Expansion Timescale Research Articles

Reassessing the Roles of PIN Proteins and Anticlinal Microtubules during Pavement Cell Morphogenesis.

The leaf epidermis is a biomechanical shell that influences the size and shape of the organ. Its morphogenesis is a multiscale process in which nanometer-scale cytoskeletal protein complexes, individual cells, and groups of cells pattern growth and define macroscopic leaf traits. Interdigitated growth of neighboring cells is an evolutionarily conserved developmental strategy. Understanding how signaling pathways and cytoskeletal proteins pattern cell walls during this form of tissue morphogenesis is an important research challenge. The cellular and molecular control of a lobed cell morphology is currently thought to involve PIN-FORMED (PIN)-type plasma membrane efflux carriers that generate subcellular auxin gradients. Auxin gradients were proposed to function across cell boundaries to encode stable offset patterns of cortical microtubules and actin filaments between adjacent cells. Many models suggest that long-lived microtubules along the anticlinal cell wall generate local cell wall heterogeneities that restrict local growth and specify the timing and location of lobe formation. Here, we used Arabidopsis (Arabidopsis thaliana) reverse genetics and multivariate long-term time-lapse imaging to test current cell shape control models. We found that neither PIN proteins nor long-lived microtubules along the anticlinal wall predict the patterns of lobe formation. In fields of lobing cells, anticlinal microtubules are not correlated with cell shape and are unstable at the time scales of cell expansion. Our analyses indicate that anticlinal microtubules have multiple functions in pavement cells and that lobe initiation is likely controlled by complex interactions among cell geometry, cell wall stress patterns, and transient microtubule networks that span the anticlinal and periclinal walls.

Fast ion acceleration in a foil plasma heated by a multi-picosecond high intensity laser

We study the one-dimensional expansion of a thin foil plasma irradiated by a high intensity laser with multi-picosecond (ps) pulse durations by using particle-in-cell simulation. Electrons are found to recirculate around the expanding plasma for many times, which results in stochastic heating leading to increase of the electron temperature in the multi-ps time scale beyond the ponderomotive scaling. The conventional isothermal model cannot describe such an expansion of plasmas in the long time scale. We here developed a non-isothermal plasma expansion theory that takes the time dependence of electron temperature into account for describing the multi-ps interactions in one-dimensional geometry. By assuming that the time scale of electron temperature evolution is slow compared with the plasma expansion time scale, we derived a non-self-similar solution. The time evolution of ion maximum energy obtained by the non-isothermal theory explains the details of that observed in the simulation.

Implications for Post-processing Nucleosynthesis of Core-collapse Supernova Models with Lagrangian Particles

Abstract We investigate core-collapse supernova (CCSN) nucleosynthesis with self-consistent, axisymmetric (2D) simulations performed using the neutrino hydrodynamics code Chimera. Computational costs have traditionally constrained the evolution of the nuclear composition within multidimensional CCSN models to, at best, a 14-species α-network capable of tracking only reactions from 4He to 60Zn. Such a simplified network limits the ability to accurately evolve detailed composition and neutronization or calculate the nuclear energy generation rate. Lagrangian tracer particles are commonly used to extend the nuclear network evolution by incorporating more realistic networks into post-processing nucleosynthesis calculations. However, limitations such as poor spatial resolution of the tracer particles; inconsistent thermodynamic evolution, including misestimation of expansion timescales; and uncertain determination of the multidimensional mass cut at the end of the simulation impose uncertainties inherent to this approach. We present a detailed analysis of the impact of such uncertainties for four self-consistent axisymmetric CCSN models initiated from solar-metallicity, nonrotating progenitors of 12, 15, 20, and 25 and evolved with the smaller α-network to more than 1 s after the launch of an explosion.

