordinated filament elongation and crosslinking can alter the architecture of bundled actin networks, highlighting the importance of maintaining precise control over filament length during the assembly of specialized actin structures.

Investigation of Collagen-Incorporated Sodium Alginate Bioprinting Hydrogel for Tissue Engineering

Tissue engineering is a promising area that is aimed at tissue regeneration and wound repair. Sodium alginate (SA) has been widely used as one of the most biocompatible materials for tissue engineering. The cost-efficiency and rapid gel ability made SA attractive in would healing and regeneration area. To improve printability and elasticity, many hydrogel-based bioinks were developed by mixing SA with other natural or synthetic polymers. In this paper, composite SA/COL bioink was used for the bioprinting of artificial cartilage tissue mimicries. The results showed that the concentration of both SA and COL has significant effects on filament diameter and merging. A higher concentration of the bioink solution led to better printing fidelity and less deformation. Overall, a higher SA concentration and a lower COL concentration contributed to a lower shrinkage ratio after crosslinking. In summary, the SA/COL composite bioink has favorable rheological properties and this study provided material composition optimization for future bioprinting of engineered tissues.

Sympathetic Quiet and Active Region Filament Eruptions

We present the observations of three sympathetic filament eruptions occurring on 19 July 2015 namely F1, F2, and F3. The events were observed in UV/EUV wavelengths by Atmospheric Imaging Assembly onboard the Solar Dynamics Observatory and by Global Oscillation Network Group telescope in H{\alpha} line. As filament F1 starts to erupt, a part of it falls close to the location of the F2 and F3 filaments. This causes the eruption of F2 and F3 during which the two filaments merge together and trigger a medium-class CME and a long-duration GOES C2.1 class flare. We discuss the dynamics and kinematics of these three filament eruptions and related phenomena.

Merging filaments I: a race against collapse

ABSTRACT The interstellar medium is characterized by an intricate filamentary network that exhibits complex structures. These show a variety of different shapes (e.g. junctions, rings, etc.) deviating strongly from the usually assumed cylindrical shape. A possible formation mechanism are filament mergers that we analyse in this study. Indeed, the proximity of filaments in networks suggests mergers to be rather likely. As the merger has to be faster than the end dominated collapse of the filament along its major axis, we expect three possible results: (a) The filaments collapse before a merger can happen, (b) the merged filamentary complex shows already signs of cores at the edges, or (c) the filaments merge into a structure which is not end-dominated. We develop an analytic formula for the merging and core-formation time-scale at the edge and validate our model via hydrodynamical simulations with the adaptive-mesh-refinement-code ramses. This allows us to predict the outcome of a filament merger, given different initial conditions which are the initial distance and the respective line-masses of each filament as well as their relative velocities.

Nozzle-integrated pre-deposition and post-deposition heating of previously deposited layers in polymer extrusion based additive manufacturing

The adhesion and merging of adjacent filaments in polymer extrusion based additive manufacturing (AM) plays a key role in determining the thermal and mechanical properties of the built part. It is well known that maintaining the deposited filaments at a high temperature aids in the process of adhesion and merging. While external mechanisms such as laser and infrared heating have been used in the past to heat up deposited filaments, this paper presents a simpler, less invasive and in situ mechanism for heating of previously deposited layers using a hot metal block integrated with and rastering together with the filament-dispensing nozzle. Infrared thermography based quantitative measurement of temperature field along the raster line is carried out for two configurations – a preheater and a postheater traveling ahead of or behind the nozzle respectively. In each case, significant temperature rise in the deposited filaments is shown. A configuration comprising both preheater and postheater is shown to result in additional thermal benefits. The measured temperature rise is shown to be a function of process parameters such as raster speed and heater-to-base gap. Experimental measurements are shown to agree well with theoretical and simulation models. Cross-section imaging of samples printed without and with the in situ heating clearly show significant improvement in neck growth and filament-to-filament merging compared to the baseline case. Improvement in thermal and structural performance of printed samples is also demonstrated. Compared to other techniques proposed in the past, the heating approach presented in this work is passive and requires minimal additional costs or complexity. The improved temperature field and consequently enhanced filament adhesion reported here may help design and build parts with superior thermal and mechanical properties using polymer AM.

