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DOE OSTI2026

Development of MOSCATO: A CFD-Level Electrochemistry and Corrosion Simulator for Molten Salt Systems

For both coolant and fueled variants of molten salt reactors (MSRs), the corrosion of structural materials is a significant challenge. The corrosion stems from chemical and electrochemical reactions initiated by fissile material, fission products, and impurities in the salt. Lower-fidelity models rely on empirical correlations for mass transfer, simplified lumped temperature profiles, and similar assumptions. They do not capture detailed spatial variations in complex geometries, creating the need for high-fidelity modeling to bridge this gap.As we approach the demonstration and possible deployment of MSRs in this decade, the development of a high-fidelity, high-performance simulator becomes imperative. To simulate the complex electrochemical environment and corrosion within molten salt systems, we have developed the Molten Salt Chemistry And TranspOrt (MOSCATO) code. This endeavor is comprised of three essential components. First, mass transfer equations are coupled with the Navier-Stokes equations in order to account for the transport of species in the salt. Second, the diffusion of alloy constituents, such as Cr, Fe, Ni, etc. is simulated within the structural metals. Third, the alloy and salt domains are coupled to account for the heterogeneous chemical and electrochemical reactions that occur at the salt-alloy interface.MOSCATO manages all three components within the framework of the highly scalable, open-source spectral element method computational fluid dynamics code Nek5000/NekRS. This integration enables MOSCATO to harness the immense computational power of modern high-performance computing resources, ensuring both high fidelity and computational speed.In addition to code development, we have initiated a comprehensive verification and validation campaign, utilizing data from diverse sources. First, MOSCATO's electrochemical solver was verified with reference numerical data. Then validation occurred against experiments: one of a thermal galvanic cell and the other for corrosion in flowing molten salt of FLiNaK (LiF-NaF-KF). This campaign verified and validated MOSCATO as a reliable tool for simulating electrochemical environments and corrosion in molten salt systems.

22 GENERAL STUDIES OF NUCLEAR REACTORS↗
DOE OSTI2025

Thermal-Fluid and Thermal-Structural Response of the T-Tube Modular Divertor to Spatiotemporally Varying Heat Loads

Tungsten (W) is the leading candidate for divertor target plates because of its high melting point (>3000°C), thermal conductivity, and ultimate tensile stress. While W and its alloys are the only solid materials that can survive the high heat fluxes incident on the divertor, W’s low-ductility high ductile-to-brittle transition temperature of ~600°C and relatively low recrystallization temperature (RT) of ~1300°C pose structural (among other) challenges. The objective of this work is to estimate the thermal-fluid and thermal-structural performance of the helium (He)-cooled T-tube divertor, which was originally developed by the Advanced Reactor Innovation and Evaluation Study (ARIES) using numerical simulations. Here, predictions of temperature distributions across the plasma-facing structural component and surface pressures from computational fluid dynamics simulations are used to determine stress distributions using commercial structural finite element modeling software over a range of fusion-relevant conditions. The maximum allowable incident heat fluxes are determined based on the temperature limits imposed by the ITER elastic Structural Design Criteria for In-vessel Components (SDC-IC) and the maximum RT over a range of He mass flow rates and presented in the form of performance design charts. Our recent work found that thermal- structural criteria accounting for the low ductility of W in a finger-type modular divertor constrain the maximum incident heat fluxes to values well below the ITER specifications, and those based on considering only the RT demonstrate that integrated thermal-fluid and elastic structural performance evaluation are required for accurate assessment of divertor performance. This novel analysis of the T-tube considers how nonuniform and transient incident heat fluxes affect its thermal-fluid and thermal-structural performance, as well as the effect of volumetric heating, which can be as great as 27% of the power incident on the divertor surface. The W tile of the T-tube, with its relatively large plasma-facing area of ~15 cm 2 , will likely experience significant spatial variations in incident heat flux. This work therefore assesses whether steady-state incident heat flux profiles with a peak of 10 MW/m 2 and maximum heat flux gradients of 200 MW/m 2 per m exceed the structural limits imposed by the ITER elastic SDC-IC and the maximum RT over a range of fusion-relevant conditions. The effect of transient heat fluxes typical of plasma detachment and reattachment from the target plate due, for example, to gas injection are also evaluated