Effects of dimensionality on kinetic simulations of laser-ion acceleration in the transparency regime

A particle-in-cell study of laser-ion acceleration mechanisms in the transparency regime illustrates how two-dimensional (2D) S and P simulations (laser polarization in and out of the simulation plane, respectively) capture different physics characterizing these systems, visible in their entirety often in cost-prohibitive three-dimensional (3D) simulations. The electron momentum anisotropy induced in the target by a laser pulse is dramatically different in the two 2D cases, manifested in differences in target expansion timescales, electric field strengths, and density thresholds for the onset of relativistically induced transparency. In particular, 2D-P simulations exhibit dramatically greater electron heating in the simulation plane, whereas 2D-S ones show a much more isotropic energy distribution, similar to 3D. An ion trajectory analysis allows one to isolate the fields responsible for ion acceleration and to characterize the acceleration regimes in time and space. The artificial longitudinal electron heating in 2D-P exaggerates the effectiveness of target-normal sheath acceleration into its dominant acceleration mechanism throughout the laser-plasma interaction, whereas 2D-S and 3D both have sizable populations accelerated preferentially during transparency.

The dependence of Ammonium-Nitrate Fuel-Oil (ANFO) detonation on confinement

As detonation is a coupled fluid-chemical process, flow divergence inside the detonation reaction zone can strongly influence detonation velocity and energy release. Such divergence is responsible for the diameter-effect and failure-diameter phenomena in condensed-phase explosives and particularly dominant in detonation of nonideal explosives such as Ammonium Nitrate and Fuel Oil (ANFO). In this study, the effect of reaction zone flow divergence on ANFO detonation was explored through variation of the inert confinement and explosive diameter in the rate-stick geometry with cylinder expansion experiments. New tests are discussed and compared to prior experiments. Presented results include the detonation velocity as a function of diameter and confinement, reaction zone times, detonation product isentropes and energies, as well as sonic surface pressures and velocities. Product energy densities and isentropes were found to increase with detonation velocity, indicating more complete chemical reaction with increased detonation velocity. Detonation reaction zone times were found to scale with the acoustic transit time of the confiner wall and used to show that the ANFO diameter effect scaled with the reaction zone time for a particle along the flow centerline, regardless of the confinement. Such a result indicates that the ANFO reaction mechanisms are sufficiently slow that the centerline fluid expansion timescale is a limiting factor controlling detonation velocity and energy release.

PROTON HEATING BY PICK-UP ION DRIVEN CYCLOTRON WAVES IN THE OUTER HELIOSPHERE: HYBRID EXPANDING BOX SIMULATIONS

ABSTRACT Using a one-dimensional hybrid expanding box model, we investigate properties of the solar wind in the outer heliosphere. We assume a proton–electron plasma with a strictly transverse ambient magnetic field and, aside from the expansion, we take into account the influence of a continuous injection of cold pick-up protons through the charge-exchange process between the solar wind protons and hydrogen of interstellar origin. The injected cold pick-up protons form a ring distribution function, which rapidly becomes unstable, and generate Alfvén cyclotron waves. The Alfvén cyclotron waves scatter pick-up protons to a spherical shell distribution function that thickens over that time owing to the expansion-driven cooling. The Alfvén cyclotron waves heat solar wind protons in the perpendicular direction (with respect to the ambient magnetic field) through cyclotron resonance. At later times, the Alfvén cyclotron waves become parametrically unstable and the generated ion-acoustic waves heat protons in the parallel direction through Landau resonance. The resulting heating of the solar wind protons is efficient on the expansion timescale.

G345.45+1.50: an expanding ring-like structure with massive star formation

Ring-like structures in the ISM are commonly associated with high-mass stars. Kinematic studies of large structures in GMCs toward these ring-like structures may help us to understand how massive stars form. The origin and properties of the ring-like structure G345.45+1.50 is investigated through observations of the 13CO(3-2) line. The aim of the observations is to determine the kinematics in the region and to compare physical characteristics estimated from gas emission with those previously determined using dust continuum emission. The 13CO(3-2) line was mapped toward the whole ring using the APEX telescope. The ring is found to be expanding with a velocity of 1.0 km/s, containing a total mass of 6.9e3 Msun, which agrees well with that determined using 1.2 mm dust continuum emission. An expansion timescale of 3e6 yr and a total energy of 7e46 erg are estimated. The origin of the ring might have been a supernova explosion, since a 35.5 cm source, J165920-400424, is located at the center of the ring without an infrared counterpart. The ring is fragmented, and 104 clumps were identified with diameters of between 0.3 and 1.6 pc, masses of between 2.3 and 7.5e2 Msun, and densities of between 1.0e2 and 1.0e4 cm^-3. At least 18% of the clumps are forming stars, as is shown in infrared images. Assuming that the clumps can be modeled as Bonnor-Ebert spheres, 13 clumps are collapsing, and the rest of them are in hydrostatic equilibrium with an external pressure with a median value of 4e4 K cm^-3. In the region, the molecular outflow IRAS 16562-3959 is identified, with a velocity range of 38.4 km/s, total mass of 13 Msun, and kinematic energy of 7e45 erg. Finally, five filamentary structures were found at the edge of the ring with an average size of 3 pc, a width of 0.6 pc, a mass of 2e2 Msun, and a column density of 6e21 cm^-2.