Effect of phase front modulation on the merging of multiple regularized femtosecond filaments

Comparative experimental data on filamentation of a powerful femtosecond laser beams with amplitude or phase front modulation is presented. We show that phase discontinuities and zero intensity lines prevented filament merging and superfilament formation.

Merger and reconnection of Weibel separated relativistic electron beam

The relativistic electron beam (REB) propagation in a plasma is fraught with beam plasma instabilities. The prominent amongst them is the collisionless Weibel destabilization which spatially separates the forward propagating REB and the return shielding currents. This results in the formation of REB current filaments which are typically of the size of electron skin depth during the linear stage of the instability. It has been observed that in the nonlinear stage, the size of filaments increases as they merge with each other. With the help of 2-D particle-in-cell simulations in the plane perpendicular to the REB propagation, it is shown that these mergers occur in two distinct nonlinear phases. In the first phase, the total magnetic energy increases. Subsequently, however, during the second phase, one observes a reduction in magnetic energy. It is shown that the transition from one nonlinear regime to another occurs when the typical current associated with individual filaments hits the Alfvén threshold. In the second nonlinear regime, therefore, the filaments can no longer permit any increase in current. Magnetic reconnection events then dissipate the excess current (and its associated magnetic energy) that would result from a merger process leading to the generation of energetic electron jets in the perpendicular plane. At later times when there are only few filaments left, the individual reconnection events can be clearly identified. It is observed that in between such events, the magnetic energy remains constant and shows a sudden drop as and when two filaments merge. The electron jets released in these reconnection events are thus responsible for the transverse heating which has been mentioned in some previous studies [Honda et al., Phys. Plasmas 7, 1302 (2000)].

On the interaction of scrape off layer filaments

The interaction of L-mode and inter-ELM scrape-off layer (SOL) filaments is investigated by simulating the dynamics of two seeded density perturbations with different initial separation. A numerical campaign of 2D and 3D simulations is presented to explore the effect that different filaments’ perpendicular size and relative distance have on their reciprocal action. It is shown that the interaction is weak unless the filaments are in close proximity, i.e. with a separation of the order of the filaments’ diameter in the drift plane. The interaction occurs when the dipolar electrostatic fields associated with the filaments merge or cancel each other and it leads to changes in the radial velocity in the centre of mass of the system that are modest () and quickly decaying with separation. These results show that filaments travelling in the SOL can be considered to a good approximation as independent entities.

EVOLUTION OF GLOBAL RELATIVISTIC JETS: COLLIMATIONS AND EXPANSION WITH kKHI AND THE WEIBEL INSTABILITY

ABSTRACT In the study of relativistic jets one of the key open questions is their interaction with the environment. Here we study the initial evolution of both electron–proton ( e – − p + ?> ) and electron–positron (e ±) relativistic jets, focusing on their lateral interaction with ambient plasma. We follow the evolution of toroidal magnetic fields generated by both the kinetic Kelvin–Helmholtz and Mushroom instabilities. For an e – − p + ?> jet, the induced magnetic field collimates the jet and electrons are perpendicularly accelerated. As the instabilities saturate and subsequently weaken, the magnetic polarity switches from clockwise to counterclockwise in the middle of the jet. For an e ± jet, we find strong mixing of electrons and positrons with the ambient plasma, resulting in the creation of a bow shock. The merging of current filaments generates density inhomogeneities that initiate a forward shock. Strong jet–ambient plasma mixing prevents a full development of the jet (on the scale studied), revealing evidence for both jet collimation and particle acceleration in the forming bow shock. Differences in the magnetic field structure generated by e – − p + ?> and e ± jets may contribute to the polarization properties of the observed emission in AGN jets and gamma-ray bursts.

