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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 1,009 records · Page 56
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DOE OSTI2013

Effects of enhanced stratification on equatorward dynamo wave propagation

We present results from simulations of rotating magnetized turbulent convection in spherical wedge geometry representing parts of the latitudinal and longitudinal extents of a star. Here we consider a set of runs for which the density stratification is varied, keeping the Reynolds and Coriolis numbers at similar values. In the case of weak stratification, we find quasi-steady dynamo solutions for moderate rotation and oscillatory ones with poleward migration of activity belts for more rapid rotation. For stronger stratification, the growth rate tends to become smaller. Furthermore, a transition from quasi-steady to oscillatory dynamos is found as the Coriolis number is increased, but now there is an equatorward migrating branch near the equator. The breakpoint where this happens corresponds to a rotation rate that is about three to seven times the solar value. The phase relation of the magnetic field is such that the toroidal field lags behind the radial field by about π/2, which can be explained by an oscillatory α{sup 2} dynamo caused by the sign change of the α-effect about the equator. We test the domain size dependence of our results for a rapidly rotating run with equatorward migration by varying the longitudinal extent of our wedge. The energy of the axisymmetric mean magnetic field decreases as the domain size increases and we find that an m = 1 mode is excited for a full 2π azimuthal extent, reminiscent of the field configurations deduced from observations of rapidly rotating late-type stars.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2013

PROBING THE ROSETTE NEBULA STELLAR BUBBLE WITH FARADAY ROTATION

We report the results of Faraday rotation measurements of 23 background radio sources whose lines of sight pass through or close to the Rosette Nebula. We made linear polarization measurements with the Karl G. Jansky Very Large Array (VLA) at frequencies of 4.4 GHz, 4.9 GHz, and 7.6 GHz. We find the background Galactic contribution to the rotation measure in this part of the sky to be +147 rad m{sup -2}. Sources whose lines of sight pass through the nebula have an excess rotation measure of 50-750 rad m{sup -2}, which we attribute to the plasma shell of the Rosette Nebula. We consider two simple plasma shell models and how they reproduce the magnitude and sign of the rotation measure, and its dependence on distance from the center of the nebula. These two models represent different modes of interaction of the Rosette Nebula star cluster with the surrounding interstellar medium. Both can reproduce the magnitude and spatial extent of the rotation measure enhancement, given plausible free parameters. We contend that the model based on a stellar bubble more closely reproduces the observed dependence of rotation measure on distance from the center of the nebula.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2022

Energy balance and Alfvén Mach numbers in compressible magnetohydrodynamic turbulence with a large-scale magnetic field

Energy equipartition is a powerful theoretical tool for understanding astrophysical plasmas. It is invoked, for example, to measure magnetic fields in the interstellar medium (ISM), as evidence for small-scale turbulent dynamo action, and, in general, to estimate the energy budget of star-forming molecular clouds. Here, in this study, we motivate and explore the role of the volume-averaged root-mean-squared (rms) magnetic coupling term between the turbulent, δB , and large-scale, B 0 , fields, $\langle$(δB·B 0 ) 2 $\rangle$$_{V}^{1/2}$. By considering the second moments of the energy balance equations we show that the rms coupling term is in energy equipartition with the volume-averaged turbulent kinetic energy for turbulence with a sub-Alfvénic large-scale field. Under the assumption of exact energy equipartition between these terms, we derive relations for the magnetic and coupling term fluctuations, which provide excellent, parameter-free agreement with time-averaged data from 280 numerical simulations of compressible magnetohydrodynamic (MHD) turbulence. Furthermore, we explore the relation between the turbulent mean field and total Alfvén Mach numbers, and demonstrate that sub-Alfvénic turbulence can only be developed through a strong, large-scale magnetic field, which supports an extremely super-Alfvénic turbulent magnetic field. This means that the magnetic field fluctuations are significantly subdominant to the velocity fluctuations in the sub-Alfvénic large-scale field regime. Throughout our study, we broadly discuss the implications for observations of magnetic fields and understanding the dynamics in the magnetized ISM.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2015