Hubbleimaging of V1331 Cygni: proper motion study of its circumstellar structures

The young star V1331 Cyg received previous attention because it is surrounded by an optical, arc-like reflection nebula. V1331 Cyg is commonly considered to be a candidate for an object that has undergone an FU-Ori (FUOR) the outbreak in the past. This in turn could lead to a time-varying appearance of the dusty arcs that may be revealed by multi-epoch imaging. In particular, a radial colour analysis of the dust arcs can then be attempted to check whether radial grain size distribution was modified by a previous FUOR wind. Second-epoch imaging of V1331 Cyg was obtained by us in 2009 using the Hubble Space Telescope (HST). By comparing this to archival HST data from 2000, we studied the time evolution of the circumstellar nebulae. After a point spread function subtraction using model point spread functions, we used customised routines to perform a proper motion analysis. The nebula expansion was first derived by deconvolving and correlating the two-epoch radial brightness profiles. Additional data from other facilities TLS, UKIDSS, SPITZER, and HERSCHEL were also incorporated to improve our understanding of the star in terms of environment, viewing angle, bipolar outflow length, and the FUOR phenomenon. The derived radial colour profiles do not indicate a spatial separation of the dust grain sizes. The HERSCHEL 160 micron images show for the time thermal emission from dust probably residing in the outer arc. By viewing V1331 Cyg almost pole-on, the length of the bipolar outflow exceeds previous estimates by far. The outer arc expansion timescale is consistent with the implantation time of the CO torus, which supports the hypothesis of an outburst that occurred a few thousand years ago. The azimuthal colour variation of the outer arc is probably due to changes of the scattering angle, imposed by a tilt or helical geometry of the dust configuration.

Solar r-process-constrained actinide production in neutrino-driven winds of supernovae

Long-lived radioactive nuclei play an important role as nucleo-cosmochronometers and as cosmic tracers of nucleosynthetic source activity. In particular nuclei in the actinide region like thorium, uranium, and plutonium can testify to the enrichment of an environment by the still enigmatic astrophysical sources that are responsible for the production of neutron-rich nuclei by the rapid neutron-capture process (r-process). Supernovae and merging neutron-star (NS) or NS-black hole binaries are considered as most likely sources of the r-nuclei. But arguments in favour of one or the other or both are indirect and make use of assumptions; they are based on theoretical models with remaining simplifications and shortcomings. An unambiguous observational determination of a production event is still missing. In order to facilitate searches in this direction, e.g.\ by looking for radioactive tracers in stellar envelopes, the interstellar medium or terrestrial reservoirs, we provide improved theoretical estimates and corresponding uncertainty ranges for the actinide production ($^{232}$Th, $^{235,236,238}$U, $^{237}$Np, $^{244}$Pu, and $^{247}$Cm) in neutrino-driven winds of core-collapse supernovae. Since state-of-the-art supernova models do not yield r-process viable conditions ---but still lack, for example, the effects of strong magnetic fields--- we base our investigation on a simple analytical, Newtonian, adiabatic and steady-state wind model and consider the superposition of a large number of contributing components, whose nucleosynthesis-relevant parameters (mass weight, entropy, expansion time scale, and neutron excess) are constrained by the assumption that the integrated wind nucleosynthesis closely reproduces the solar system distribution of r-process elements. We also test the influence of uncertain nuclear physics.

