Finger-like Features Research Articles

Galactic ‘Snake’ IRDC G11.11−0.12: a site of multiple hub–filament systems and colliding filamentary clouds

ABSTRACT To probe star formation processes, we present a multiscale and multiwavelength investigation of the ‘Snake’ nebula/infrared dark cloud G11.11−0.12 (hereafter, G11; length ∼27 pc). Spitzer images hint at the presence of subfilaments (in absorption), and reveal four infrared-dark hub–filament system (HFS) candidates (extent < 6 pc) towards G11, where massive clumps (> 500 M⊙) and protostars are identified. The 13CO(2–1), C18O(2–1), and NH3(1,1) line data reveal a noticeable velocity oscillation towards G11, as well as its left part (or part-A) around Vlsr of 31.5 km s−1, and its right part (or part-B) around Vlsr of 29.5 km s−1. The common zone of these cloud components is investigated towards the centre of G11 housing one HFS. Each cloud component hosts two subfilaments. In comparison to part-A, more APEX Telescope Large Area Survey of the Galaxy (ATLASGAL) clumps are observed towards part-B. The JWST near-infrared images discover one infrared-dark HFS candidate (extent ∼0.55 pc) around the massive protostar G11P1 (i.e. G11P1-HFS). Hence, the infrared observations reveal multiple infrared-dark HFS candidates at multiscale in G11. The Atacama Large Millimeter/submillimeter Array (ALMA) 1.16-mm continuum map shows multiple finger-like features (extent ∼3500–10 000 au) surrounding a dusty envelope-like feature (extent ∼18 000 au) towards the central hub of G11P1-HFS. Signatures of forming massive stars are found towards the centre of the envelope-like feature. The ALMA H13CO+ line data show two cloud components with a velocity separation of ∼2 km s−1 towards G11P1. Overall, the collision process, the ‘fray and fragment’ mechanism, and the ‘global non-isotropic collapse’ scenario seem to be operational in G11.

ALMA Reveals Hidden Morphologies in the Molecular Envelope of VY Canis Majoris

The J = 2 → 1 transition of CO near 230 GHz and the J = 3 → 2 line of HCN at 265 GHz have been imaged in the envelope of the red hypergiant star, VY Canis Majoris (VY CMa), using the Atacama Large Millimeter Array (ALMA) with angular resolutions 0.″2–1.″5; single-dish data were added to provide sensitivity up to 30″. These images reveal a far more complex envelope, with previously unseen outflows extending 4″–9″ from the star. These new structures include an arc-like outflow with an angular separation of ∼9″ northeast from the stellar position (“NE Arc”), twin fingerlike features approximately 4″ to the north/northeast (“NE Extension”), and a roughly spherical region observed ∼7″ E of the star (“E Bubble”). The NE Arc appears to be decelerating from base (V LSR ∼ 7 km s−1) to tip (V LSR ∼ 18 km s−1), while the NE Extension is blueshifted with V LSR ∼ −7 km s−1. Among the new features, HCN is only detected in the NE Arc. In addition, known structures Arc 1, Arc 2, and NW Arc, as well as other features closer to the star, are closely replicated in CO, suggesting that the gas and dust are well mixed. The CO spectra are consistent with the kinematic picture of VY CMa derived from HST data. Arc 2, however, has added complexity. Preliminary results from CO suggest 12C/13C ∼ 22–38 across the envelope. The additional presence of at least three major episodic mass ejection events significantly broadens the current perspective of the envelope structure and mass-loss history of VY CMa.

Geometry-controlled instabilities for soft-soft adhesive interfaces.

Soft materials interfaces can develop complex morphologies, such as cavities or finger-like features, during separation as a result of a mechanical instability. While the onset and growth of these instabilities have been investigated previously for interfaces between rigid and soft materials, no existing predictive model provides insight for controlling the separation morphology associated with these instabilities when both "sides" of the interface are soft. Here, we expand previous models to account for the geometry and materials properties of two soft materials that form an interface. The total compliance of the system, which depends nonlinearly on the thickness of each contacting soft material, plays a primary role in governing the morphology of the separating interface. We validate this model with experimental measurements using a series of soft elastomers with varying layer thicknesses and fixed materials properties, in order to emphasize the geometry alone can give rise to the observed differences in the interface separation process. This model also demonstrates that the degree of geometric asymmetry, or the ratio of the layer thicknesses that form an interface, influences the stress experienced in either layer, thus providing a rich means of controlling how unstable interface separations develop and propagate. This framework is a powerful tool to understand and control adhesion mechanisms in fields ranging from biology to soft robotics, and provides intuition for engineering a separation mode for a desired end result.