THERMAL STABILITY OF MAGNETIZED, OPTICALLY THIN, RADIATIVE COOLING-DOMINATED ACCRETION DISKS

We investigate the thermal stability of optically thin, two-temperature, radiative cooling-dominated accretion disks. Our linear analysis shows that the disk is thermally unstable without magnetic fields, which agrees with previous stability analysis on the Shapiro-Lightman-Eardley disk. By taking into account the effects of magnetic fields, however, we find that the disk can be, or partly be, thermally stable. Our results may be helpful to understand the outflows in optically thin flows. Moreover, such radiative cooling-dominated disks may provide a new explanation of the different behaviors between black hole and neutron star X-ray binaries on the radio/X-ray correlation.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2014

The properties and fate of the galactic center G2 cloud

The object G2 was recently discovered descending into the gravitational potential of the supermassive black hole (BH) Sgr A*. We test the photoionized cloud scenario, determine the cloud properties, and estimate the emission during the pericenter passage. The incident radiation is computed starting from the individual stars at the locations of G2. The radiative transfer calculations are conducted with CLOUDY code and 2011 broadband and line luminosities are fitted. The spherically symmetric, tidally distorted, and magnetically arrested cloud shapes are tested with both the interstellar medium dust and 10 nm graphite dust. The best-fitting magnetically arrested model has the initial density n {sub init} = 1.8 × 10{sup 5} cm{sup –3}, initial radius R {sub init} = 2.2 × 10{sup 15} cm = 17 mas, mass m {sub cloud} = 4 M {sub Earth}, and dust relative abundance A = 0.072. It provides a good fit to 2011 data, is consistent with the luminosities in 2004 and 2008, and reaches an agreement with the observed size. We revise down the predicted radio and X-ray bow shock luminosities to be below the quiescent level of Sgr A*, which readily leads to non-detection in agreement to observations. The magnetic energy dissipation in the cloud at the pericenter coupled with more powerful irradiation may lead to an infrared source with an apparent magnitude m{sub L{sup ′}}≈13.0. No shock into the cloud and no X-rays are expected from cloud squeezing by the ambient gas pressure. Larger than previously estimated cloud mass m {sub cloud} = (4-20) M {sub Earth} may produce a higher accretion rate and a brighter state of Sgr A* as the debris descend onto the BH.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2013

COLLAPSE AND FRAGMENTATION OF MAGNETIC MOLECULAR CLOUD CORES WITH THE ENZO AMR MHD CODE. I. UNIFORM DENSITY SPHERES

Magnetic fields are important contributors to the dynamics of collapsing molecular cloud cores, and can have a major effect on whether collapse results in a single protostar or fragmentation into a binary or multiple protostar system. New models are presented of the collapse of magnetic cloud cores using the adaptive mesh refinement code Enzo2.0. The code was used to calculate the ideal magnetohydrodynamics (MHD) of initially spherical, uniform density, and rotation clouds with density perturbations, i.e., the Boss and Bodenheimer standard isothermal test case for three-dimensional (3D) hydrodynamics codes. After first verifying that Enzo reproduces the binary fragmentation expected for the non-magnetic test case, a large set of models was computed with varied initial magnetic field strengths and directions with respect to the cloud core axis of rotation (parallel or perpendicular), density perturbation amplitudes, and equations of state. Three significantly different outcomes resulted: (1) contraction without sustained collapse, forming a denser cloud core; (2) collapse to form a single protostar with significant spiral arms; and (3) collapse and fragmentation into binary or multiple protostar systems, with multiple spiral arms. Comparisons are also made with previous MHD calculations of similar clouds with a barotropic equations of state. These results for the collapse of initially uniform density spheres illustrate the central importance of both magnetic field direction and field strength for determining the outcome of dynamic protostellar collapse.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2014

Rotational and radial velocities of 1.3-2.2 M {sub ☉} red giants in open clusters