SOLAR WIND COLLISIONAL AGE FROM A GLOBAL MAGNETOHYDRODYNAMICS SIMULATION

ABSTRACT Simple estimates of the number of Coulomb collisions experienced by the interplanetary plasma to the point of observation, i.e., the “collisional age”, can be usefully employed in the study of non-thermal features of the solar wind. Usually these estimates are based on local plasma properties at the point of observation. Here we improve the method of estimation of the collisional age by employing solutions obtained from global three-dimensional magnetohydrodynamics simulations. This enables evaluation of the complete analytical expression for the collisional age without using approximations. The improved estimation of the collisional timescale is compared with turbulence and expansion timescales to assess the relative importance of collisions. The collisional age computed using the approximate formula employed in previous work is compared with the improved simulation-based calculations to examine the validity of the simplified formula. We also develop an analytical expression for the evaluation of the collisional age and we find good agreement between the numerical and analytical results. Finally, we briefly discuss the implications for an improved estimation of collisionality along spacecraft trajectories, including Solar Probe Plus.

Intracellular Pressure Dynamics in Blebbing Cells

Blebs are pressure-driven protrusions that play an important role in cell migration, particularly in three-dimensional environments. A bleb is initiated when the cytoskeleton detaches from the cell membrane, resulting in the pressure-driven flow of cytosol toward the area of detachment and local expansion of the cell membrane. Recent experiments involving blebbing cells have led to conflicting hypotheses regarding the timescale of intracellular pressure propagation. The interpretation of one set of experiments supports a poroelastic model of the cytoplasm that leads to slow pressure equilibration when compared to the timescale of bleb expansion. A different study concludes that pressure equilibrates faster than the timescale of bleb expansion. To address this discrepancy, a dynamic computational model of the cell was developed that includes mechanics of and the interactions among the cytoplasm, the actin cortex, the cell membrane, and the cytoskeleton. The model results quantify the relationship among cytoplasmic rheology, pressure, and bleb expansion dynamics, and provide a more detailed picture of intracellular pressure dynamics. This study shows the elastic response of the cytoplasm relieves pressure and limits bleb size, and that both permeability and elasticity of the cytoplasm determine bleb expansion time. Our model with a poroelastic cytoplasm shows that pressure disturbances from bleb initiation propagate faster than the timescale of bleb expansion and that pressure equilibrates slower than the timescale of bleb expansion. The multiple timescales in intracellular pressure dynamics explain the apparent discrepancy in the interpretation of experimental results.

SMA observations of the W3(OH) complex: Dynamical differentiation between W3(H2O) and W3(OH)

We present Submillimeter Array (SMA) observations of the HCN\,(3--2) and HCO$^+$\,(3--2) molecular lines toward the W3(H$_2$O) and W3(OH) star-forming complexes. Infall and outflow motions in the W3(H$_2$O) have been characterized by observing HCN and HCO$^+$ transitions. High-velocity blue/red-shifted emission, tracing the outflow, show multiple knots, which might originate in episodic and precessing outflows. `Blue-peaked' line profiles indicate that gas is infalling onto the W3(H$_2$O) dust core. The measured large mass accretion rate, 2.3$\times$10$^{-3}$~M$_{\odot}$~yr$^{-1}$, together with the small free-fall time scale, 5$\times$10$^{3}$~yr, suggest W3(H$_2$O) is in an early evolutionary stage of the process of formation of high-mass stars. For the W3(OH), a two-layer model fit to the HCN and HCO$^+$ spectral lines and Spizter/IRAC images support that the W3(OH) H{\sc ii} region is expanding and interacting with the ambient gas, with the shocked neutral gas being expanding with an expansion timescale of 6.4$\times$10$^{3}$~yr. The observations suggest different kinematical timescales and dynamical states for the W3(H$_2$O) and W3(OH).