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2015

PANCHROMATIC HUBBLE ANDROMEDA TREASURY. XII. MAPPING STELLAR METALLICITY DISTRIBUTIONS IN M31

We present a study of spatial variations in the metallicity of old red giant branch stars in the Andromeda galaxy. Photometric metallicity estimates are derived by interpolating isochrones for over seven million stars in the Panchromatic Hubble Andromeda Treasury (PHAT) survey. This is the first systematic study of stellar metallicities over the inner 20 kpc of Andromeda’s galactic disk. We see a clear metallicity gradient of −0.020 ± 0.004 dex kpc{sup −1} from ∼4–20 kpc assuming a constant red giant branch age. This metallicity gradient is derived after correcting for the effects of photometric bias and completeness and dust extinction, and is quite insensitive to these effects. The unknown age gradient in M31's disk creates the dominant systematic uncertainty in our derived metallicity gradient. However, spectroscopic analyses of galaxies similar to M31 show that they typically have small age gradients that make this systematic error comparable to the 1σ error on our metallicity gradient measurement. In addition to the metallicity gradient, we observe an asymmetric local enhancement in metallicity at radii of 3–6 kpc that appears to be associated with Andromeda’s elongated bar. This same region also appears to have an enhanced stellar density and velocity dispersion.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2011

THE ASYMMETRY OF SUNSPOT CYCLES AND WALDMEIER RELATIONS AS A RESULT OF NONLINEAR SURFACE-SHEAR SHAPED DYNAMO

The paper presents a study of a solar dynamo model operating in the bulk of the convection zone with the toroidal magnetic field flux concentrated in the subsurface rotational shear layer. We explore how this type of dynamo may depend on spatial variations of turbulent parameters and on the differential rotation near the surface. The mean-field dynamo model takes into account the evolution of magnetic helicity and describes its nonlinear feedback on the generation of large-scale magnetic field by the {alpha}-effect. We compare the magnetic cycle characteristics predicted by the model, including the cycle asymmetry (associated with the growth and decay times) and the duration-amplitude relation (Waldmeier's effects), with the observed sunspot cycle properties. We show that the model qualitatively reproduces the basic properties of the solar cycles.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2011

MULTI-ZONE MODELING OF THE PULSAR WIND NEBULA HESS J1825-137

The pulsar wind nebula associated with PSR J1826-1334, HESS J1825-137, is a bright very high energy (VHE) source with an angular extent of {approx}1 Degree-Sign and spatially resolved spectroscopic TeV measurements. The gamma-ray spectral index is observed to soften with increasing distance from the pulsar, likely the result of cooling losses as electrons traverse the nebula. We describe analysis of X-ray data of the extended nebula, as well as three-dimensional time-dependent spectral energy distribution modeling, with emphasis on the spatial variations within HESS J1825-137. The multi-wavelength data place significant constraints on electron injection, transport, and cooling within the nebula. The large size and high nebular energy budget imply a relatively rapid initial pulsar spin period of 13 {+-} 7 ms and an age of 40 {+-} 9 kyr. The relative fluxes of each VHE zone can be explained by advective particle transport with a radially decreasing velocity profile with v(r){proportional_to}r{sup -0.5}. The evolution of the cooling break requires an evolving magnetic field which also decreases radially from the pulsar, B(r,t){proportional_to}r{sup -0.7} E-dot (t){sup 1/2}. Detection of 10 TeV flux {approx}80 pc from the pulsar requires rapid diffusion of high-energy particles, contrary to the common assumption of toroidal magnetic fields with strong magnetic confinement. The model predicts a rather uniform Fermi Large Area Telescope (LAT) surface brightness out to {approx}1 Degree-Sign from the pulsar, in good agreement with the recently discovered LAT source centered 0.{sup 0}5 southwest of PSR J1826-1334 with extension 0.{sup 0}6 {+-} 0.{sup 0}1.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2012

