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Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.

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At least 1,531 records · Page 85
All sourcesNASA NTRSDOE OSTI
DOE OSTI2026

Structured electrolytes facilitate Grotthuss-type transport for enhanced proton-coupled electron transfer reactions

Concentrated hydrogen-bonded electrolytes (CoHBEs) are structured, electrochemically stable, less-volatile alternatives to aqueous and dilute nonaqueous electrolytes, however, with high viscosities that limit molecular diffusion. This work provides an understanding of the proton conduction mechanism in CoHBEs based on mixtures of acids and azoles and establishes a link between the structurally dictated transport properties and the proton-coupled electron transfer (PCET) reaction rates that can be leveraged for enhancing electrochemical reactions. Diffusion and relaxation NMR studies suggest a breaking of the viscosity–conductivity tradeoff, where at high azole concentrations (>45 mol%), Grotthuss transport is more likely with lowered proton transfer energy barriers between the azole and the acid according to the machine learning (ML) accelerated ab initio path integral MD (AI-PIMD) simulations. Proton conduction pathways are found to be switchable between the hydrogen bonding networks of the acid and the azole, with imidazole chain forming structures better facilitating Grotthuss hopping. Supported by small-angle neutron scattering studies, the chains are found to have six member molecules on average with maximum of 3 to 4 imidazole/imidazoliums at 50 to 60 mol%. Despite their high viscosities, the measured PCET rates for quinones and phenazines measured in the protic CoHBEs present relatively high electron transfer rate constants (k 0 ~ 10 −4 cm/s), validated by rotating disc electrode and scanning electrochemical microscopy measurements. The results demonstrate that strategic tuning of hydrogen-bond donor–acceptor interactions enables the decoupling of proton transport and viscosity, thereby impacting PCET reactions.

electrokinetics↗
DOE OSTI2023

Data-Driven RANS Turbulence Closures for Forced Convection Flow in Reactor Downcomer Geometry

Recent progress in data-driven turbulence modeling has shown its potential to enhance or replace traditional equation-based Reynolds-averaged Navier-Stokes (RANS) turbulence models. Here, this work utilizes invariant neural network (NN) architectures to model Reynolds stresses and turbulent heat fluxes in forced convection flows (when the models can be decoupled). As the considered flow is statistically one dimensional, the invariant NN architecture for the Reynolds stress model reduces to the linear eddy viscosity model. To develop the data-driven models, direct numerical and RANS simulations in vertical planar channel geometry mimicking a part of the reactor downcomer are performed. Different conditions and fluids relevant to advanced reactors (sodium, lead, unitary-Prandtl-number fluid, and molten salt) constitute the training database. The models enabled accurate predictions of velocity and temperature, and compared to the baseline k–τ turbulence model with the simple gradient diffusion hypothesis, do not require tuning of the turbulent Prandtl number. The data-driven framework is implemented in the open-source graphics processing unit–accelerated spectral element solver nekRS and has shown the potential for future developments and consideration of more complex mixed convection flows.

21 SPECIFIC NUCLEAR REACTORS AND ASSOCIATED PLANTS↗
DOE OSTI2018

Limiting Surface Density Method Adapted to Large Arrays of Heterogeneous Shipping Packages with Nonlinear Responses

The classic Limiting Surface Density (LSD) method is an empirical calculation technique for analyzing and setting mass limits for fissile items in storage arrays. LSD is a desirable method because it can reduce or eliminate the need for lengthy detailed Monte Carlo models of storage arrays. The original (or classic) method was developed based on idealized arrays of bare spherical metal items in air-spaced cubic units in a water-reflected cubic array. In this case, the geometric and material-based surface densities were acceptably correlated by linear functions. Later updates to the method were made to allow for concrete reflection rather than water, cylindrical masses rather than spheres, different material forms, and noncubic arrays. However, in the intervening four decades since those updates, little work has been done to update the method, especially for use with contemporary highly heterogeneous shipping packages that are noncubic and stored in noncubic arrays. In this work, the LSD method is reevaluated for application to highly heterogeneous shipping packages for fissile material. The package modeled is the 9975 shipping package, currently the primary package used to store fissile material at Savannah River Site’s K-Area Complex. The package is neither cubic nor rectangular but resembles nested cylinders of stainless steel, lead, aluminum, and Celotex. The fissile content is assumed to be a cylinder of plutonium metal. The packages may be arranged in arrays with both an equal number of packages per side (package cubic) and an unequal number of packages per side (noncubic). The cubic arrangements are used to derive the 9975-specific material and geometry constants for the classic linear form LSD method. The linear form of the LSD, with noncubic array adjustment, is applied and evaluated against computational models for these packages to determine the critical unit fissile mass. Sensitivity equations are derived from the classic method, and these are also used to make projections of the critical unit fissile mass. It was discovered that the heterogeneous packages have a nonlinear surface density versus critical mass relationship compared to the acceptably linear response of bare spherical fissile masses. Methodology is developed to address the nonlinear response. In so doing, the solution to the nonlinear LSD method becomes decoupled from the critical mass of a single unit, adding to its flexibility. The ability of the method to predict changes in neutron multiplication due to perturbations in a parameter is examined to provide a basis for analyzing upset conditions. In conclusion, a full rederivation of the classic LSD method from diffusion theory is also included as this was found to be lacking in the available literature.