R-PROCESS LANTHANIDE PRODUCTION AND HEATING RATES IN KILONOVAE

r-Process nucleosynthesis in material ejected during neutron star mergers may lead to radioactively powered transients called kilonovae. The timescale and peak luminosity of these transients depend on the composition of the ejecta, which determines the local heating rate from nuclear decays and the opacity. Kasen et al. (2013, ApJ, 774, 25) and Tanaka & Hotokezaka (2013, ApJ, 775, 113) pointed out that lanthanides can drastically increase the opacity in these outflows. We use the new general-purpose nuclear reaction network SkyNet to carry out a parameter study of r-process nucleosynthesis for a range of initial electron fractions $Y_e$, initial specific entropies $s$, and expansion timescales $\tau$. We find that the ejecta is lanthanide-free for $Y_e \gtrsim 0.22 - 0.30$, depending on $s$ and $\tau$. The heating rate is insensitive to $s$ and $\tau$, but certain, larger values of $Y_e$ lead to reduced heating rates, due to individual nuclides dominating the heating. We calculate approximate light curves with a simplified gray radiative transport scheme. The light curves peak at about a day (week) in the lanthanide-free (-rich) cases. The heating rate does not change much as the ejecta becomes lanthanide-free with increasing $Y_e$, but the light curve peak becomes about an order of magnitude brighter because it peaks much earlier when the heating rate is larger. We also provide parametric fits for the heating rates between 0.1 and $100\,\text{days}$, and we provide a simple fit in $Y_e$, $s$, and $\tau$ to estimate whether the ejecta is lanthanide-rich or not.

Self-heterodyne detection of thein situphase of an atomic superconducting quantum interference device

We present theoretical and experimental analysis of an interferometric measurement of the {\it in-situ} phase drop across and current flow through a rotating barrier in a toroidal Bose-Einstein condensate (BEC). This experiment is the atomic analog of the rf-superconducting quantum interference device (SQUID). The phase drop is extracted from a spiral-shaped density profile created by the spatial interference of the expanding toroidal BEC and a reference BEC after release from all trapping potentials. We characterize the interferometer when it contains a single particle, which is initially in a coherent superposition of a torus and reference state, as well as when it contains a many-body state in the mean-field approximation. The single-particle picture is sufficient to explain the origin of the spirals, to relate the phase-drop across the barrier to the geometry of a spiral, and to bound the expansion times for which the {\it in-situ} phase can be accurately determined. Mean-field estimates and numerical simulations show that the inter-atomic interactions shorten the expansion time scales compared to the single-particle case. Finally, we compare the mean-field simulations with our experimental data and confirm that the interferometer indeed accurately measures the {\it in-situ} phase drop.

Expansion of a cold non-neutral plasma slab

Expansion of the ion and electron fronts of a cold non-neutral plasma slab with a quasi-neutral core bounded by layers containing only ions is investigated analytically and exact solutions are obtained. It is found that on average, the plasma expansion time scales linearly with the initial inverse ion plasma frequency as well as the degree of charge imbalance, and no expansion occurs if the cold plasma slab is stationary and overall neutral. However, in both cases, there can exist prominent oscillations on the electron front.

Growth-induced breaking and unbreaking of ergodicity in fully-connected spin systems

Two canonical models of statistical mechanics, the fully-connected voter and Glauber–Ising models, are modified to incorporate growth via the addition or replication of spins. The resulting behaviour is examined in a regime where the timescale of expansion cannot be separated from that of the internal dynamics. Depending on the model specification, growth radically alters the long-time dynamical behaviour by breaking or unbreaking ergodicity.

Simulating the effect of centrifugal forces in Jupiter's magnetosphere

AbstractJupiter's large scale size and rapid planetary rotation period combine to produce the strong centrifugal force responsible for many unique properties of its magnetosphere. It was previously proposed that this centrifugal force and nonadiabatic field line stretching could cause the observed dawn‐dusk asymmetry of Jupiter's plasma sheet, which is thickest near dusk. As flux tubes rotate and stretch between noon and dusk, particles bouncing along the field gain parallel energy and create pressure anisotropy. Because bounce times can be long compared with the outward expansion timescale, particles may respond nonadiabatically, and the resulting pressure anisotropy can drive the plasma sheet to instability. We used a large‐scale kinetic simulation to follow a collection of rotating particles as they move in a time‐varying, rotating magnetic field designed to represent flux tube expansion in Jupiter's magnetosphere. The analysis quantifies the response of trapped particles by characterizing the pressure anisotropy and energy changes. We compare results of nonadiabatic and adiabatic outward expansions and find that the nonadiabatic case leads to a large pitch angle anisotropy and higher total energy than for adiabatic expansion. Although the calculation was not handled fully self‐consistently, the results support the proposition that plasma pressure changes lead to changes in the magnetic field structure with local time. Our findings are consistent with the idea that nonadiabatic effects in Jupiter's magnetosphere contribute to field dipolarization and the observed plasma sheet thickening between noon and dusk.