THE ROLE OF DRAG IN THE ENERGETICS OF STRONGLY FORCED EXOPLANET ATMOSPHERES

In contrast to the Earth, where frictional heating is typically negligible, we show that drag mechanisms could act as an important heat source in the strongly forced atmospheres of some exoplanets, with the potential to alter the circulation. We modify the standard formalism of the atmospheric energy cycle to explicitly track the loss of kinetic energy and the associated frictional (re)heating, for application to exoplanets such as the asymmetrically heated 'hot Jupiters' and gas giants on highly eccentric orbits. We establish that an understanding of the dominant drag mechanisms and their dependence on local atmospheric conditions is critical for accurate modeling, not just in their ability to limit wind speeds, but also because they could possibly change the energetics of the circulation enough to alter the nature of the flow. We discuss possible sources of drag and estimate the strength necessary to significantly influence the atmospheric energetics. As we show, the frictional heating depends on the magnitude of kinetic energy dissipation as well as its spatial variation, so that the more localized a drag mechanism is, the weaker it can be and still affect the circulation. We also use the derived formalism to estimate the rate of numerical loss of kinetic energy in a few previously published hot Jupiter models with and without magnetic drag and find it to be surprisingly large, at 5%-10% of the incident stellar irradiation.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2012

VELOCITY POWER SPECTRA FROM CROSS-FIELD TURBULENCE IN THE PROTON KINETIC REGIME

Numerical hybrid simulations with particle protons and fluid electrons are conducted for turbulent fluctuations with spatial variations in a plane perpendicular to the background magnetic field. In the turbulent phase, the proton bulk velocity spectrum has a dissipation range starting at a smaller wavenumber than the magnetic spectrum dissipation range. The steepened portion of the proton bulk velocity spectrum is constrained to a smaller wavenumber with an increasing ratio of background proton plasma to magnetic pressure {beta}{sub p}. The form of the magnetic spectrum does not depend on {beta}{sub p}. The collisionless proton and fluctuation interaction which heats protons mainly across the magnetic field is deemed to be the result of a viscous-like interaction based, in part, on the dependence of the velocity spectrum on {beta}{sub p}.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2012

A HIGH-RESOLUTION STUDY OF THE H I-H{sub 2} TRANSITION ACROSS THE PERSEUS MOLECULAR CLOUD

To investigate the fundamental principles of H{sub 2} formation in a giant molecular cloud, we derive the H I and H{sub 2} surface density ({Sigma}{sub HI} and {Sigma}{sub H2}) images of the Perseus molecular cloud on sub-pc scales ({approx}0.4 pc). We use the far-infrared data from the Improved Reprocessing of the IRAS Survey and the V-band extinction image provided by the COMPLETE Survey to estimate the dust column density image of Perseus. In combination with the H I data from the Galactic Arecibo L-band Feed Array H I Survey and an estimate of the local dust-to-gas ratio, we then derive the {Sigma}{sub H2} distribution across Perseus. We find a relatively uniform {Sigma}{sub HI}{approx} 6-8 M{sub Sun} pc{sup -2} for both dark and star-forming regions, suggesting a minimum H I surface density required to shield H{sub 2} against photodissociation. As a result, a remarkably tight and consistent relation is found between {Sigma}{sub H2}/{Sigma}{sub HI} and {Sigma}{sub HI} + {Sigma}{sub H2}. The transition between the H I- and H{sub 2}-dominated regions occurs at N(H I) + 2N(H{sub 2}) {approx} (8-14) Multiplication-Sign 10{sup 20} cm{sup -2}. Our findings are consistent with predictions for H{sub 2} formation in equilibrium, suggesting that turbulence may not be of primary importance for H{sub 2} formation. However, the importance of a warm neutral medium for H{sub 2} shielding, an internal radiation field, and the timescale of H{sub 2} formation still remain as open questions. We also compare H{sub 2} and CO distributions and estimate the fraction of 'CO-dark' gas, f{sub DG} {approx} 0.3. While significant spatial variations of f{sub DG} are found, we do not find a clear correlation with the mean V-band extinction.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2012

GAS METALLICITIES IN THE EXTENDED DISKS OF NGC 1512 AND NGC 3621. CHEMICAL SIGNATURES OF METAL MIXING OR ENRICHED GAS ACCRETION?