42 ENGINEERING↗
DOE OSTI2020

Nonlinear Elimination Applied to Radiation Diffusion

We apply a nonlinearly preconditioned, quasi-Newton framework to accelerate the numerical solution of the thermal radiative transfer (TRT) equations. This framework was inspired by the unpublished method that has existed for years in Teton, Lawrence Livermore National Laboratory’s deterministic TRT code. In this paper, we cast this iteration scheme within a formal nonlinear preconditioning framework and compare its performance against other iteration schemes in the framework. With proper choices of iteration controls for the various levels of the solver, we can recover the standard linearized one-step method, a full nonlinear Newton scheme, as well as the method in Teton. In brief, the nonlinear preconditioning TRT scheme formally eliminates the material temperature equation from the nonlinear system in a nonlinear analog of a Schur complement. This nonlinear elimination step involves solving a decoupled nonlinear equation for each spatial degree of freedom and is therefore inexpensive. By applying a quasi-Newton iteration scheme on the new system, we obtain a three-level iteration scheme that is at least as efficient as commonly used TRT schemes. The new method allows full convergence to the nonlinear backward Euler time-discretized system, increasing accuracy and robustness, while using a similar number of linear iterations as the more common linearized one-step methods Eq. (4).

77 NANOSCIENCE AND NANOTECHNOLOGY↗
DOE OSTI2017

Vacuum System and Modeling for the Materials Plasma Exposure Experiment

Understanding the science of plasma-material interactions (PMI) is essential for the future development of fusion facilities. The design of divertors and first walls for the next generation of long-pulse fusion facilities, such as a Fusion Nuclear Science Facility (FNSF) or a DEMO, requires significant PMI research and development. In order to meet this need, a new linear plasma facility, the Materials Plasma Exposure Experiment (MPEX) is proposed, which will produce divertor relevant plasma conditions for these next generation facilities. The device will be capable of handling low activation irradiated samples and be able to remove and replace samples without breaking vacuum. A Target Exchange Chamber (TEC) which can be disconnected from the high field environment in order to perform in-situ diagnostics is planned for the facility as well. The vacuum system for MPEX must be carefully designed in order to meet the requirements of the different heating systems, and to provide conditions at the target similar to those expected in a divertor. An automated coupling-decoupling (“autocoupler”) system is designed to create a high vacuum seal, and will allow the TEC to be disconnected without breaking vacuum in either the TEC or the primary plasma materials interaction chamber. This autocoupler, which can be actuated remotely in the presence of the high magnetic fields, has been designed and prototyped, and shows robustness in a variety of conditions. The vacuum system has been modeled using a simplified finite element analysis, and indicates that the design goals for the pressures in key regions of the facility are achievable.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2023

Final Design of the Material Plasma Exposure eXperiment

The Material Plasma Exposure eXperiment (MPEX) has completed its design phase. MPEX will be a unique facility to investigate plasma material interactions (PMIs) under fusion prototypic divertor conditions in steady state. This includes plasma exposure conditions expected in a fusion reactor divertor. Materials to be investigated will include solids, liquids, and neutron pre-irradiated materials. Electron and ion heating will allow for a large operational domain ranging from erosion-dominated PMI conditions to deposition-dominated PMI conditions. An overview of the final design for all MPEX systems is given. Here, in particular, it is shown how mission-driven project requirements have led to detailed design choices with innovative solutions. Examples are the water-cooled helicon window, the electron cyclotron heating launcher, the target holder and manipulator, and the autonomous decouplers.