TETHER-CUTTING RECONNECTION BETWEEN TWO SOLAR FILAMENTS TRIGGERING OUTFLOWS AND A CORONAL MASS EJECTION

ABSTRACT Triggering mechanisms of solar eruptions have long been a challenge. A few previous case studies have indicated that preceding gentle filament merging via magnetic reconnection may launch following intense eruption, according to the tether-cutting (TC) model. However, the detailed process of TC reconnection between filaments has not been exhibited yet. In this work, we report the high-resolution observations from the Interface Region Imaging Spectrometer (IRIS) of TC reconnection between two sheared filaments in NOAA active region 12146. The TC reconnection commenced on ∼15:35 UT on 2014 August 29 and triggered an eruptive GOES C4.3-class flare ∼8 minutes later. An associated coronal mass ejection appeared in the field of view of the Solar and Heliospheric Observatory/LASCO C2 about 40 minutes later. Thanks to the high spatial resolution of IRIS data, bright plasma outflows generated by the TC reconnection are clearly observed, which moved along the subarcsecond fine-scale flux tube structures in the erupting filament. Based on the imaging and spectral observations, the mean plane-of-sky and line-of-sight velocities of the TC reconnection outflows are separately measured to be ∼79 and 86 km s−1, which derives an average real speed of ∼120 km s−1. In addition, it is found that spectral features, such as peak intensities, Doppler shifts, and line widths in the TC reconnection region are evidently enhanced compared to those in the nearby region just before the flare.

Nonlinear dynamics of filamentation instability and current filament merging in a high density current-driven plasma

The magnetic field generation due to the filamentation instability (FI) of a high density current-driven plasma is studied through a new nonlinear diffusion equation. This equation is obtained on the basis of quantum hydrodynamic model and numerically solved by applying the Crank–Nicolson method. The spatiotemporal evolution of the magnetic field and the electron density distribution exhibits the current filament merging as a nonlinear phase of the FI which is responsible for the strong magnetic fields in the current-driven plasmas. It is found that the general behaviour of the FI is the same as that of the classical case but the instability growth rate, its magnitude, and the saturation time are affected by the quantum effects. It is eventually concluded that the quantum effects can play a stabilizing role in such situation.

SHOCK FORMATION IN ELECTRON–ION PLASMAS: MECHANISM AND TIMING

We analyse the full shock formation process in electron-ion plasmas in theory and simulations. It is accepted that electromagnetic shocks in initially unmagnetised relativistic plasmas are triggered by the filamentation instability. However, the transition from the first unstable phase to the quasi-steady shock is still missing. We derive a theoretical model for the shock formation time, taking into account the filament merging in the non-linear phase of the filamentation instability. This process is much slower than in electron-positron pair shocks, so that the shock formation is longer by a factor proportional to sqrt(m_i/m_e) ln(m_i/m_e).

MAGNETIC FIELDS IN HIGH-MASS INFRARED DARK CLOUDS

High-mass Stars are cosmic engines known to dominate the energetics in the Milky Way and other galaxies. However, their formation is still not well understood. Massive, cold, dense clouds, often appearing as Infrared Dark Clouds (IRDCs), are the nurseries of massive stars. No measurements of magnetic fields in IRDCs in a state prior to the onset of high-mass star formation (HMSF) have previously been available, and prevailing HMSF theories do not consider strong magnetic fields. Here, we report observations of magnetic fields in two of the most massive IRDCs in the Milky Way. We show that IRDCs G11.11-0.12 and G0.253+0.016 are strongly magnetized and that the strong magnetic field is as important as turbulence and gravity for HMSF. The main dense filament in G11.11-0.12 is perpendicular to the magnetic field, while the lower density filament merging onto the main filament is parallel to the magnetic field. The implied magnetic field is strong enough to suppress fragmentation sufficiently to allow HMSF. Other mechanisms reducing fragmentation, such as the entrapment of heating from young stars via high mass surface densities, are not required to facilitate HMSF.

ON THE FEEDING ZONE OF PLANETESIMAL FORMATION BY THE STREAMING INSTABILITY

The streaming instability is a promising mechanism to overcome the barriers in direct dust growth and lead to the formation of planetesimals. Most previous studies of the streaming instability, however, were focused on a local region of a protoplanetary disk with a limited simulation domain such that only one filamentary concentration of solids has been observed. The characteristic separation between filaments is therefore not known. To address this, we conduct the largest-scale simulations of the streaming instability to date, with computational domains up to 1.6 gas scale heights both horizontally and vertically. The large dynamical range allows the effect of vertical gas stratification to become prominent. We observe more frequent merging and splitting of filaments in simulation boxes of high vertical extent. We find multiple filamentary concentrations of solids with an average separation of about 0.2 local gas scale heights, much higher than the most unstable wavelength from linear stability analysis. This measures the characteristic separation of planetesimal forming events driven by the streaming instability and thus the initial feeding zone of planetesimals.