We have obtained spectra of 135 H II regions located in the inner and extended disks of the spiral galaxies NGC 1512 and NGC 3621, spanning the range of galactocentric distances 0.2-2 Multiplication-Sign R{sub 25} (from {approx}2-3 kpc to {approx}18-25 kpc). We find that the excitation properties of nebulae in the outer (R > R{sub 25}) disks are similar to those of the inner disks, but on average younger H II regions tend to be selected in the bright inner disks. Reddening by dust is not negligible in the outer disks and subject to significant large-scale spatial variations. For both galaxies, the radial abundance gradient flattens to a constant value outside of the isophotal radius. The outer disk O/H abundance ratio is highly homogeneous, with a scatter of only {approx}0.06 dex. In the case of the interacting galaxy NGC 1512 we find a number of H II regions with peculiar metallicity for their radius, a result which can be interpreted by gas flows activated by the gravitational encounter with NGC 1510. Based on the excitation and chemical (N/O ratio) analysis, we find no compelling evidence for variations in the upper initial mass function of ionizing clusters of extended disks. The O/H abundance in the outer disks of the target galaxies corresponds to {approx}35% of the solar value (or higher, depending on the metallicity diagnostic). This agrees with our earlier measurements in M83 and NGC 4625, and conflicts with the notion that metallicities in extended disks of spiral galaxies are low and on the order of {approx}0.1 Multiplication-Sign Z{sub Sun }. We show that, in general, the observed metal enrichment cannot be produced with the current level of star formation, even if the latter extends over a Hubble time. We discuss the possibility that metal transport mechanisms from the inner disks lead to metal pollution of the outer disks. Gas accretion from the intergalactic medium, enriched by outflows, offers an alternative solution, justified within the framework of hydrodynamic simulations of galaxy evolution. Specific model predictions of the chemical enrichment and the flat gradients in extended disks of nearby galaxies will be valuable to discriminate between these different scenarios.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2012

SPITZER IMAGING OF THE NEARBY RICH YOUNG CLUSTER, Cep OB3b

We map the full extent of a rich massive young cluster in the Cep OB3b association with the Infrared Array Camera and Multi-band Imaging Photometer System instruments aboard the Spitzer Space Telescope and the ACIS instrument aboard the Chandra X-Ray Observatory. At 700 pc, it is revealed to be the second nearest large (>1000 member), young (<5 Myr) cluster known. In contrast to the nearest large cluster, the Orion Nebula Cluster, Cep OB3b is only lightly obscured and is mostly located in a large cavity carved out of the surrounding molecular cloud. Our infrared and X-ray data sets, as well as visible photometry from the literature, are used to take a census of the young stars in Cep OB3b. We find that the young stars within the cluster are concentrated in two sub-clusters; an eastern sub-cluster, near the Cep B molecular clump, and a western sub-cluster, near the Cep F molecular clump. Using our census of young stars, we examine the fraction of young stars with infrared excesses indicative of circumstellar disks. We create a map of the disk fraction throughout the cluster and find that it is spatially variable. Due to these spatial variations, the two sub-clusters exhibit substantially different average disk fractions from each other: 32% {+-} 4% and 50% {+-} 6%. We discuss whether the discrepant disk fractions are due to the photodestruction of disks by the high mass members of the cluster or whether they result from differences in the ages of the sub-clusters. We conclude that the discrepant disk fractions are most likely due to differences in the ages.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2012

A STUDY OF HEATING AND COOLING OF THE ISM IN NGC 1097 WITH HERSCHEL-PACS AND SPITZER-IRS

NGC 1097 is a nearby Seyfert 1 galaxy with a bright circumnuclear starburst ring, a strong large-scale bar, and an active nucleus. We present a detailed study of the spatial variation of the far-infrared (FIR) [C II]158 {mu}m and [O I]63 {mu}m lines and mid-infrared H{sub 2} emission lines as tracers of gas cooling, and of the polycyclic aromatic hydrocarbon (PAH) bands as tracers of the photoelectric heating, using Herschel-PACS and Spitzer-IRS infrared spectral maps. We focus on the nucleus and the ring, and two star-forming regions (Enuc N and Enuc S). We estimated a photoelectric gas heating efficiency ([C II]158 {mu}m+[O I]63 {mu}m)/PAH in the ring about 50% lower than in Enuc N and S. The average 11.3/7.7 {mu}m PAH ratio is also lower in the ring, which may suggest a larger fraction of ionized PAHs, but no clear correlation with [C II]158 {mu}m/PAH(5.5-14 {mu}m) is found. PAHs in the ring are responsible for a factor of two more [C II]158 {mu}m and [O I]63 {mu}m emission per unit mass than PAHs in the Enuc S. spectral energy distribution (SED) modeling indicates that at most 25% of the FIR power in the ring and Enuc S can come from high-intensity photodissociation regions (PDRs), in which case G{sub 0} {approx} 10{sup 2.3} and n{sub H} {approx} 10{sup 3.5} cm{sup -3} in the ring. For these values of G{sub 0} and n{sub H}, PDR models cannot reproduce the observed H{sub 2} emission. Much of the H{sub 2} emission in the starburst ring could come from warm regions in the diffuse interstellar medium that are heated by turbulent dissipation or shocks.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2012