Fusion reactor↗
DOE OSTI2015

VLBI IMAGING OF THE DOUBLE PEAKED EMISSION LINE SEYFERT KISSR 1494

We present here the results from dual-frequency phase-referenced Very Long Baseline Interferometry observations of the Seyfert galaxy KISSR 1494, which exhibits double peaked emission lines in its Sloan Digital Sky Survey spectrum. We detect a single radio component at 1.6 GHz, but not at 5 GHz, implying a spectral index steeper than –1.5 ± 0.5 (S {sub ν}∝ν{sup α}). The high brightness temperature of the radio component (∼1.4 × 10{sup 7} K) and the steep radio spectrum support a non-thermal synchrotron origin. A crude estimate of the black hole mass derived from the M {sub BH}-σ{sub *} relation is ∼1.4 ± 1.0 × 10{sup 8} M {sub ☉}; it is accreting at an Eddington rate of ∼0.02. The radio data are consistent with either the radio emission coming from the parsec-scale base of a synchrotron wind originating in the magnetized corona above the accretion disk, or from the inner ionized edge of the accretion disk or torus. In the former case, the narrow line region (NLR) clouds may form a part of the broad outflow, while in the latter, the NLR clouds may form a part of an extended disk beyond the torus. The radio and NLR emission may also be decoupled so that the radio emission originates in an outflow while the NLR is in a disk and vice versa. While with the present data it is not possible to clearly distinguish between these scenarios, there appears to be greater circumstantial evidence supporting the coronal wind picture in KISSR 1494. From the kiloparsec-scale radio emission, the time-averaged kinetic power of this outflow is estimated to be Q ≈ 1.5 × 10{sup 42} erg s{sup –1}, which is typical of radio outflows in low-luminosity active galactic nuclei. This supports the idea that radio ''jets'' and outflowing coronal winds are indistinguishable in Seyfert galaxies.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2015

STELLAR KINEMATICS AND STRUCTURAL PROPERTIES OF VIRGO CLUSTER DWARF EARLY-TYPE GALAXIES FROM THE SMAKCED PROJECT. III. ANGULAR MOMENTUM AND CONSTRAINTS ON FORMATION SCENARIOS

We analyze the stellar kinematics of 39 dwarf early-type galaxies (dEs) in the Virgo Cluster. Based on the specific stellar angular momentum λ{sub Re} and the ellipticity, we find 11 slow rotators and 28 fast rotators. The fast rotators in the outer parts of the Virgo Cluster rotate significantly faster than fast rotators in the inner parts of the cluster. Moreover, 10 out of the 11 slow rotators are located in the inner 3° (D < 1 Mpc) of the cluster. The fast rotators contain subtle disk-like structures that are visible in high-pass filtered optical images, while the slow rotators do not exhibit these structures. In addition, two of the dEs have kinematically decoupled cores and four more have emission partially filling in the Balmer absorption lines. These properties suggest that Virgo Cluster dEs may have originated from late-type star-forming galaxies that were transformed by the environment after their infall into the cluster. The correlation between λ{sub Re} and the clustercentric distance can be explained by a scenario where low luminosity star-forming galaxies fall into the cluster, their gas is rapidly removed by ram-pressure stripping, although some of it can be retained in their core, their star formation is quenched but their stellar kinematics are preserved. After a long time in the cluster and several passes through its center, the galaxies are heated up and transformed into slow rotating dEs.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2015

THE DYNAMICAL EVOLUTION OF STELLAR BLACK HOLES IN GLOBULAR CLUSTERS

Our current understanding of the stellar initial mass function and massive star evolution suggests that young globular clusters (GCs) may have formed hundreds to thousands of stellar-mass black holes (BHs), the remnants of stars with initial masses from ∼20-100 M {sub ☉}. Birth kicks from supernova explosions may eject some BHs from their birth clusters, but most should be retained. Using a Monte Carlo method we investigate the long-term dynamical evolution of GCs containing large numbers of stellar BHs. We describe numerical results for 42 models, covering a broad range of realistic initial conditions, including up to 1.6 × 10{sup 6} stars. In almost all models we find that significant numbers of BHs (up to ∼10{sup 3}) are retained all the way to the present. This is in contrast to previous theoretical expectations that most BHs should be ejected dynamically within a few gigayears The main reason for this difference is that core collapse driven by BHs (through the Spitzer {sup m}ass segregation instability{sup )} is easily reverted through three-body processes, and involves only a small number of the most massive BHs, while lower-mass BHs remain well-mixed with ordinary stars far from the central cusp. Thus the rapid segregation of stellar BHs does not lead to a long-term physical separation of most BHs into a dynamically decoupled inner core, as often assumed previously. Combined with the recent detections of several BH X-ray binary candidates in Galactic GCs, our results suggest that stellar BHs could still be present in large numbers in many GCs today, and that they may play a significant role in shaping the long-term dynamical evolution and the present-day dynamical structure of many clusters.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2015