This study presents the rotational distribution of red giant (RG) stars in 11 old to intermediate age open clusters. The masses of these stars are all above the Kraft break, so they lose negligible amounts of their birth angular momentum (AM) during the main-sequence (MS) evolution. However, they do span a mass range with quite different AM distributions imparted during formation, with the stars less massive than ∼1.6M {sub ☉} arriving on the MS with lower rotation rates than the more massive stars. The majority of RGs in this study are slow rotators across the entire red giant branch regardless of mass, supporting the picture that intermediate-mass stars rapidly spin down when they evolve off the MS and develop convection zones capable of driving a magnetic dynamo. Nevertheless, a small fraction of RGs in open clusters show some level of enhanced rotation, and faster rotators are as common in these clusters as in the field RG population. Most of these enhanced rotators appear to be red clump stars, which is also true of the underlying stellar sample, while others are clearly RGs that are above or below the clump. In addition to rotational velocities, the radial velocities (RVs) and membership probabilities of individual stars are also presented. Cluster heliocentric RVs for NGC 6005 and Pismis 18 are reported for the first time.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2015

Pulsar J0453+1559: A double neutron star system with a large mass asymmetry

To understand the nature of supernovae and neutron star (NS) formation, as well as binary stellar evolution and their interactions, it is important to probe the distribution of NS masses. Until now, all double NS (DNS) systems have been measured as having a mass ratio close to unity (q ≥ 0.91). Here, we report the measurement of the individual masses of the 4.07-day binary pulsar J0453+1559 from measurements of the rate of advance of periastron and Shapiro delay: the mass of the pulsar is M{sub p} = 1.559 ± 0.005 M{sub ⊙} and that of its companion is M{sub c}=1.174±0.004 M{sub ⊙}; q = 0.75. If this companion is also an NS, as indicated by the orbital eccentricity of the system (e = 0.11), then its mass is the smallest precisely measured for any such object. The pulsar has a spin period of 45.7 ms and a spin period derivative of P-dot = (1.8616±0.0007)×10{sup −19} s s{sup −1}; from these, we derive a characteristic age of ∼ 4.1×10{sup 9} years and a magnetic field of ∼ 2.9×10{sup 9} G, i.e., this pulsar was mildly recycled by the accretion of matter from the progenitor of the companion star. This suggests that it was formed with (very approximately) its current mass. Thus, NSs form with a wide range of masses, which is important for understanding their formation in supernovae. It is also important for the search for gravitational waves released during an NS–NS merger: it is now evident that we should not assume that all DNS systems are symmetric.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2017

Laminar and Turbulent Dynamos in Chiral Magnetohydrodynamics. I. Theory

The magnetohydrodynamic (MHD) description of plasmas with relativistic particles necessarily includes an additional new field, the chiral chemical potential associated with the axial charge (i.e., the number difference between right- and left-handed relativistic fermions). This chiral chemical potential gives rise to a contribution to the electric current density of the plasma ( chiral magnetic effect ). We present a self-consistent treatment of the chiral MHD equations , which include the back-reaction of the magnetic field on a chiral chemical potential and its interaction with the plasma velocity field. A number of novel phenomena are exhibited. First, we show that the chiral magnetic effect decreases the frequency of the Alfvén wave for incompressible flows, increases the frequencies of the Alfvén wave and of the fast magnetosonic wave for compressible flows, and decreases the frequency of the slow magnetosonic wave. Second, we show that, in addition to the well-known laminar chiral dynamo effect, which is not related to fluid motions, there is a dynamo caused by the joint action of velocity shear and chiral magnetic effect. In the presence of turbulence with vanishing mean kinetic helicity, the derived mean-field chiral MHD equations describe turbulent large-scale dynamos caused by the chiral alpha effect, which is dominant for large fluid and magnetic Reynolds numbers. The chiral alpha effect is due to an interaction of the chiral magnetic effect and fluctuations of the small-scale current produced by tangling magnetic fluctuations (which are generated by tangling of the large-scale magnetic field by sheared velocity fluctuations). These dynamo effects may have interesting consequences in the dynamics of the early universe, neutron stars, and the quark–gluon plasma.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2013