THE COSMOLOGICAL IMPACT OF LUMINOUS TeV BLAZARS. II. REWRITING THE THERMAL HISTORY OF THE INTERGALACTIC MEDIUM

The universe is opaque to extragalactic very high energy gamma rays (VHEGRs, E > 100 GeV) because they annihilate and pair produce on the extragalactic background light. The resulting ultrarelativistic pairs are commonly assumed to lose energy primarily through inverse Compton scattering of cosmic microwave background (CMB) photons, reprocessing the original emission from TeV to GeV energies. In Broderick et al., we argued that this is not the case; powerful plasma instabilities driven by the highly anisotropic nature of the ultrarelativistic pair distribution provide a plausible way to dissipate the kinetic energy of the TeV-generated pairs locally, heating the intergalactic medium (IGM). Here, we explore the effect of this heating on the thermal history of the IGM. We collate the observed extragalactic VHEGR sources to determine a local VHEGR heating rate. Given the pointed nature of VHEGR observations, we estimate the correction for the various selection effects using Fermi observations of high- and intermediate-peaked BL Lac objects. As the extragalactic component of the local VHEGR flux is dominated by TeV blazars, we then estimate the evolution of the TeV blazar luminosity density by tying it to the well-observed quasar luminosity density and producing a VHEGR heating rate as a function of redshift. This heating is relatively homogeneous for z {approx}< 4, but there is greater spatial variation at higher redshift (order unity at z {approx} 6) because of the reduced number of blazars that contribute to local heating. We show that this new heating process dominates photoheating in the low-redshift evolution of the IGM and calculate the effect of this heating in a one-zone model. As a consequence, the inclusion of TeV blazar heating qualitatively and quantitatively changes the structure and history of the IGM. Due to the homogeneous nature of the extragalactic background light, TeV blazars produce a uniform volumetric heating rate. This heating is sufficient to increase the temperature of the mean density IGM by nearly an order of magnitude, and at low densities by substantially more. It also naturally produces the inverted temperature-density relation inferred by recent observations of the high-redshift Ly{alpha} forest, a feature that is difficult to reconcile with standard reionization models. Finally, we close with a discussion on the possibility of detecting this hot low-density IGM suggested by our model either directly or indirectly via the local Ly{alpha} forest, the Comptonized CMB, or free-free emission, but we find that such measurements are currently not feasible.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2012

ANALYSIS OF WMAP 7 YEAR TEMPERATURE DATA: ASTROPHYSICS OF THE GALACTIC HAZE

We perform a joint analysis of the cosmic microwave background (CMB) and Galactic emission from the WMAP 7 year temperature data. Using the Commander code, based on Gibbs sampling, we simultaneously derive the CMB and Galactic components on scales larger than 1 Degree-Sign with improved sensitivity over previous work. We conduct a detailed study of the low-frequency Galactic foreground, focusing on the 'microwave haze' emission around the Galactic center. We demonstrate improved performance in quantifying the diffuse Galactic emission when including Haslam 408 MHz data and when jointly modeling the spinning and thermal dust emission. We examine whether the hypothetical Galactic haze can be explained by a spatial variation of the synchrotron spectral index, and find that the excess of emission around the Galactic center is stable with respect to variations of the foreground model. Our results demonstrate that the new Galactic foreground component-the microwave haze-is indeed present.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2012