Poynting-flux-dominated jets challenged by their photospheric emission

One of the key open question in the study of jets, in general, and jets in gamma-ray bursts (GRBs), in particular, is the magnetization of the outflow. Here we consider the photospheric emission of Poynting-flux-dominated outflows, when the dynamics is mediated by magnetic reconnection. We show that thermal three-particle processes, responsible for the thermalization of the plasma, become inefficient at a radius of r{sub sup}∼10{sup 9.5} cm, far below the photosphere, at ∼10{sup 11.5} cm. Conservation of the total photon number above r{sub sup} combined with Compton scattering below the photosphere enforces kinetic equilibrium between electrons and photons. This, in turn, leads to an increase in the observed photon temperature, which reaches ≳8 MeV (observed energy) when decoupling the plasma at the photosphere. This result is weakly dependent on the free model parameters. We show that in this case, the expected thermal luminosity is a few percent of the total luminosity, and could therefore be detected. The predicted peak energy is more than an order of magnitude higher than the observed peak energy of most GRBs, which puts strong constraints on the magnetization of these outflows.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2015

The impact of dust evolution and photoevaporation on disk dispersal

Protoplanetary disks are dispersed by viscous evolution and photoevaporation in a few million years; in the interim small, sub-micron-sized dust grains must grow and form planets. The time-varying abundance of small grains in an evolving disk directly affects gas heating by far-ultraviolet (FUV) photons, while dust evolution affects photoevaporation by changing the disk opacity and resulting penetration of FUV photons in the disk. Photoevaporative flows, in turn, selectively carry small dust grains, leaving the larger particles—which decouple, from the gas—behind in the disk. We study these effects by investigating the evolution of a disk subject to viscosity, photoevaporation by EUV, FUV, and X-rays, dust evolution, and radial drift using a one-dimensional (1D) multi-fluid approach (gas + different dust grain sizes) to solve for the evolving surface density distributions. The 1D evolution is augmented by 1+1D models constructed at each epoch to obtain the instantaneous disk structure and determine photoevaporation rates. The implementation of a dust coagulation/fragmentation model results in a marginal decrease in disk lifetimes when compared to models with no dust evolution; the disk lifetime is thus found to be relatively insensitive to the evolving dust opacity. We find that photoevaporation can cause significant reductions in the gas/dust mass ratio in the planet-forming regions of the disk as it evolves, and may result in a corresponding increase in heavy element abundances relative to hydrogen. We discuss implications for theories of planetesimal formation and giant planet formation, including the formation of gas-poor giants. After gas disk dispersal, ∼3×10{sup −4} M {sub ⊙} of mass in solids typically remain, comparable to the solids inventory of our solar system.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2015

ASYMMETRIC MAGNETIC RECONNECTION IN WEAKLY IONIZED CHROMOSPHERIC PLASMAS

Realistic models of magnetic reconnection in the solar chromosphere must take into account that the plasma is partially ionized and that plasma conditions within any two magnetic flux bundles undergoing reconnection may not be the same. Asymmetric reconnection in the chromosphere may occur when newly emerged flux interacts with pre-existing, overlying flux. We present 2.5D simulations of asymmetric reconnection in weakly ionized, reacting plasmas where the magnetic field strengths, ion and neutral densities, and temperatures are different in each upstream region. The plasma and neutral components are evolved separately to allow non-equilibrium ionization. As in previous simulations of chromospheric reconnection, the current sheet thins to the scale of the neutral–ion mean free path and the ion and neutral outflows are strongly coupled. However, the ion and neutral inflows are asymmetrically decoupled. In cases with magnetic asymmetry, a net flow of neutrals through the current sheet from the weak-field (high-density) upstream region into the strong-field upstream region results from a neutral pressure gradient. Consequently, neutrals dragged along with the outflow are more likely to originate from the weak-field region. The Hall effect leads to the development of a characteristic quadrupole magnetic field modified by asymmetry, but the X-point geometry expected during Hall reconnection does not occur. All simulations show the development of plasmoids after an initial laminar phase.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2015

CO–0.30–0.07: A peculiar molecular clump with an extremely broad velocity width in the central molecular zone of the Milky Way