THE W40 REGION IN THE GOULD BELT: AN EMBEDDED CLUSTER AND H II REGION AT THE JUNCTION OF FILAMENTS

We present a multiwavelength study of W40 star-forming region using IR observations in UKIRT JHK bands, Spitzer IRAC bands & Herschel PACS bands; 2.12 micron H2 narrow-band imaging; & radio observations from GMRT (610 & 1280 MHz), in a FoV of ~34'x40'. Spitzer observations along with NIR observations are used to identify 1162 Class II/III & 40 Class I sources in the FoV. The NN stellar surface density analysis shows that majority of these YSOs constitute the embedded cluster centered on the source IRS1A South. Some YSOs, predominantly younger population, are distributed along & trace the filamentary structures at lower stellar surface density. The cluster radius is obtained as 0.44pc - matching well with the extent of radio emission - with a peak density of 650pc^-2. The JHK data is used to map the extinction which is subsequently used to compute the cloud mass. It has resulted in 126 Msun & 71 Msun for the central cluster & the northern IRS5 region, respectively. H2 narrow-band imaging displays significant emission, which prominently resembles fluorescent emission arising at the borders of dense regions. Radio analysis shows this region as having blister morphology, with the radio peak coinciding with a protostellar source. Free-free emission SED analysis is used to obtain physical parameters of the overall region & the IRS5 sub-region. This multiwavelength scenario is suggestive of star formation having resulted from merging of multiple filaments to form a hub. Star formation seems to have taken place in two successive epochs, with the first epoch traced by the central cluster & the high-mass star(s) - followed by a second epoch which is spreading into the filaments as uncovered by the Class I sources & even younger protostellar sources along the filaments. The IRS5 HII region displays indications of swept-up material which has possibly led to the formation of protostars.

Destabilization, interaction, and eruption of filaments on the sun and associated phenomena

A brief overview of the observation results of filament interactions on the Sun both from the Earth and from space at the TRACE, SOHO, STEREO, Hinode, and Yohkoh stations in white light, Hα, extreme UV (EUV), X rays, and a radio range is presented. Various filament collision cases, where filaments merge (or do not merge) or change their forms as a result of reconnection, are considered. Filament interactions are associated with flares, filament eruptions, and coronal mass ejections. The theoretical aspects of the observed phenomena are also considered.

Complex, quiescent kinematics in a highly filamentary infrared dark cloud★

Infrared Dark Clouds (IRDCs) host the initial conditions under which massive stars and stellar clusters form. We have obtained high sensitivity and high spectral resolution observations with the IRAM 30m antenna, which allowed us to perform detailed analysis of the kinematics within one IRDC, G035.39-00.33. We focus on the 1-0 and 3-2 transitions of N2H+, C18O (1-0), and make comparison with SiO (2-1) observations and extinction mapping. Three interacting filaments of gas are found. We report large-scale velocity coherence throughout the cloud, evidenced through small velocity gradients and relatively narrow line widths. This suggests that the merging of these filaments is somewhat "gentle", possibly regulated by magnetic fields. This merging of filaments may be responsible for the weak parsec-scale SiO emission detected by Jimenez-Serra et al. 2010, via grain mantle vaporization. A systematic velocity shift between the N2H+ (1-0) and C18O (1-0) gas throughout the cloud of 0.18 +/- 0.04 kms^{-1} is also found, consistent with a scenario of collisions between filaments which is still ongoing. The N2H+ (1-0) is extended throughout the IRDC and it does not only trace dense cores, as found in nearby low-mass star-forming regions. The average H2 number density across the IRDC is ~ 5 x 10^4 cm^{-3}, at least one order of magnitude larger than in nearby molecular clouds where low-mass stars are forming. A temperature gradient perpendicular to the filament is found. From our study, we conclude that G035.39-00.33 (clearly seen in the extinction map and in N2H+) has been formed via the collision between two relatively quiescent filaments with average densities of ~ 5 x 10^3 cm^{-3}, moving with relative velocities of ~ 5 kms^{-1}. The accumulation of material at the merging points started > 1 Myr ago and it is still ongoing.