Structure of the pellet-cladding interaction layer of a high-burnup Zr-Nb-O nuclear fuel cladding

Structure of the pellet/cladding-interaction (PCI) layer of a high-burnup nuclear fuel with niobium-bearing cladding was investigated using modern transmission electron microscopy (TEM) techniques. Morphology of the PCI consisted of uniform zirconia layer on the cladding side and finger-like features towards to the ceramic fuel. The PCI layer showed a complex microstructure that constitutes three distinct zirconia layers formed by two zirconia phases. From cladding to fuel, monoclinic, tetragonal, and another tetragonal phase were determined via TEM diffraction patterns and precession electron diffraction (PED) using ASTAR microscopy platform (AMP). Characterizations showed that monoclinic and tetragonal phases close to the cladding were crack-free, but the tetragonal layer adjacent to the fuel contained cracks and pores. Presence of the monoclinic layer suggested that its formation involved the gaseous diffusion of oxygen below threshold dose of fission product damage before the fuel/cladding contact occurred. Furthermore, high-resolution TEM revealed compound phase of (U,Zr)O2 at fringes of zirconia fingers on the fuel side.

Primary breakup of power-law biofuel sprays in pressurized gaseous crossflow

The spray characteristics of biofuels have attracted broad interests, due to its bright prospect as one of the alternative fuels. The rheology of some biofuels was found to be power-law behavior. The spray performance of these fuels still remains unclear. Besides, biofuel atomization in pressurized gaseous crossflow can be of interest for combustion engines. The influential factors, namely the momentum flux ratios, Weber number and liquid-gas density ratios, are investigated employing controlling variable approach in the present. Power-law behavior fuel has shown great advantages in bringing a better atomization compared to the Newtonian one with case-control method. Jet column surface disturbance and the deformation magnitude during the injection process is emphasized. The penetration core lengths and breakup points are identified to illustrate the column deformation and breakup times. Besides, the column furcation processes which form finger-like features, as well as surface wavelengths are detected and compared along the studies of the influential factors. A larger injection momentum brings a smaller column deflection while the deflection is independent of Weber number. Besides, the locations of the furcation points increase with the raise of the momentum flux ratios while reduce with the increase of Weber number. The significant effect for increasing the density ratios is on promoting jet atomization. Column breakup processes tend to take place when keep increasing the gas densities. The column surface fluctuations are enhanced while the quantity of the ligaments raises and the shapes become richer.

Gravitational instabilities in a protosolar-like disc III: molecular line detection and sensitivities

At the time of formation, protoplanetary discs likely contain a comparable mass to their host protostars. As a result, gravitational instabilities (GIs) are expected to play a pivotal role in the early phases of disc evolution. However, as these young objects are heavily embedded, confirmation of GIs has remained elusive. Therefore, we use the radiative transfer code LIME to produce line images of a $0.17\,\mathrm{M}_{\odot}$ self-gravitating protosolar-like disc. We note the limitations of using LIME and explore methods to improve upon the default gridding. We synthesise noiseless observations to determine the sensitivities required to detect the spiral flux, and find that the line flux distribution does not necessarily correlate to the abundance density distribution; hence performing radiative transfer calculations is imperative. Moreover, the spiral features are seen in absorption, due to the GI-heated midplane and high extinction, which could be indicative of GI activity. If a small beamsize and appropriate molecular line are used then spatially resolving spirals in a protosolar-like disc should be possible with ALMA for an on-source time of 30 hr. Spectrally resolving non-axisymmetric structure takes only a tenth as long for a reasonable noise level, but attributing this structure to GI-induced activity would be tentative. Finally, we find that identifying finger-like features in PV diagrams of nearly edge-on discs, which are a direct indicator of spirals, is feasible with an on-source time of 19 hr, and hence likely offers the most promising means of confirming GI-driven spiral structure in young, embedded protoplanetary discs.