The high velocity dispersion compact cloud CO–0.30–0.07 is a peculiar molecular clump discovered in the central molecular zone of the Milky Way, which is characterized by its extremely broad velocity emissions (∼145 km s{sup −1}) despite the absence of internal energy sources. We present new interferometric maps of the cloud in multiple molecular lines in frequency ranges of 265–269 GHz and 276–280 GHz obtained using the Submillimeter Array, along with the single-dish images previously obtained with the ASTE 10 m telescope. The data show that the characteristic broad velocity emissions are predominantly confined in two parallel ridges running through the cloud center. The central ridges are tightly anticorrelated with each other in both space and velocity, thereby sharply dividing the entire cloud into two distinct velocity components (+15 and +55 km s{sup −1}). This morphology is consistent with a model in which the two velocity components collide with a relative velocity of 40 km s{sup −1} at the interface defined by the central ridges, although an alternative explanation with a highly inclined expanding-ring model is yet to be fully invalidated. We have also unexpectedly detected several compact clumps (≲0.1 pc in radius) likely formed by shock compression. The clumps have several features in common with typical star-forming clouds: high densities (10{sup 6.5−−7.5} cm{sup −3}), rich abundances of hot-core-type molecular species, and relatively narrow velocity widths apparently decoupled from the furious turbulence dominating the cloud. The cloud CO–0.30–0.07 is possibly at an early phase of star formation activity triggered by the shock impact.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2015

The kinematic and chemical properties of a potential core-forming clump: Perseus B1-E

We present {sup 13}CO and C{sup 18}O (1-0), (2-1), and (3-2) maps toward the core-forming Perseus B1-E clump using observations from the James Clerk Maxwell Telescope, the Submillimeter Telescope of the Arizona Radio Observatory, and the IRAM 30 m telescope. We find that the {sup 13}CO and C{sup 18}O line emission both have very complex velocity structures, indicative of multiple velocity components within the ambient gas. The (1–0) transitions reveal a radial velocity gradient across B1-E of ∼1 km s{sup −1} pc{sup −1} that increases from northwest to southeast, whereas the majority of the Perseus cloud has a radial velocity gradient increasing from southwest to northeast. In contrast, we see no evidence of a velocity gradient associated with the denser Herschel-identified substructures in B1-E. Additionally, the denser substructures have much lower systemic motions than the ambient clump material, which indicates that they are likely decoupled from the large-scale gas. Nevertheless, these substructures themselves have broad line widths (∼0.4 km s{sup −1}) similar to that of the C{sup 18}O gas in the clump, which suggests they inherited their kinematic properties from the larger-scale, moderately dense gas. Finally, we find evidence of C{sup 18}O depletion only toward one substructure, B1-E2, which is also the only object with narrow (transonic) line widths. We suggest that as prestellar cores form, their chemical and kinematic properties are linked in evolution, such that these objects must first dissipate their turbulence before they deplete in CO.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2015

The physical nature of the cosmic accretion of baryons and dark matter into halos and their galaxies

The cosmic accretion of both dark matter and baryons into halos is typically measured using some evolving virial relation, but recent work suggests that most halo growth at late cosmic time (z≲2) is not physical but is rather the by-product of an evolving virial radius (“pseudo-evolution”). Using Omega25, a suite of cosmological simulations that incorporate both dark matter and gas dynamics with differing treatments of gas cooling, star formation, and thermal feedback, we systematically explore the physics that governs cosmic accretion into halos and their galaxies. Physically meaningful cosmic accretion of both dark matter and baryons occurs at z≳1 across our halo mass range: M{sub 200m}=10{sup 11−14} M{sub ⊙}. However, dark matter, because it is dissipationless, is deposited (in a time-average sense) at ≳R{sub 200m}(z) in a shell-like manner, such that dark matter mass and density experience little-to-no physical growth at any radius within a halo at z<1. In contrast, gas, because it is able to cool radiatively, experiences significant accretion at all radii, at a rate that roughly tracks the accretion rate at R{sub 200m}, at all redshifts. Infalling gas starts to decouple from dark matter at ≈2 R{sub 200m} and continues to accrete to smaller radii until the onset of strong angular-momentum support at ≈0.1 R{sub 200m}. Thus, while the growth of dark matter is subject to pseudo-evolution, the growth of baryons is not. The fact that the accretion rate of gas on galactic scales tracks the accretion rate near R{sub 200m} provides insight into the tight relations between the masses/sizes of galaxies and those of their host halos across cosmic time.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2015