Insights into Cytoplasmic Rheology Gained from Modeling Cellular Blebbing

Blebbing occurs when the cytoskeleton detaches from the cell membrane, resulting in the pressure-driven flow of cytosol towards the area of detachment and local expansion of the cell membrane. Recent experiments involving blebbing cells have led to conflicting hypotheses regarding the timescale of intracellular pressure propagation. The interpretation of one set of experiments supports a poroelastic cytoplasmic model which leads to slow pressure equilibration when compared to the timescale of bleb expansion. A different study concludes that pressure equilibrates faster than the timescale of bleb expansion. To address this, a dynamic computational model of the cell was developed that includes mechanics of and the interactions between the intracellular fluid, the actin cortex, the cell membrane, and the cytoskeleton. The model results quantify the relative importance of cytoskeletal elasticity and drag in bleb expansion dynamics. This study also shows that recent multi-bleb experimental results can be explained by the combination of cytoskeletal poroelasticity with either dynamic membrane-cortex adhesion or cortical reformation.

Evolution of antibaryon abundances in the early universe and in heavy-ion collisions

We study the kinetics of antibaryon production and annihilation in an expanding system, assuming that it is spatially homogeneous and chemically equilibrated at the initial stage. By solving simplified rate equations for (anti)baryon abundances we study the deviations from chemical equilibrium at late stages. The calculations are done for different expansion rates and net-baryon-to-entropy ratios, covering the conditions from early Universe to heavy-ion collisions. Our analysis includes both stable (anti)baryons and resonances. We conclude that residual antibaryon abundances are very sensitive to the time scales of expansion. Our calculations naturally explain noticeable deviations of antiproton-to-pion and proton-to-pion ratios observed in nuclear collisions at the LHC energy from the thermal model predictions. We conclude that at high bombarding energies the chemical freeze-out of (anti)baryons should occur at lower temperatures as compared to mesons.

THE EFFECT OF THE PRE-DETONATION STELLAR INTERNAL VELOCITY PROFILE ON THE NUCLEOSYNTHETIC YIELDS IN TYPE Ia SUPERNOVA

A common model of the explosion mechanism of Type Ia supernovae is based on a delayed detonation of a white dwarf. A variety of models differ primarily in the method by which the deflagration leads to a detonation. A common feature of the models, however, is that all of them involve the propagation of the detonation through a white dwarf that is either expanding or contracting, where the stellar internal velocity profile depends on both time and space. In this work, we investigate the effects of the pre-detonation stellar internal velocity profile and the post-detonation velocity of expansion on the production of alpha-particle nuclei, including Ni56, which are the primary nuclei produced by the detonation wave. We perform one-dimensional hydrodynamic simulations of the explosion phase of the white dwarf for center and off-center detonations with five different stellar velocity profiles at the onset of the detonation. We observe two distinct post-detonation expansion phases: rarefaction and bulk expansion. Almost all the burning to Ni56 occurs only in the rarefaction phase, and its expansion time scale is influenced by pre-existing flow structure in the star, in particular by the pre-detonation stellar velocity profile. We find that the mass fractions of the alpha-particle nuclei, including Ni56, are tight functions of the empirical physical parameter rho_up/v_down, where rho_up is the mass density immediately upstream of the detonation wave front and v_down is the velocity of the flow immediately downstream of the detonation wave front. We also find that v_down depends on the pre-detonation flow velocity. We conclude that the properties of the pre-existing flow, in particular the internal stellar velocity profile, influence the final isotopic composition of burned matter produced by the detonation.