RAYLEIGH-TAYLOR TYPE INSTABILITIES IN THE RECONNECTION EXHAUST JET AS A MECHANISM FOR SUPRA-ARCADE DOWNFLOWS IN THE SUN

Supra-arcade downflows (hereafter referred to as SADs) are low-emission, elongated, finger-like features usually observed in active-region coronae above post-eruption flare arcades. Observations exhibit downward moving SADs intertwined with bright upward moving spikes. Whereas SADs are dark voids, spikes are brighter, denser structures. Although SADs have been observed for decades, the mechanism of formation of SADs remains an open issue. In our three-dimensional resistive magnetohydrodynamic simulations, we demonstrate that secondary Rayleigh-Taylor type instabilities develop in the downstream region of a reconnecting current sheet. The instability results in the formation of low-density coherent structures that resemble SADs, and high-density structures that appear to be spike-like. Comparison between the simulation results and observations suggests that secondary Rayleigh-Taylor type instabilities in the exhaust of reconnecting current sheets provide a plausible mechanism for observed SADs and spikes.

Stress Corrosion Cracking of Sensitized Type 304 Stainless Steel in High-Temperature Water with Anionic Impurities Contamination

This paper describes some results selected from a larger program that was aimed at understanding the stress corrosion cracking (SCC) initiation of Type 304 stainless steel (UNS S30400) in high-temperature deaerated water. Out of a large number of statically loaded samples, only a small minority of the tested samples underwent SCC. The occurrence of SCC indicates a synergism between sensitization, ionic impurities (mainly chloride and sulfate), and/or superficial defects and cold work. In fact, none of the nonsensitized materials initiated cracking (within the time scale of the tests), while only three sensitized samples underwent extensive SCC. The crack morphology of the fractured sample was predominantly inter-granular with some transgranular regions. Transmission electron microscopic samples containing crack tips were, in most respect, in line with the literature: a magnetite/spinel duplex layer on the crack surfaces, a Cr-rich oxide at the crack tip, and Ni enrichment at the metal/oxide interface and oxidized deformation bands intercepting the crack flanks. Also, finger-like features protruding several hundreds of nanometers along the slip planes intersecting the intergranular crack were found on grain boundaries with a high degree of localized deformation. These results support the theory that cracking initiation and propagation might be associated with the formation of oxide on crystallographic planes inside the material.

The mechanics of continental lithosphere–asthenosphere coupling

The physical interaction of the convecting asthenosphere and the highly viscous lower lithosphere determines the transmission of plate driving stresses from the mantle to the plates, and controls the stress state of deep cratonic roots, which is important for cratonic diamond formation models. A growing number of studies have highlighted a high degree of heterogeneity in root zones, however, little work has been done on how a heterogenous lower lithosphere affects plate-mantle coupling. The purpose of this work is to assess the degree that physical and rheological heterogeneity in the lowermost lithosphere affects the coupling and transmission of stress from the asthenosphere to the lithosphere. Southern Africa is an area where fine scale variations in the seismic and lithological structure have been identified. Here we present numerical models of the dynamics of stylised and Southern Africa-like cratonic roots in a convecting mantle. We find lithosphere structure exerts a strong control on asthenospheric flow and this dominates the lower lithosphere stress field. Weak metasomatic zones do not systematically affect the stress regime, though they do mitigate the effects of stress extremes on root zones. Short length-scale finger-like features as seen in Southern African tomography are not stable over geological time periods for plausible rheologies, which suggests such small-scale variations in the seismic structure may have a chemical/metasomatic origin. Such chemical heterogeneities can be remarkably long-lived (order of hundreds of Myrs), despite their lack of mechanical integrity.

Influence of Surface Layers Modified by Proton Exchange on Domain Kinetics of Lithium Niobate

The abnormal domain kinetics in congruent lithium niobate single crystals caused by surface modification using proton exchange was studied experimentally. The dependence of domain kinetics on polarity of the growing domains at the surface with proton exchange was revealed. The abnormal smooth motion of irregularly shaped domain walls with finger-like features and correlated nucleation was observed for growth of domains with Z + at surface with proton exchange. Such behavior drastically differs from jump-like motion of plane domain walls oriented along Y directions observed both in conventional lithium niobate and for growth of domains with Z – at surface with proton exchange.