The Prometheus Magneto-Inertial Fusion Research Facility [White Paper]

A next generation pulsed power magnetized high energy density laboratory plasma (MHEDLP) research facility, named Prometheus, which will enable demonstration of Q=1 (DT equivalent) low-convergence ratio pulsed-power magneto-inertial fusion (MIF) plasmas. The USA presently leads the world in pulsed power MHED research, with efforts primarily at AFRL’s Shiva Star Facility (FRCHX) and Sandia (Z-Machine MagLIF). Magneto-inertial fusion combines features of both magnetic and inertial fusion approaches, creating an intrinsically pulsed hybrid approach to fusion in the laboratory which solves some of the pressing problems that the two conventional (MCF and ICF) approaches face individually. Physics demonstrations of kilovolt MHED plasmas are expected in the next few years on these present facilities.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2015

SPATIAL AND SPECTRAL MODELING OF THE GAMMA-RAY DISTRIBUTION IN THE LARGE MAGELLANIC CLOUD

We perform spatial and spectral analyses of the LMC gamma-ray emission collected over 66 months by the Fermi Gamma-ray Space Telescope. In our spatial analysis, we model the LMC cosmic-ray distribution and gamma-ray production using observed maps of the LMC interstellar medium, star formation history, interstellar radiation field, and synchrotron emission. We use bootstrapping of the data to quantify the robustness of spatial model performance. We model the LMC gamma-ray spectrum using fitting functions derived from the physics of π{sup 0} decay, Bremsstrahlung, and inverse Compton scattering. We find the integrated gamma-ray flux of the LMC from 200 MeV to 20 GeV to be 1.37 ± 0.02 × 10{sup −7} ph cm{sup −2} s{sup −1}, of which we attribute about 6% to inverse Compton scattering and 44% to Bremsstrahlung. From our work, we conclude that the spectral index of the LMC cosmic-ray proton population is 2.4 ± 0.2, and we find that cosmic-ray energy loss through gamma-ray production is concentrated within a few 100 pc of acceleration sites. Assuming cosmic-ray energy equipartition with magnetic fields, we estimate LMC cosmic rays encounter an average magnetic field strength ∼3 μG.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2020

HAWC+ Far-infrared Observations of the Magnetic Field Geometry in M51 and NGC 891

Stratospheric Observatory for Infrared Astronomy High-resolution Airborne Wideband Camera Plus polarimetry at 154 μm is reported for the face-on galaxy M51 and the edge-on galaxy NGC 891. For M51, the polarization vectors generally follow the spiral pattern defined by the molecular gas distribution, the far-infrared (FIR) intensity contours, and other tracers of star formation. The fractional polarization is much lower in the FIR-bright central regions than in the outer regions, and we rule out loss of grain alignment and variations in magnetic field strength as causes. When compared with existing synchrotron observations, which sample different regions with different weighting, we find the net position angles are strongly correlated, the fractional polarizations are moderately correlated, but the polarized intensities are uncorrelated. We argue that the low fractional polarization in the central regions must be due to significant numbers of highly turbulent segments across the beam and along lines of sight in the beam in the central 3 kpc of M51. For NGC 891, the FIR polarization vectors within an intensity contour of 1500 MJy sr{sup −1} are oriented very close to the plane of the galaxy. The FIR polarimetry is probably sampling the magnetic field geometry in NGC 891 much deeper into the disk than is possible with NIR polarimetry and radio synchrotron measurements. In some locations in NGC 891, the FIR polarization is very low, suggesting we are preferentially viewing the magnetic field mostly along the line of sight, down the length of embedded spiral arms. There is tentative evidence for a vertical field in the polarized emission off the plane of the disk.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2014