SPATIALLY RESOLVED CHEMISTRY IN NEARBY GALAXIES. II. THE NUCLEAR BAR IN MAFFEI 2

We present 2''-10'' imaging of 11 transitions from 9 molecular species across the nuclear bar in Maffei 2. The data were obtained with the BIMA and OVRO interferometers. The 10 detected transitions are compared with existing CO isotopologues, HCN, CS, and millimeter continuum data. Dramatic spatial variations among the mapped species are observed across the nuclear bar. A principal component analysis is performed to characterize correlations between the transitions, star formation, and molecular column density. The analysis reveals that HCN, HNC, HCO{sup +}, and 3 mm continuum are tightly correlated, indicating a direct connection to massive star formation. We find two main morphologically distinct chemical groups, CH{sub 3}OH, SiO, and HNCO comprising the grain chemistry molecules, versus HCN, HNC, HCO{sup +}, and C{sub 2}H, molecules strong in the presence of star formation. The grain chemistry molecules, HNCO, CH{sub 3}OH, and SiO, trace hydrodynamical bar shocks. The near constancy of the HNCO/CH{sub 3}OH, SiO/CH{sub 3}OH, and SiO/HNCO ratios argues that shock properties are uniform across the nucleus. HCN/HCO{sup +}, HCN/HNC, HCN/CS, and HCN/CO ratios are explained primarily by variations in density. High HCO{sup +}/N{sub 2}H{sup +} ratios are correlated with the C{sub 2}H line, suggesting that this ratio may be a powerful new dense photon-dominated region probe in external galaxies. C{sub 2}H reveals a molecular outflow along the minor axis. The morphology and kinematics of the outflow are consistent with an outflow age of 6-7 Myr.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2013

CHANGES OF DUST OPACITY WITH DENSITY IN THE ORION A MOLECULAR CLOUD

We have studied the opacity of dust grains at submillimeter wavelengths by estimating the optical depth from imaging at 160, 250, 350, and 500 {mu}m from the Herschel Gould Belt Survey and comparing this to a column density obtained from the Two Micron All Sky Survey derived color excess E(J - K {sub s}). Our main goal was to investigate the spatial variations of the opacity due to 'big' grains over a variety of environmental conditions and thereby quantify how emission properties of the dust change with column (and volume) density. The central and southern areas of the Orion A molecular cloud examined here, with N {sub H} ranging from 1.5 Multiplication-Sign 10{sup 21} cm{sup -2} to 50 Multiplication-Sign 10{sup 21} cm{sup -2}, are well suited to this approach. We fit the multi-frequency Herschel spectral energy distributions (SEDs) of each pixel with a modified blackbody to obtain the temperature, T, and optical depth, {tau}{sub 1200}, at a fiducial frequency of 1200 GHz (250 {mu}m). Using a calibration of N {sub H}/E(J - K{sub s} ) for the interstellar medium (ISM) we obtained the opacity (dust emission cross-section per H nucleon), {sigma}{sub e}(1200), for every pixel. From a value {approx}1 Multiplication-Sign 10{sup -25} cm{sup 2} H{sup -1} at the lowest column densities that is typical of the high-latitude diffuse ISM, {sigma}{sub e}(1200) increases as N {sup 0.28} {sub H} over the range studied. This is suggestive of grain evolution. Integrating the SEDs over frequency, we also calculated the specific power P (emission power per H) for the big grains. In low column density regions where dust clouds are optically thin to the interstellar radiation field (ISRF), P is typically 3.7 Multiplication-Sign 10{sup -31} W H{sup -1}, again close to that in the high-latitude diffuse ISM. However, we find evidence for a decrease of P in high column density regions, which would be a natural outcome of attenuation of the ISRF that heats the grains, and for localized increases for dust illuminated by nearby stars or embedded protostars.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2013