Plasma physical parameters along CME-driven shocks. II. Observation–simulation comparison

In this work, we compare the spatial distribution of the plasma parameters along the 1999 June 11 coronal mass ejection (CME)-driven shock front with the results obtained from a CME-like event simulated with the FLIPMHD3D code, based on the FLIP-MHD particle-in-cell method. The observational data are retrieved from the combination of white-light coronagraphic data (for the upstream values) and the application of the Rankine–Hugoniot equations (for the downstream values). The comparison shows a higher compression ratio X and Alfvénic Mach number M{sub A} at the shock nose, and a stronger magnetic field deflection d toward the flanks, in agreement with observations. Then, we compare the spatial distribution of M{sub A} with the profiles obtained from the solutions of the shock adiabatic equation relating M{sub A}, X, and θ{sub Bn} (the angle between the upstream magnetic field and the shock front normal) for the special cases of parallel and perpendicular shock, and with a semi-empirical expression for a generically oblique shock. The semi-empirical curve approximates the actual values of M{sub A} very well, if the effects of a non-negligible shock thickness δ{sub sh} and plasma-to magnetic pressure ratio β{sub u} are taken into account throughout the computation. Moreover, the simulated shock turns out to be supercritical at the nose and sub-critical at the flanks. Finally, we develop a new one-dimensional Lagrangian ideal MHD method based on the GrAALE code, to simulate the ion-electron temperature decoupling due to the shock transit. Two models are used, a simple solar wind model and a variable-γ model. Both produce results in agreement with observations, the second one being capable of introducing the physics responsible for the additional electron heating due to secondary effects (collisions, Alfvén waves, etc.).

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2015

Spin and magnetism of white dwarfs

The magnetism and rotation of white dwarf (WD) stars are investigated in relation to a hydromagnetic dynamo operating in the progenitor during shell burning phases. The downward pumping of angular momentum in the convective envelope, in combination with the absorption of a planet or tidal spin-up from a binary companion, can trigger strong dynamo action near the core–envelope boundary. Several arguments point to the outer core as the source for a magnetic field in the WD remnant: the outer third of a ∼0.55 M{sub ⊙} WD is processed during the shell burning phase(s) of the progenitor; the escape of magnetic helicity through the envelope mediates the growth of (compensating) helicity in the core, as is needed to maintain a stable magnetic field in the remnant; and the intense radiation flux at the core boundary facilitates magnetic buoyancy within a relatively thick tachocline layer. The helicity flux into the growing core is driven by a dynamical imbalance with a latitude-dependent rotational stress. The magnetic field deposited in an isolated massive WD is concentrated in an outer shell of mass ≲0.1 M{sub ⊙} and can reach ∼10 MG. A buried toroidal field experiences moderate ohmic decay above an age ∼0.3 Gyr, which may lead to growth or decay of the external magnetic field. The final WD spin period is related to a critical spin rate below which magnetic activity shuts off and core and envelope decouple; it generally sits in the range of hours to days. WD periods ranging up to a year are possible if the envelope re-expands following a late thermal pulse.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2015

XMM-Newton observations of SGR 1806–20 over seven years following the 2004 giant flare

We report on the study of 14 XMM-Newton observations of the magnetar SGR 1806–20 spread over a period of 8 years, starting in 2003 and extending to 2011. We find that in mid 2005, a year and a half after a giant flare (GF), the torques on the star increased to the largest value yet seen, with a long term average rate between 2005 and 2011 of | ν-dot |≈1.35×10{sup −11} Hz s{sup −1}, an order of magnitude larger than its historical level measured in 1995. The pulse morphology of the source is complex in the observations following the GF, while its pulsed-fraction remained constant at about 7% in all observations. Spectrally, the combination of a blackbody (BB) and power-law (PL) components is an excellent fit to all observations. The BB and PL fluxes increased by a factor of 2.5 and 4, respectively, while the spectra hardened, in concordance with the 2004 major outburst that preceded the GF. The fluxes decayed exponentially back to quiescence with a characteristic timescale of τ∼1.5 years, although they did not reach a constant value until at least 3.5 years later (2009). The long-term timing and spectral behavior of the source point to a decoupling between the mechanisms responsible for their respective behavior. We argue that low level seismic activity causing small twists in the open field lines can explain the long lasting large torques on the star, while the spectral behavior is due to a twist imparted onto closed field lines after the 2004 large outburst.

79 ASTRONOMY AND ASTROPHYSICS↗