THE ORTHOGONAL GAMMA-RAY BURST MODEL

We explore the analogy between a rotating magnetized black hole and an axisymmetric pulsar and derive the black hole's electromagnetic spindown after its formation in the core collapse of a supermassive star. The spindown shows two characteristic phases: an early Blandford-Znajek phase that lasts a few hundred seconds and a late pulsar-like afterglow phase that lasts much longer. During the first phase, the spindown luminosity decreases almost exponentially, whereas during the afterglow phase it decreases as t {sup –a} with 1 ≲ a ≲ 1.5. We associate our findings with long duration gamma-ray bursts and compare them with observations.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2015

TOWARD AN EMPIRICAL THEORY OF PULSAR EMISSION. X. ON THE PRECURSOR AND POSTCURSOR EMISSION

Precursors and postcursors (PPCs) are rare emission components, which appear beyond the main pulse emission, in some cases far away from it, and are detected in a handful of pulsar. In this paper we attempt to characterize the PPC emission in relation to the pulsar main pulse geometry. In our analysis we find that PPC components have properties very different from that of outer conal emission. The separation of the PPC components from the main pulse center remains constant with frequency. In addition the beam opening angles corresponding to the separation of PPC components from the pulsar center are much larger than the largest encountered in conal emission. Pulsar radio emission is believed to originate within the magnetic polar flux tubes due to the growth of instabilities in the outflowing relativistic plasma. Observationally, there is strong evidence that the main pulse emission originates at altitudes of about 50 neutron star radii for a canonical pulsar. Currently, the most plausible radio emission model that can explain main pulse emission is the coherent curvature radiation mechanism, wherein relativistic charged solitons are formed in a non-stationary electron-positron-pair plasma. The wider beam opening angles of PPC require the emission to emanate from larger altitudes as compared to the main pulse, if both these components originate by the same emission mechanism. We explore this possibility and find that this emission mechanism is probably inapplicable at the height of the PPC emission. We propose that the PPC emission represents a new type of radiation from pulsars with a mechanism different from that of the main pulse.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2016

A SEARCH FOR VERY HIGH ENERGY GAMMA RAYS FROM THE MISSING LINK BINARY PULSAR J1023+0038 WITH VERITAS

The binary millisecond radio pulsar PSR J1023+0038 exhibits many characteristics similar to the gamma-ray binary system PSR B1259–63/LS 2883, making it an ideal candidate for the study of high-energy nonthermal emission. It has been the subject of multiwavelength campaigns following the disappearance of the pulsed radio emission in 2013 June, which revealed the appearance of an accretion disk around the neutron star. We present the results of very high energy (VHE) gamma-ray observations carried out by the Very Energetic Radiation Imaging Telescope Array System before and after this change of state. Searches for steady and pulsed emission of both data sets yield no significant gamma-ray signal above 100 GeV, and upper limits are given for both a steady and pulsed gamma-ray flux. These upper limits are used to constrain the magnetic field strength in the shock region of the PSR J1023+0038 system. Assuming that VHE gamma rays are produced via an inverse Compton mechanism in the shock region, we constrain the shock magnetic field to be greater than ∼2 G before the disappearance of the radio pulsar and greater than ∼10 G afterward.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2018

Characteristics of a Degenerate Neutron Gas in a Magnetic Field with Allowance for the Anomalous Magnetic Moment of the Neutron

General expressions for the dependence of the Fermi energy, pressure, and total energy of a degenerate neutron gas in a magnetic field on the magnitude of the field and the neutron concentration with allowance for the anomalous magnetic moment of the neutron have been obtained in implicit form, and the dependence of these quantities on the field is presented in graphical form for the neutron concentration C = 10{sup 38} cm{sup –3}, which is typical for neutron stars. Analytical estimates of the pressure have been made for the magnitude of the fields possible in neutron stars ~10{sup 17}–10{sup 19} G and this neutron concentration ~10{sup 38} cm{sup –3}, including when the neutron gas is close to its saturated state with preferred orientation of the anomalous magnetic moment of all the neutrons in alignment with the field. It is found that even such fields ~10{sup 17} G have practically no effect on the pressure in comparison with the case when the field is absent, an effect being possible only for В ~ 10{sup 18}–10{sup 19} G. The analytical dependence on the neutron concentration of the corresponding field B{sub S} at which the neutron gas transitions to the saturated state has been found in explicit form. It is established that for B > B{sub S} the indicated characteristics of the neutron gas, and likewise its state, no longer change.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
DOE OSTI2022