COSMOLOGICAL MAGNETOHYDRODYNAMIC SIMULATIONS OF GALAXY CLUSTER RADIO RELICS: INSIGHTS AND WARNINGS FOR OBSERVATIONS

Non-thermal radio emission from cosmic-ray electrons in the vicinity of merging galaxy clusters is an important tracer of cluster merger activity, and is the result of complex physical processes that involve magnetic fields, particle acceleration, gas dynamics, and radiation. In particular, objects known as radio relics are thought to be the result of shock-accelerated electrons that, when embedded in a magnetic field, emit synchrotron radiation in the radio wavelengths. In order to properly model this emission, we utilize the adaptive mesh refinement simulation of the magnetohydrodynamic evolution of a galaxy cluster from cosmological initial conditions. We locate shock fronts and apply models of cosmic-ray electron acceleration that are then input into radio emission models. We have determined the thermodynamic properties of this radio-emitting plasma and constructed synthetic radio observations to compare observed galaxy clusters. We find a significant dependence of the observed morphology and radio relic properties on the viewing angle of the cluster, raising concerns regarding the interpretation of observed radio features in clusters. We also find that a given shock should not be characterized by a single Mach number. We find that the bulk of the radio emission comes from gas with T > 5 Multiplication-Sign 10{sup 7} K, {rho} {approx} 10{sup -28}-10{sup -27} g cm{sup -3}, with magnetic field strengths of 0.1-1.0 {mu}G, and shock Mach numbers of M {approx} 3-6. We present an analysis of the radio spectral index which suggests that the spatial variation of the spectral index can mimic synchrotron aging. Finally, we examine the polarization fraction and position angle of the simulated radio features, and compare to observations.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2013

PSR J0357+3205: THE TAIL OF THE TURTLE

Using a new XMM-Newton observation, we have characterized the X-ray properties of the middle-aged radio-quiet {gamma}-ray pulsar J0357+3205 (named Morla) and its tail. The X-ray emission from the pulsar is consistent with a magnetospheric non-thermal origin plus a thermal emission from a hot spot (or hot spots). The lack of a thermal component from the whole surface makes Morla the coldest neutron star in its age range. We found marginal evidence for a double-peaked modulation of the X-ray emission. The study of the 9' long tail confirmed the lack of extended emission near the pulsar itself. The tail shows a very asymmetric brightness profile and its spectrum lacks any spatial variation. We found the nebular emission to be inconsistent with a classical bow shock, ram-pressure-dominated pulsar wind nebula. We propose thermal bremsstrahlung as an alternative mechanism for Morla's tail emission. In this scenario, the tail emission comes from the shocked interstellar medium (ISM) material heated up to X-ray temperatures. This can fully explain the peculiar features of the tail, assuming a hot, moderately dense ISM around the pulsar. For a bremsstrahlung-emitting tail, we can estimate the pulsar distance to be between 300 and 900 pc. A pulsar velocity of {approx}1900 km s{sup -1} is required, which would make Morla the pulsar with the largest velocity, and high inclination angles (>70 Degree-Sign ) are preferred. We propose Morla's nebula as the first example of a new 'turtle's tail' class of thermally emitting nebulae associated with high-velocity pulsars.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2013

A CR-HYDRO-NEI MODEL OF MULTI-WAVELENGTH EMISSION FROM THE VELA JR. SUPERNOVA REMNANT (SNR RX J0852.0-4622)

Based largely on energy budget considerations and the observed cosmic-ray (CR) ionic composition, supernova remnant (SNR) blast waves are the most likely sources of CR ions with energies at least up to the 'knee' near 10{sup 15} eV. Shocks in young shell-type TeV-bright SNRs are surely producing TeV particles, but the emission could be dominated by ions producing {pi}{sup 0}-decay emission or electrons producing inverse Compton gamma rays. Unambiguously identifying the GeV-TeV emission process in a particular SNR will not only help pin down the origin of CRs, it will also add significantly to our understanding of the diffusive shock acceleration (DSA) mechanism and improve our understanding of supernovae and the impact SNRs have on the circumstellar medium. In this study, we investigate the Vela Jr. SNR, an example of TeV-bright non-thermal SNRs. We perform hydrodynamic simulations coupled with nonlinear DSA and non-equilibrium ionization near the forward shock to confront currently available multi-wavelength data. We find, with an analysis similar to that used earlier for SNR RX J1713.7-3946, that self-consistently modeling the thermal X-ray line emission with the non-thermal continuum in our one-dimensional model strongly constrains the fitting parameters, and this leads convincingly to a leptonic origin for the GeV-TeV emission for Vela Jr. This conclusion is further supported by applying additional constraints from observation, including the radial brightness profiles of the SNR shell in TeV gamma rays, and the spatial variation of the X-ray synchrotron spectral index. We will discuss implications of our models on future observations by the next-generation telescopes.

79 ASTRONOMY AND ASTROPHYSICS↗