Defects in multilayer plastic films I: Interface defects in extrusion

This paper deals with defects in multilayer plastic films in the extrusion process. A methodology to link process parameters and fluid rheology to onset of certain types of defects is developed. The defects considered include zigzag features that can form on the surface or interfaces in the extruded multilayer films, and parabolic and finger-like features that can form at the interfaces. The zigzag defects occur due to excessive shear and the parabolic and viscous-finger defects develop in response to flow perturbations. Novel approaches to predict the formation of the latter defects based on steady-state flow calculations are presented in this paper. Tendency of formation of parabolic defects can be evaluated based on steady-state calculations, and possibility of formation of viscous fingers can be explored based on energy considerations. It has also been shown that a process window can be developed by simultaneously considering the process requirements for elimination of these defects.

Initiation and propagation of localized deformation in elasto-plastic strips under uniaxial tension

This paper deals with the evolution of inhomogeneous deformation in shape memory alloy strips and mild steel strips under uniaxial tension. New experiments on NiTi strips, which initially are in an austenitic phase, show that at a critical stress level martensite nucleates in sharp bands inclined at 55° to the axis of loading. Under prescribed end displacement martensite subsequently spreads either by steady-state propagation of inclined transition fronts or via a criss-cross pattern of finger-like features. Similar events have been reported in the literature regarding the evolution of Luders bands in fine grained steel strips and wires. The similarity of macroscopic events, despite the different mechanisms of instability at the micro-level, prompted us to approximate the material behavior as a finitely deforming elasto-plastic solid with a trilinear up-down-up nominal stress-strain response. Two such stress-strain responses were used in finite element simulations of strip tension tests. In the first the true stress-strain response maintains its stability and in the second the intermediate branch has a negative slope. While both material models produced inhomogeneous deformations with features similar to those of the experiments, the larger initiation peak associated with the second gave results which closely resembled specific experiments. The numerical simulations confirmed that the evolution of events seen in experiments on SMAs and mild steels is strongly influenced by overall geometric (structural) effects. Furthermore, the success of this simple continuum constitutive model strongly suggests that continuum level events remain dominant players in such fine grained materials.

Fabry-Perot imaging of molecular hydrogen emission on Orion-KL region

view Abstract Citations (21) References (34) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Fabry-Perot Imaging of Molecular Hydrogen Emission in Orion-KL Region Sugai, Hajime ; Usuda, Tomonori ; Kataza, Hirokazu ; Tanaka, Masuo ; Inoue, Motoko Y. ; Takami, Hideki ; Aoki, Tetsuo ; Hiromoto, Norishisa ; Kawavata, Hironobu Abstract A continuum-subtracted image of the H2 nu = 1 goes to 0 S(1) line covering the central 4 min square portion of the Orion-KL region, has been obtained using a wide-field Fabry-Perot imager at the Communication Research Laboratoy 1.5 m telescope. Nine H2 peaks (including four newly identified) are found in te core, as well as a curved, bridgelike structure that enroaches on the inner edge of the disk structure observed in CS (J = 1 goes to 0) emission. This existence of the bridge indicates that the outflow from IRc2 occurs not only toward the bipolar axis but also toward the disk, and suggests channeling by the disk of a more or less isotropics flow from IRc2 into a bipolar flow. Outside the core, where finger-like features of H2 emission are seen, we identify 43 hot H2 clumps. These are located along filamentary structures seen in CS (J = 1 goes to 0) maps and avoid peaks of CS intensity. The geometrical thickness of the hot H2 in these features is much smaller than their typical dimension. We conclude that the H2 emission from the clumps is radiated at the surface of the CS filaments, implying that outflow form IRc2 shocks the H2 at the surfaces. From comparison of polarization measurements of Burton et al. (1991) with the present meausrements, as well as from discussion of the energetics, we rule out the possibility that the H2 clumps are due to reflection. Publication: The Astrophysical Journal Pub Date: January 1994 DOI: 10.1086/173599 Bibcode: 1994ApJ...420..746S Keywords: Hydrogen Clouds; Interstellar Matter; Orion Nebula; Data Reduction; Emission Spectra; Fabry-Perot Spectrometers; Herbig-Haro Objects; Isotropic Media; Shock Waves; Astrophysics; ISM: INDIVIDUAL NAME: ORION NEBULA; ISM: JETS AND OUTFLOWS; ISM: MOLECULES; SHOCK WAVES full text sources ADS | data products SIMBAD (58)