Black Hole to Photosphere: 3D GRMHD Simulations of Collapsars Reveal Wobbling and Hybrid Composition Jets

Long-duration γ-ray bursts (GRBs) accompany the collapse of massive stars and carry information about the central engine. However, no 3D models have been able to follow these jets from their birth via black hole (BH) to the photosphere. We present the first such 3D general-relativity magnetohydrodynamic simulations, which span over six orders of magnitude in space and time. The collapsing stellar envelope forms an accretion disk, which drags inwardly the magnetic flux that accumulates around the BH, becomes dynamically important, and launches bipolar jets. The jets reach the photosphere at ~10 12 cm with an opening angle θ j ~ 6° and a Lorentz factor Γ j ≲ 30, unbinding ≳90% of the star. We find that (i) the disk–jet system spontaneously develops misalignment relative to the BH rotational axis. As a result, the jet wobbles with an angle θ t ~ 12°, which can naturally explain quiescent times in GRB lightcurves. The effective opening angle for detection θ j + θ t suggests that the intrinsic GRB rate is lower by an order of magnitude than standard estimates. This suggests that successful GRBs are rarer than currently thought and emerge in only ~0.1% of supernovae Ib/c, implying that jets are either not launched or choked inside most supernova Ib/c progenitors. (ii) The magnetic energy in the jet decreases due to mixing with the star, resulting in jets with a hybrid composition of magnetic and thermal components at the photosphere, where ~10% of the gas maintains magnetization σ ≳ 0.1. This indicates that both a photospheric component and reconnection may play a role in the prompt emission.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2013

Photometric variability in a warm, strongly magnetic DQ white dwarf, SDSS J103655.39+652252.2

We present the discovery of photometric variability in the DQ white dwarf SDSS J103655.39+652252.2 (SDSS J1036+6522). Time-series photometry reveals a coherent monoperiodic modulation at a period of 1115.64751(67) s with an amplitude 0.442% ± 0.024%; no other periodic modulations are observed with amplitudes ≳ 0.13%. The period, amplitude, and phase of this modulation are constant within errors over 16 months. The spectrum of SDSS J1036+6522 shows magnetic splitting of carbon lines, and we use Paschen-Back formalism to develop a grid of model atmospheres for mixed carbon and helium atmospheres. Our models, while reliant on several simplistic assumptions, nevertheless match the major spectral and photometric properties of the star with a self-consistent set of parameters: T {sub eff} ≈ 15, 500 K, log g ≈ 9, log (C/He) = –1.0, and a mean magnetic field strength of 3.0 ± 0.2 MG. The temperature and abundances strongly suggest that SDSS J1036+6522 is a transition object between the hot, carbon-dominated DQs and the cool, helium-dominated DQs. The variability of SDSS J1036+6522 has characteristics similar to those of the variable hot carbon-atmosphere white dwarfs (DQVs), however, its temperature is significantly cooler. The pulse profile of SDSS J1036+6522 is nearly sinusoidal, in contrast with the significantly asymmetric pulse shapes of the known magnetic DQVs. If the variability in SDSS J1036+6522 is due to the same mechanism as other DQVs, then the pulse shape is not a definitive diagnostic on the absence of a strong magnetic field in DQVs. It remains unclear whether the root cause of the variability in SDSS J1036+6522 and the other hot DQVs is the same.

79 ASTRONOMY AND ASTROPHYSICS↗